September
2018
Updated: 17 September 2018
Welcome to the night skies of Spring, featuring Scorpius, Sagittarius, Aquila, Lyra, Cygnus, Jupiter, Saturn and Mars
Note: Some parts of this webpage may be formatted incorrectly by older browsers.
The Alluna RC-20 Ritchey Chrétien telescope was installed in March, 2016. Seen on the video monitor is the same image that is the Lunar Feature for this Month (see below).
The 20-inch telescope is able to locate and track any sky object (including Earth satellites and the International Space Station) with software called TheSkyX Professional, into which is embedded a unique T-Point model developed for our site with our equipment over the past year.
Explanatory Notes:
Rise and set times are given for the theoretical horizon, which is a flat horizon all the
way round the compass, with no mountains, hills, trees or buildings to obscure the view. Observers will have to make allowance for their own actual horizon.
Transient phenomena are provided for the current month and the next. Geocentric phenomena are calculated as if the Earth were fixed in space
as the ancient Greeks believed. This viewpoint is useful, as otherwise rising and setting times would be meaningless. In the list of geocentric events, the nearer object is
given first.
When a planet is referred to as ‘stationary’, it means that its movement across the stellar background appears to have ceased, not that
the planet itself has stopped. With inferior planets (those inside the Earth’s orbit, Mercury and Venus), this is caused by the planet heading either directly
towards or directly away from the Earth. With superior planets (Mars out to Pluto), this phenomenon is caused by the planet either beginning or ending its
retrograde loop due to the Earth’s overtaking it.
Apogee and perigee: Maximum and minimum distances of the Moon or artificial satellite from the Earth.
Aphelion and perihelion: Maximum and minimum distances of a planet, asteroid or comet from the Sun.
The zenith is the point in the
sky directly overhead from the observer. The
meridian is a semicircle starting from a point on the horizon that is exactly
due north from the observer, and arching up into the sky to the zenith and
continuing down to a point on the horizon that is exactly due south. On the way
down it passes through the South Celestial Pole which is 26.6 degrees above the
horizon at Nambour. The elevation of the South Celestial Pole is exactly the
same as the observer's latitude, e.g. from Cairns it is 16.9 degrees above the
horizon, and from Melbourne it is 37.8 degrees. The Earth's axis points to this
point in the sky in the southern hemisphere, and to an equivalent point in the
northern hemisphere, near the star Polaris
All astronomical objects rise until they reach the meridian, then they begin to
set. The act of crossing or 'transitting' the meridian is called 'culmination'.
Objects closer to the South Celestial Pole than its altitude above the southern
horizon do not rise or set, but are always above the horizon, constantly
circling once each sidereal day. They are called 'circumpolar'. A handspan at arm's length with fingers spread covers an angle of approximately 18 - 20 degrees.
mv = visual magnitude or brightness. Magnitude 1 stars are very bright, magnitude 2 less so, and
magnitude 6 stars are so faint that the unaided eye can only just detect them under good, dark conditions. Binoculars will allow us to see down to magnitude
8, and the Observatory telescope can reach visual magnitude 17 or 22 photographically. The world's biggest telescopes have detected stars and galaxies as faint as
magnitude 30. The sixteen very brightest stars are assigned magnitudes of 0 or even -1. The brightest star, Sirius, has a magnitude of -1.44. Jupiter can reach -2.4, and
Venus can be more than 6 times brighter at magnitude -4.7, bright enough to cast shadows. The Full Moon can reach magnitude -12 and the
overhead Sun is magnitude -26.5. Each
magnitude step is 2.51 times brighter or fainter than the next one, i.e. a magnitude 3.0 star is 2.51 times brighter than a magnitude 4.0. Magnitude 1.0
stars are exactly 100 times brighter than magnitude 6.0 (5 steps each of 2.51 times, 2.51x2.51x2.51x2.51x2.51 = 2.515 = 100).
The Four Minute Rule:
How long does it take the Earth to complete one rotation? No, it's not 24 hours - that is the time taken for the Sun to cross the meridian on successive days. (The meridian is an imaginary semicircular line running from the due south point on the horizon and arching overhead through the zenith, and coming down to the horizon again at its due north point.) This 24 hours is a little longer than one complete rotation, as the curve in the Earth's orbit means that it needs to turn a fraction more (~1 degree of angle) in order for the Sun to cross the meridian again. It is called a 'solar day'. The stars, clusters, nebulae and galaxies are so distant that most appear to have fixed positions in the night sky on a human time-scale, and for a star to return to the same point in the sky relative to a fixed observer takes 23 hours 56 minutes 4.0916 seconds. This is the time taken for the Earth to complete exactly one rotation, and is called a 'sidereal day'.
As our clocks and lives are organised to run on solar days of 24 hours, and the stars circulate in 23 hours 56 minutes approximately, there is a four minute difference between the movement of the Sun and the movement of the stars. This causes the following phenomena:
1. The Sun slowly moves in the sky relative to the stars by four minutes of time or one degree of angle per day. Over the course of a year it moves ~4 minutes X 365 days = 24 hours, and ~1 degree X 365 = 360 degrees or a complete circle. Together, both these facts mean that after the course of a year the Sun returns to exactly the same position relative to the stars, ready for the whole process to begin again.
2. For a given clock time, say 8:00 pm, the stars on consecutive evenings are ~4 minutes or ~1 degree further on than they were the previous night. This means that the stars, as well as their nightly movement caused by the Earth's rotation, also drift further west for a given time as the weeks pass. The stars of autumn, such as Orion are lost below the western horizon by mid-June, and new constellations, such as Sagittarius, have appeared in the east. The stars change with the seasons, and after a year, they are all back where they started, thanks to the Earth's having completed a revolution of the Sun and returned to its theoretical starting point.
We can therefore say that the star patterns we see in the sky at 11:00 pm tonight will be identical to those we see at 10:32 pm this day next week (4 minutes X 7 = 28 minutes earlier), and will be identical to those of 9:00 pm this date next month or 7:00 pm the month after. All the above also includes the Moon and planets, but their movements are made more complicated, for as well as the Four Minute Drift with the stars, they also drift at different rates against the starry background, the closest ones drifting the fastest (such as the Moon or Venus), and the most distant ones (such as Saturn or Neptune) moving the slowest.
Solar System
Sun:
The Sun begins the month in the constellation of Leo, the Lion. It leaves Leo and passes into Virgo, the Virgin on September 17.
Moon Phases: Lunations (Brown series): #1183, 1184, 1185
New Moon:
First Quarter:
Full Moon:
Lunar Orbital Elements:
September 08: Moon at perigee (361 352 km) at 11:36 hrs, diameter = 33.1'
September 20: Moon at apogee (404 876 km) at 11:16 hrs, diameter = 29.5'
September 20: Moon at descending node at 19:33 hrs, diameter = 29.5'
October 06: Moon at perigee (366 399 km) at 08:41 hrs, diameter = 32.6'
October 17: Moon at descending node at 22:06 hrs, diameter = 29.6'
October 18: Moon at apogee (404 215 km) at 05:21 hrs, diameter = 29.6'
October 31: Moon at ascending node at 13:46 hrs, diameter = 32.3'
Moon at 8 days after New, as on September 18.
The photograph above shows the
Moon when approximately eight days after New, just after First Quarter. A detailed map of the Moon's near side is available here. A rotatable view of the Moon, with ability to zoom in close to the surface
(including the far side), and giving detailed
information on each feature, may be downloaded here.
Click
here for a photographic animation showing the lunar phases. It also shows the Moon's wobble or libration, and how its apparent size changes as it moves from perigee to apogee each month. It takes a little while to load, but once running is very cool ! All these downloads are freeware, although the authors do accept donations if the user feels inclined to support their work.
Lunar Feature for this Month
Each month we describe a lunar crater, cluster of craters, valley, mountain range or other object, chosen at random, but one with interesting attributes. A recent photograph from our Alluna RC20 telescope will illustrate the object. As all large lunar objects are named, the origin of the name will be given if it is important. This month we will look at the crater Gutenberg and its neighbours Gaudibert and Goclenius.
This image of the south-western margin of the Moon's Mare Fecunditatis (Sea of Fertility) was taken at 5:19 pm on 18 July 2018,
just five minutes after sunset. A deep red filter was used to counteract the
light of the sky. North is to the top, east (where the Sun is
rising) is to the right. The central crater is 77 km diameter Gutenberg. To its south-east is the 56 km crater Goclenius, which
is crossed by numerous clefts. Towards the lower left corner of the image is the 34 km crater Gauldibert, which has a very unusual volcanic floor.
One craterlet clearly visible in this image, just north of
the small hill at the centre of Goclenius, is only 830 metres across.
Even smaller ones in the flat area due west of Goclenius may also be detected.
Gutenberg is quite an old crater. It has been deformed by a 28 km crater, almost as old, on its eastern side, which is known as Gutenberg E. Floods of molten lava
from the asteroid impact which created the Mare Fecunditatis have burst through and carried away some of the wall of Gutenberg E, and filled up that crater to the same level
of the outside Mare. Lava has then burst through the western wall of Gutenberg E and pooled on the floor of Gutenberg itself,
where it has swamped the bases of the mountain ranges and isolated peaks existing there, so that only their summits are visible today. On the outside of
Gutenberg's western wall is a much more recent crater, Gutenberg A, which is still perfectly shaped as a circular bowl crater with a diameter of 15 km. South
of Gutenberg and adjoining it is an ancient 45 km crater, Gutenberg C. Its northern ramparts have been obliterated by the later impact of Gutenberg itself,
and debris from that event has almost completely filled Gutenberg C, making it look more like a mountain mass.
In the lower right corner is an interesting crater, Goclenius, 56 km in diameter. Its walls are not very high, and its floor is depressed well below the level of
the surrounding Mare. Goclenius also has several mountain summits protruding above the floor, but it also has an interesting network of clefts crossing the
floor. The main cleft begins near the south wall and trends fin a north-north-western direction. Just before it meets the north wall, it turns
into a rille or graben, which is where a pair of parallel faults pull apart, allowing the space between them to drop 100 metres or more. This produces a
feature like a rift valley on Earth, averaging a width between the parallel walls of 2 to 5 km.
Some rilles stretch across the lunar surface for hundreds of
kilometres. The rille that starts in Goclenius can be seen continuing past Gutenberg to the top margin. There are two similar but shallower rilles running
parallel to it on its eastern side. A similar rille crosses the floor of Gutenberg,
and trending in the same direction. The third crater worthy of mention in this image is Gaudibert, 34 km in diameter and near the lower left corner. This crater does not have a bowl-shaped
or flat floor, but the whole crater and its surroundings is a pyroclastic area, which means that it was not caused by an impact, but was the centre of widespread explosive
volcanic activity. Features showing Gaudibert's tortured appearance are very uncommon on the Moon.
In the actual lower left corner can be seen part of the Mare Nectaris, or
Sea of Nectar. In the upper right corner is part of the Mare Fecunditatis, or
Sea of Fertility. The smoothness of the Mares or Seas shows that they are much more recent features than the heavily-cratered Terrae or Lands.
Johannes Gutenberg
The first documents were written on papyrus, made from the stalks of the papyrus plant, crushed and pressed together. Egyptian examples from 2550 BCE
describing the building of the pyramids of Giza are the oldest in existence. As sheets of papyrus were prone to cracking, parchment (named after the town
of Pergamum) became the writing material of choice from the Hellenistic period to medieval times, as it was pliable and could be made into scrolls. Parchment
was made of scraped and dried animal skin. The best quality parchment was made out of calfskin and called ‘vellum’.
Paper (from the word ‘papyrus’) first appeared in China around 150 BCE and was made of the crushed fibres of mulberry tree bark, hemp waste and old rags. It
spread to the Islamic world through Samarkand around AD 750, and reached Europe around AD 1000. By this time, paper had become cheaper than vellum, and was made
by hand from hemp fibres and discarded linen. By 1400 paper mills were set up around Europe, but the machines were very rudimentary and could only crush
the raw materials. The production of paper sheets was still very labour-intensive. Books were only affordable by institutions or the well-off.
Johannes Gutenberg (1398-1468) was born in Mainz and moved to Strasbourg in 1428. By 1438 he was working with three partners on a secret project, the development of a new printing process.
Gutenberg’s great invention was to work out a way to cast individual letters by hand, out of a soft alloy of 82% lead, 9% tin and 6% antimony. The individual letters, called ‘sorts’ (for they needed to be sorted, we call them ‘slugs’ today) were made in reverse, so that they would print the right way round. Because of this, typesetters were admonished, “Mind your p’s and q’s”. They could also have been warned to “watch their b’s and d’s.” The letters were arranged into words, lines and paragraphs and clamped together, the lines also being in reverse. The paragraphs were then assembled into a frame or ‘forme’ to make complete pages for printing. If the typesetter ran out of cast letters, the work would be held up until more were cast and he could become irritable, leading to the saying, “he is out of sorts”.
After the run of pages had been printed, the letters were removed for later use. They were stored in a wooden case of 'pigeon-holes', the top rows being used for capital letters and the lower rows being used to house the small letters. This is the origin of the terms "upper case" and "lower case". The letters had to be consistent in size and shape so that they could be locked securely together for use in the printing press, so a crucial part of the process was the precision casting of the letters so that each line of type would match exactly with the other lines. From the beginning, this was regarded as an essential requirement. The typeface selected was a form of gothic script then used in Germany for writing liturgical texts and still widely used today.
As water-based inks would not adhere or ‘take’ to the metal letters, Gutenberg developed new inks which would, using lampblack and soot dissolved in linseed oil. This concept of re-usable metal type, the use of new waterproof inks made of vegetable oil and carbon, and the use of a wooden printing press based on a typical woodblock screw press, combined to produce a truly epochal invention – a practical mechanical printing system. Gutenberg had by this time returned to his hometown, Mainz, and there he produced his first publication, a German poem, in 1450.
In 1455 (possibly starting in 1454), Gutenberg brought out 180 copies of a beautifully executed folio Biblia Sacra (Holy Bible). It consisted of two volumes of 300 pages each, with 42 lines on each page. Especial care was taken in ensuring that the registration of the print on each page was precise. It is estimated that Gutenberg and his workmen had to cast as many as 46 000 individual sorts for this job. After printing the pages of text on a kind of paper made from rag-cotton and linen, each book was hand illustrated in the same elegant way as Bibles from the same period handwritten by scribes were. The paper pages, requiring the importation of 50 000 sheets from Italy, when printed were supplied as loose sets, but 30 copies were printed and bound using vellum, requiring an estimated 5000 calfskins. Most owners had the pages bound together elsewhere by book binders, in a style to match their other books. The date of 1455 was documented on the spine of a copy bound in Paris.
The ink used contained carbon, copper, lead and other metallic compounds that gave the printed text a lustrous, reflective sheen. The Gutenberg Bible as it came to be known, sold for up to 30 florins, which was roughly three years’ wages for an average clerk. Nonetheless, it was significantly cheaper than a handwritten Bible that could take a single scribe over a year to prepare. It immediately sold out, some of the first copies going to England. They were mostly bought by monasteries, libraries, cathedrals, and by wealthy individuals who wished to donate a copy to an institution. In the fifteenth century, only one Gutenberg Bible was in private hands.
Today, forty-eight substantially complete copies are known to exist, including two at the British Library that can be viewed and compared online. Although the text lacks modern features such as pagination, indentation of first lines and paragraph breaks, it is acclaimed for its high aesthetic and technical quality, the printed letters being clearly defined and with consistent spacing. In fact, the Gutenberg Bible is nothing short of spectacular, as online images will attest.
The introduction of Gutenberg’s printing method and oil-based inks made possible the development in Germany of intaglio engraving, in which artists improved on the traditional woodcut methods by engraving copper sheets to make printing plates for illustrations, diagrams and maps. Copper is a soft metal and easy to engrave, but it had a disadvantage in that, after striking the first few hundred copies, the plates would show signs of wear and deterioration. The engravers would then have to go over the grooves on the plates to sharpen the images produced. This is why printing runs were usually restricted to about 500 copies. The introduction of these new technologies revolutionised book production – news and books began to spread across Europe much faster than ever before. Also, their cheaper prices made them accessible to impoverished scholars, as well as the wealthy.
There were more books produced in the fifty years after the Gutenberg Bible than had been produced in the 1000 years preceding it. Gutenberg’s invention is considered to be the key factor in facilitating Luther’s Reformation, the Renaissance, the Scientific Revolution and the modern world. Paper made from pulped wood fibre only came into universal use when steam-powered machines made it economically feasible in the 1840s.
The photograph of Gutenberg covers the area inside the rectangle above.
Click
Geocentric Events:
September 2: Venus 1.2º south of the star Spica (Alpha Virginis, mv= 0.98) at 03:27 hrs
October 1: Limb of Moon 37 arcminutes south of the star Zeta Tauri (mv= 2.97) at 18:43 hrs
The Planets for this month:
It should be remembered that close approaches of Moon, planets and stars are only perspective effects as seen from the Earth - that is why they are called
'geocentric or Earth-centred phenomena'. The Moon, planets and stars do not really approach and dance around each other as it appears to us from the vantage
point of our speeding planet.
September 2: Mercury at perihelion at 19:29 hrs (diameter = 6.1")
September 3: Moon 1.6º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 12:59 hrs
September 4: Limb of Moon 37 arcminutes south of the star Zeta Tauri (mv= 2.97) at 14:53 hrs
September 5: Limb of Moon 47 arcminutes south of the star Mu Geminorum (mv= 2.87) at 07:41 hrs
September 5: Venus at aphelion at 19:43 hrs (diameter = 31.0")
September 6: Saturn at eastern stationary point at 19:25 hrs (diameter of globe = 17.1")
September 6: Mercury 1º north of the star Regulus (Alpha Leonis, mv= 1.36) at 13:41 hrs
September 8: Neptune at opposition at 04:06 hrs (diameter = 2.3")
September 9: Moon 2º north of Regulus (Alpha Leonis, mv= 1.36) at 00:18 hrs
September 9: Moon 1.6º north of Mercury at 08:07 hrs
September 13: Moon 10.6º north of Venus at 07:22 hrs
September 14: Moon 4.5º north of Jupiter at 14:09 hrs
September 16: Mars at perihelion at 21:42 hrs (diameter = 18.1")
September 18: Moon 2.6º north of Saturn at 03:28 hrs
September 19: Limb of Moon 35 arcminutes north of the star Pi Sagittarii (mv= 2.88) at 06:25 hrs
September 19: Moon 1.9º north of Pluto at 09:43 hrs
September 20: Moon 5.1º north of Mars at 12:26 hrs
September 21: Mercury in superior conjunction at 11:39 hrs (diameter = 4.8")
September 22: Moon 1.5º north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 09:36 hrs
September 23: Spring equinox at 11:48 hrs
September 24: Moon 1.6º south of Neptune at 04:35 hrs
September 26: Saturn at eastern quadrature at 09:46 hrs (diameter = 16.5")
September 27: Moon 4.4º south of Uranus at 18:35 hrs
September 30: Saturn 57 arcminutes north of the star 11 Sagittarii (mv= 4.96) at 13:20 hrs
September 30: Moon 1.5º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 16:48 hrs
October 2: Moon 1.4º south of the star Mu Geminorum (mv= 2.87) at 14:43 hrs
October 6: Venus at eastern stationary point at 04:50 hrs (diameter = 49.9")
October 6: Moon 2.3º north of the star Regulus (Alpha Leonis, mv= 1.36) at 10:16 hrs
October 6: Mercury 2º north of the star Spica (Alpha Virginis, mv= 0.98) at 16:44 hrs
October 10: Saturn 1.7º south of the star Mu Sagittarii (mv= 3.84) at 07:22 hrs
October 10: Moon 5.7º north of Mercury at 18:25 hrs
October 12: Moon 4.5º north of Jupiter at 07:54 hrs
October 12: Pluto at eastern quadrature at 13:58 hrs
October 15: Moon 2.2º north of Saturn at 10:56 hrs
October 16: Moon occults the star Pi Sagittarii (mv= 2.88) between 12:23 and 12:11 hrs
October 16: Moon 1.3º north of Pluto at 19:44 hrs
October 16: Mercury at aphelion at 19:06 hrs (diameter = 5.1")
October 18: Moon 2.3º north of Mars at 23:06 hrs
October 18: Moon occults the star Theta Capricorni (mv= 4.08) between 22:58 and 00:08 hrs
October 19: Limb of Moon 44 arcminutes north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 16:34 hrs
October 20: Mercury 2º south of the star Zuben Elgenubi (Alpha Librae, mv= 2.75) at 17:40 hrs
October 21: Moon 1.9º south of Neptune at 09:32 hrs
October 24: Uranus at opposition at 10:27 hrs (diameter = 3.7")
October 25: Moon 3.7º south of Uranus at 02:23 hrs
October 27:: Venus at inferior conjunction at 00:15 hrs (diameter = 61.3")
October 27: Moon 1.9º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 21:21 hrs
October 28: Moon occults the star Zeta Tauri (mv= 2.97) between 23:29 and 00:22 hrs
October 29: Mercury 3.1º south of Jupiter at 21:24 hrs
October 29: Moon 1.2º south of the star Mu Geminorum (mv= 2.87) at 18:08 hrs
October 30: Jupiter 43 arcminutes north of the star Kappa Librae (mv= 4.75) at 18:26 hrs
Mercury: On September 1, Mercury will be low in the east-north-eastern pre-dawn sky in the constellation of Leo. It will be hard to find due to its involvement with the glare of the Sun. Each successive night in the first week of September, Mercury will be harder to locate. It will pass through superior conjunction on September 21 and will move to the twilight evening sky. Mercury shines nearly as bright as Sirius, but is only visible when it is at a large angular distance from the glare of the Sun. As Mercury lies well inside the Earth's orbit and close to the Sun, it can never move more than 27.8º from the Sun. In the second week of October, Mercury will be seen close to the western horizon during twilight, and will appear close to Venus on October 16 and close to Jupiter on October 28 and 29. The thin crescent Moon will be between Mercury and Jupiter on November 9, but all three will be.
Venus: This, the brightest planet, passed through superior conjunction (on the far side of the Sun) on January 9, thereby moving from the pre-dawn eastern sky to the twilight western sky. This month it is very prominent in the early evenings as a so-called 'evening star'. In mid-September, Venus will be 42º or just over two handspans east of the Sun, quite high above the west-north-western horizon about 45 minutes after sunset. It will appear in a small telescope as a tiny 'crescent Moon' with a magnitude of -4.5 and a phase of 30%. It will be in the constellation Virgo. The waxing crescent Moon will be between Venus and Jupiter on the evening of September13. We will lose Venus from our early evening sky on October 27.
(The coloured fringes to the first and third images below are due to refractive effects in our own atmosphere, and are not intrinsic to Venus. The planet was closer to the horizon when these images were taken than it was for the second photograph, which was taken when Venus was at its greatest elongation from the Sun).
February 2018 August 2018 October 2018
Click here for a photographic animation showing the Venusian phases. Venus is always far brighter than anything else in the sky except for the Sun and Moon. For most of 2017, Venus appeared as a 'Morning Star' in the pre-dawn sky, where it stayed for about nine months. Venus passed on the far side of the Sun (superior conjunction) on January 9, 2018, and is now in the evening sky as an 'Evening Star'. It will return to the morning sky to be a 'Morning Star' once again on October 27.
Because Venus is visible as the 'Evening Star' and as the 'Morning Star', astronomers of ancient times believed that it was two different objects. They called it Hesperus when it appeared in the evening sky and Phosphorus when it was seen before dawn. They also realised that these objects moved with respect to the so-called 'fixed stars' and so were not really stars themselves, but planets (from the Greek word for 'wanderers'). When it was finally realised that the two objects were one and the same, the two names were dropped and the Greeks applied a new name Aphrodite (Goddess of Love) to the planet, to counter Ares (God of War). We use the Roman versions of these names, Venus and Mars, for these two planets.
Venus at 6.55 pm on September 7, 2018. The phase is 36 % and the angular
diameter is 32 arcseconds.
This is the year of Mars:
The red planet is ideally placed for viewing this month as it is well above the eastern horizon as night falls and is close to the zenith at mid-evening. On September 1 it is well past opposition, still a little brighter than Jupiter, and has an angular diameter of 21 arcseconds. Mars reaches a point near the zenith at 9:15 pm. It is near the border of Sagittarius and Capricornus. As the month progresses, Mars will travel eastwards against the background stars, moving deeper into Capricornus. It will pass into the next zodiacal constellation, Aquarius, on November 11.This has been a very favourable opposition, as Mars has appeared bigger than it has for many years. It has been particularly favourable for us in the southern hemisphere, as during the month of opposition it was almost directly overhead each midnight from the Sunshine Coast. This winter has been an excellent time for planet observing, with Mars, Jupiter and Saturn all available each evening and high overhead, with Venus also available in the western twilight sky. The next time that Mars will have an opposition in which it reaches a size as favourable as the current one did will be in September 2035, when Mars will be in the constellation Aquarius. The next opposition will occur in just over two years time, on October 14, 2020. Mars will become almost as big as this year (22 arcseconds), but being in the constellation Pisces will not be so favourably placed in the sky, having an altitude at culmination of only 58 degrees.
The only problem with the current apparition of Mars is that on May 31 last, a huge dust storm developed in its atmosphere. By the end of June the dust storm had completely enveloped the planet, totally obscuring the normally easily visible surface features and rendering the planet relatively featureless. The Mars rover Opportunity switched off its sensors and other facilities, as the storm turned the sky dark on Mars, and the solar panels on Opportunity were not providing enough electrical power to maintain normal operations, putting it into 'sleep' mode.
In August the storm began to dissipate, and some surface features became faintly visible, but not clear. The rover Curiosity, being nuclear powered and independent of sunlight, has continued to work, but the dust has meant that its cameras cannot operate properly. We hope that the storm dissipates before we leave Mars too far behind, so that we can clearly see details on Mars again, and enabling Opportunity to 'wake up' and resume its work.
On September 20, the waxing gibbous Moon will be just to the north (left) of Mars in the early evening sky.
In this image, the south polar cap of Mars is easily seen. Above it is a dark triangular area known as Syrtis Major. Dark Sinus Sabaeus runs off to the left, just south of the equator. Between the south polar cap and the equator is a large desert called Hellas. The desert to upper left is known as Aeria, and that to the north-east of Syrtis Major is called Isidis Regio. Photograph taken in 1971.
Mars
photographed from Starfield Observatory, Nambour on June 29 and July 9, 2016, showing two different
sides of the planet. The north polar cap is prominent.
Brilliant Mars at left, shining at magnitude 0.9, passes in front of the dark molecular clouds in Sagittarius on October 15, 2014. At the top margin is the white fourth magnitude star 44 Ophiuchi. Its type is A3 IV:m. Below it and to the left is another star, less bright and orange in colour. This is the sixth magnitude star SAO 185374, and its type is K0 III. To the right (north) of this star is a dark molecular cloud named B74. A line of more dark clouds wends its way down through the image to a small, extremely dense cloud, B68, just right of centre at the bottom margin. In the lower right-hand corner is a long dark cloud shaped like a figure 5. This is the Snake Nebula, B72. Above the Snake is a larger cloud, B77. These dark clouds were discovered by Edward Emerson Barnard at Mount Wilson in 1905. He catalogued 370 of them, hence the initial 'B'. The bright centre of our Galaxy is behind these dark clouds, and is hidden from view. If the clouds were not there, the galactic centre would be so bright that it would turn night into day.
Mars near opposition, July 24, 2018
Mars, called the red planet but usually coloured orange, has now taken on a yellowish tint and has brightened by 0.4 magnitude, making it twice as bright as previous predictions for the July 27 opposition. These phenomena have been caused by a great dust storm which has completely encircled the planet, obscuring the surface features so that they are only seen faintly through the thick curtain of dust. Although planetary photographers are mostly disappointed, many observers are interested to see that the yellow colour and increased brightness mean that a weather event on a distant planet can actually be detected with the unaided eye - a very unusual thing in itself.
The three pictures above were taken on the evening of July 24, at 9:05, 9:51 and 11:34 pm. Although the fine details that are usually seen on Mars are hidden by the dust storm,
some of the larger features can be discerned, revealing how much Mars rotates in two and a half hours. Mars' sidereal rotation period (the time taken for one complete
rotation or 'Martian day') is 24 hours 37 minutes 22 seconds - a little longer than an Earth day. The dust storm began in the Hellas Desert on May 31, and after two months
it still enshrouded the planet. In August it began to clear.
Central meridian: 295º.
The two pictures immediately above were taken on the evening of September 7, at 6:25 and 8:06 pm. The dust storm is finally abating, and some of the surface features are becoming visible once again. This pair of images also demonstrates the rotation of Mars in 1 hour 41 minutes (equal to 24.6 degrees of longitude), but this time the view is of the opposite side of the planet to the set of three above. As we are now leaving Mars behind, the images are appreciably smaller (the angular diameter of the red planet has fallen to 20 arcseconds). Well past opposition, Mars on September 7 exhibited a phase effect of 92.65 %.
Central meridian: 180º.
Jupiter: This gas giant planet is now visible in the sky only in the early evenings, in the constellation of Libra, the Scales. It spends the month quite close to the star Zuben Elgenubi, which is the brightest star in Libra. On August 7 Jupiter was at eastern quadrature, meaning that it was high in the north, crossing the meridian at sunset. In mid-September, Jupiter will be about two handspans above the western horizon at 7 pm, about a handspan above brilliant Venus. The waxing crescent Moon will be seen to the north (right) of Jupiter soon after sunset on September 14.
Jupiter as photographed from Nambour on the evening of April 25, 2017. The images were
taken, from left to right, at 9:10, 9:23, 9:49, 10:06 and 10:37 pm. The rapid rotation of this giant planet in a little under 10 hours is clearly seen. In the
southern hemisphere, the Great Red Spot (bigger than the Earth) is prominent, sitting within a 'bay' in the South Tropical Belt. South of it is one of the
numerous White Spots. All of these are features in the cloud tops of Jupiter's atmosphere.
Jupiter at opposition, May 9, 2018
The third photograph was taken at 10:20 pm, when Europa was approaching
Jupiter's central meridian. Its dark shadow is behind it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth
photograph at 10:34 pm shows Europa and its shadow well past the central meridian. Europa is the smallest of the Galilean satellites, and has a diameter of 3120 kilometres.
It is ice-covered, which accounts for its brightness and whitish colour. Jupiter's elevation above the horizon for the four photographs in order was 50º,
55º, 66º and 71º. As the evening progressed, the air temperature dropped a little and the planet gained altitude. The 'seeing' improved slightly, from
Antoniadi IV to Antoniadi III. At the time of the photographs, Europa's angular diameter was 1.57 arcseconds. Part of the final photograph is enlarged below.
Jupiter at 11:34 pm on May 18, ten days later. Changes in the rotating cloud patterns are apparent, as some cloud bands rotate faster than others and
interact. Compare with the first photograph in the line of four taken on May 9. The Great Red Spot is ploughing a furrow through the clouds of the South
Tropical Belt, and is pushing up a turbulent bow wave.
Jupiter as it appeared at 7:29 pm on July 2, 2017. The Great Red Spot is in a similar position near Jupiter's eastern limb (edge) as in the fifth picture in the series above.
It will be seen that in the past two months the position of the Spot has drifted
when compared with the festoons in the Equatorial Belt, so must rotate around the planet at a slower rate. In fact, the Belt enclosing the Great Red Spot rotates around the
planet in 9 hours 55 minutes, and the Equatorial Belt takes five minutes less. This high rate of rotation has made the planet quite oblate. The prominent 'bay'
around the Red Spot in the five earlier images appears to be disappearing, and a darker streak along the northern edge of the South Tropical Belt is moving
south. Two new white spots have developed in the South Temperate Belt, west of the Red Spot. The five upper images were taken near opposition, when the Sun was
directly behind the Earth and illuminating all of Jupiter's disc evenly. The July 2 image was taken just four days before Eastern Quadrature, when the angle
from the Sun to Jupiter and back to the Earth was at its maximum size. This angle means that we see a tiny amount of Jupiter's dark side, the shadow being
visible around the limb of the planet on the left-hand side, whereas the right-hand limb is clear and sharp. Three of Jupiter's Galilean satellites are visible, Ganymede to
the left and Europa to the right. The satellite Io can be detected in a transit of Jupiter, sitting in front of the North Tropical Belt, just to the left of its centre.
Jupiter reached opposition on May 9, 2018 at 10:21 hrs, and the above photographs were taken that evening, some ten to twelve hours later. The first image above was
taken at 9:03 pm, when the Great Red Spot was approaching Jupiter's central meridian and the satellite Europa was preparing to transit Jupiter's disc.
Europa's transit began at 9:22 pm, one minute after its shadow had touched Jupiter's cloud tops. The second photograph was taken three minutes later at 9:25
pm, with the Great Red Spot very close to Jupiter's central meridian.
Saturn:
The
photograph above was taken when Saturn was close to opposition, with the Earth between Saturn and the Sun. At that time, the shadow of Saturn's globe upon the Ring system
was directly behind the planet and hardly visible. The photograph below was taken at 7:14 pm on September 09, 2018, when Saturn was 17 days prior to eastern quadrature.
At such a time, the angle from the Sun to Saturn and back to the Earth is near its maximum, making the shadow fall at an angle across the Rings as seen from
Earth. It may be seen falling across the far side of the Ring to the left side of the globe.
Uranus:
This ice giant planet is only observable from about 10 pm on this month, as it reached conjunction with the Sun on April 18. Uranus shines at about magnitude 5.7, so a pair of binoculars or a small telescope is required to observe it. It is currently in the constellation Aries, near its south-west corner. At mid-month it rises at about 8:30 pm. The almost Full Moon will be in the vicinity of Uranus just before midnight on September 27. Uranus will reach opposition on October 24.
Neptune: The icy blue planet is located in the eastern end of the constellation Aquarius. It will reach opposition on September 8, so this month will be observable for most of the night. On September 1 it rises at 5:58 pm. The almost Full Moon will be in the vicinity of Neptune on September 23.
Neptune, photographed from Nambour on October 31, 2008
Pluto:
The movement of the dwarf planet Pluto in two days, between 13 and 15 September, 2008. Pluto is the one object that has moved.
Width of field: 200 arcseconds
This is a stack of four images, showing the movement of Pluto over the period October 22 to 25, 2014. Pluto's image for each date appears as a star-like point at the upper right corner of the numerals. The four are equidistant points on an almost-straight line. Four eleventh magnitude field stars are identified. A is GSC 6292:20, mv = 11.6. B is GSC 6288:1587, mv = 11.9. C is GSC 6292:171, mv = 11.2.
D is GSC 6292:36, mv = 11.5. (GSC = Guide Star Catalogue). The position of Pluto on October 24 (centre of image) was at Right Ascension = 18 hours 48 minutes 13 seconds, Declination = -20º 39' 11". The planet moved 2' 51" with respect to the stellar background during the three days between the first and last images, or 57 arcseconds per day, or 1 arcsecond every 25¼ minutes.Alpha
Aurigids
September 1
Waning gibbous
Moon, 70% sunlit ZHR =
10
Radiant: Near the bright star Capella
Use this
Although most meteors are found in swarms associated with debris from comets, there are numerous 'loners', meteors travelling on solitary
paths through space. When these enter our atmosphere, unannounced and at any time, they are known as 'sporadics'. Oan average clear and dark evening, an
observer can expect to see about ten meteors per hour. They burn up to ash in their passage through our atmosphere. The ash slowly settles to the ground as
meteoric dust. The Earth gains about 80 tonnes of such dust every day, so a percentage of the soil we walk on is actually interplanetary in origin. If a
meteor survives its passage through the air and reaches the ground, it is called a 'meteorite'. In the past, large meteorites (possibly comet nuclei or small asteroids) collided with the Earth and produced huge craters which still exist
today. These craters are called 'astroblemes'. Two famous ones in Australia are Wolfe Creek Crater and Gosse's Bluff. The Moon and Mercury are covered with such
astroblemes, and craters are also found on Venus, Mars, planetary satellites, minor planets, asteroids and even comets.
ZHR = zenithal hourly rate (number of meteors expected to be observed at the zenith in one hour). The
maximum phase of meteor showers usually occurs between 3 am and sunrise. The reason most meteors are observed in the pre-dawn hours is because at that time
we are on the front of the Earth as it rushes through space at 107 000 km per hour (30 km per second). We are meeting the meteors head-on, and the speed at
which they enter our atmosphere is the sum of their own speed plus ours. In the evenings, we are on the rear side of the Earth, and many meteors we see at that
time are actually having to catch us up. This means that the speed at which they enter our atmosphere is less than in the morning hours, and they burn up less
brilliantly.
Comets:
Green comets in the news
Comet PANSTARRS
(C/2017 S3)
Comet 21P/Giacobini-Zinner
On September 10, another green comet will make its closest approach to Earth in 72 years. This small but active comet is named Comet 21P/Giacobini-Zinner. The 'P' indicates that it is a periodic comet in an elliptical orbit around the Sun, and returning regularly for us to see. After it passes Earth, it will swing around the Sun and head out towards the furthest point in its orbit, just beyond Jupiter. After 36 years it will head back towards the Sun.This month it will shine at magnitude 7 so it will be easy to see in small telescopes and binoculars, but not with the unaided eye. It will only be observable in the hour or so before dawn begins to light the sky, low to the north-east horizon. On September 10 it will be gliding through the stars of the constellation Auriga about 58 million kilometres from our planet. In the week ahead, it will cross into Gemini and on September 15 it will pass right across the rich star cluster M35, providing a spectacular photo-opportunity for amateur astronomers. Visit the September 9 edition of Spaceweather for details and observing tips.
Comet Lulin
This comet, (C/2007 N3), discovered in 2007 at Lulin Observatory by a collaborative team of Taiwanese and Chinese astronomers, is now in the outer Solar System, and has faded below magnitude 15.
Comet Lulin at 11:25 pm on February 28, 2009, in Leo. The brightest star is Nu Leonis, magnitude 5.26.
The
LINEARNearly all of these programs are based in the northern hemisphere, leaving gaps in the coverage of the southern sky. These gaps are the areas of sky where amateur astronomers look for comets from their backyard observatories.
To find out more about current comets, including finder charts showing exact positions and magnitudes, click
here. To see pictures of these comets, click here.
The 3.9 metre Anglo-Australian Telescope (AAT) at the Australian Astronomical Observatory near Coonabarabran, NSW.
Deep Space
Sky Charts and Maps available on-line:
There are some useful representations of the sky available here. The sky charts linked
below show the sky as it appears to the unaided eye. Stars rise four minutes earlier each night, so at the end of a week the stars have gained about half an
hour. After a month they have gained two hours. In other words, the stars that were positioned in the sky at 8 pm at the beginning of a month will have the
same positions at 6 pm by the end of that month. After 12 months the stars have gained 12 x 2 hours = 24 hours = 1 day, so after a year the stars have returned
to their original positions for the chosen time. This accounts for the slow changing of the starry sky as the seasons progress.
The following interactive sky charts are courtesy of Sky and Telescope magazine. They can simulate a view of the sky from any location on Earth at any time of day or night between the years 1600 and 2400. You can also print an all-sky map. A Java-enabled web browser is required. You will need to specify the location, date and time before the charts are generated. The accuracy of the charts will depend on your computer’s clock being set to the correct time and date.
To produce a real-time sky chart (i.e. a chart showing the sky at the instant the chart is generated), enter the name of your nearest city and the country. You will also need to enter the approximate latitude and longitude of your observing site. For the Sunshine Coast, these are:
latitude: 26.6o South longitude: 153o East
Then enter your time, by scrolling down through the list of cities to "Brisbane: UT + 10 hours". Enter this one if you are located near this city, as Nambour is. The code means that Brisbane is ten hours ahead of Universal Time (UT), which is related to Greenwich Mean Time (GMT), the time observed at longitude 0o, which passes through London, England. Click
here to generate these charts._____________________________________
Similar real-time charts can also be generated from another source, by following this second link:
Click here for a different real-time sky chart.The first, circular chart will show the full hemisphere of sky overhead. The zenith is at the centre of the circle, and the cardinal points are shown around the circumference, which marks the horizon. The chart also shows the positions of the Moon and planets at that time. As the chart is rather cluttered, click on a part of it to show that section of the sky in greater detail. Also, click on Update to make the screen concurrent with the ever-moving sky.
The stars and constellations around the horizon to an elevation of about 40o can be examined by clicking on
The view can be panned around the horizon, 45 degrees at a time. Scrolling down the screen will reveal tables showing setup and customising options, and an Ephemeris showing the positions of the Sun, Moon and planets, and whether they are visible at the time or not. These charts and data are from YourSky, produced by John Walker.
The charts above and the descriptions below assume that the observer has a good observing site with a low, flat horizon that is not too much obscured by buildings or trees. Detection of fainter sky objects is greatly assisted if the observer can avoid bright lights, or, ideally, travel to a dark sky site. On the Sunshine Coast, one merely has to travel a few kilometres west of the coastal strip to enjoy magnificent sky views. On the Blackall Range, simply avoid streetlights. Allow your eyes about 15 minutes to become dark-adapted, a little longer if you have been watching television. Small binoculars can provide some amazing views, and with a small telescope, the sky’s the limit.
In August, the Eta Carinae Nebula is best viewed in the early evening, to the south-west of the Southern Cross.
The Stars and Constellations for this
month:
These descriptions of the night sky are for 9 pm on September 1 and 7 pm on September
30. Broadly speaking, the following description starts low in the west and
follows the horizon to the right, heading round to the east, then south, then
west, then overhead.
The Ring Nebula was ejected from the central Sun-sized star towards the end of its life, as powerful stellar winds blew away the outer layers of its atmosphere to form a ring.
Approaching due north is the constellation Cygnus, the Swan. Cygnus is also known as the
Northern
Cross, but to us in the southern hemisphere it appears this month upside-down and tilted to the left. The star at the bottom of the Cross, or the
highest one above the horizon as we see it tonight, is a beautiful double star or binary, called Albireo (see below).
Whereas most binaries are a pair of similar stars, there are many in which the two stars are very different, such as brilliant Sirius the Dog Star with its tiny white dwarf companion known as 'The Pup'. Albireo's two components have a marked colour contrast, the brighter star being a golden yellow, and the fainter companion being a vivid electric blue. It is a wonderful object to view with a small telescope.
At the top of the Northern Cross (which is the star closest to the horizon as we see it) is the bright first magnitude star
Deneb, or Alpha Cygni.
Deneb is a white star, and is the nineteenth brightest in the sky. It will be due north at about 9.00pm at mid-month. Its name means 'The Tail', as it marks the tail
of the Swan. Deneb, although bright, is one of the most distant individual stars we can see with the unaided eye. It lies at a distance of about 2600 light years. This means
that the light entering your eye tonight actually left Deneb in the 6th century BC, about the time when the world's first 'scientist', Thales of Miletus, was studying the
night sky from ancient Ionia, part of today's Turkey. Everything you see in the night sky is a view into the past, due to the finite speed of light.
Vega is the brightest star at centre left, with the stars of Lyra to its right. Deneb is the brightest star near the bottom edge. Cygnus, or the 'Northern Cross', stretches up vertically from Deneb to Albireo, above centre. We see the Northern Cross upside-down. The three bright stars of Aquila form a line at upper right, with Altair, the brightest, being the middle one. The small diamond-shaped constellation of Delphinus, the Dolphin, is above centre-right.
In the north-east, the Great Square of Pegasus has cleared the horizon, and is standing on one corner. It is very large, each side being around
15 degrees long. It is about as large as a fist held at arm's length. The Great Square is remarkable for having few naked-eye stars within it.
The names of the four stars marking the corners of the Square (starting at the uppermost one and moving in a clockwise direction) are Markab, Algenib, Alpheratz and Scheat. Although these four stars are known as the Great Square of Pegasus, only three are actually in the constellation of Pegasus, the Winged Horse. In point of fact, Alpheratz is the brightest star of the constellation Andromeda, the Chained Maiden.
In the east, a mv 2.2 star is about a handspan above the horizon. This is Beta Ceti, the brightest ordinary star in the constellation Cetus, the Whale. Its common name is Diphda, and it has a yellowish-orange colour. By rights, the star Menkar or Alpha Ceti should be brighter, but Menkar is actually more than half a magnitude fainter than Diphda. Menkar does not rise at the beginning of September until about 11 pm.
Cetus is a large constellation, running around the eastern horizon tonight, and to the unaided eye it appears unremarkable. But it does contain a most interesting star, which even men of old noticed, naming it Mira, the Wonderful (see below). Between Cetus and Pegasus is the faint zodiacal constellation of Pisces, the Fishes, which contains a faint ring of stars known as the 'Circlet'. Near this asterism, this year, Pisces is distinguished by the presence of the planet Uranus, about a handspan east of the Circlet.
Above Diphda is Fomalhaut, a bright, white first magnitude star in the faint constellation Piscis Austrinus, the Southern Fish. The first planet to be photographed circling another star is embedded in the dust ring surrounding Fomalhaut. Above Fomalhaut and to the right is a large, flattened triangle of stars, Grus, the Crane.
A little more than a handspan above the south-eastern horizon is
Achernar, which is the ninth brightest star. From locations south of
Newcastle, Achernar is circumpolar, i.e. it never dips below the horizon but is always in the sky. A hot blue-white star, Achernar is the main star in the
constellation Eridanus the River, which winds its way from Achernar towards the south-eastern horizon and then turns along the horizon to the east towards
Cetus. It then continues below the horizon all the way to Orion, which this month will not rise above the eastern horizon until a little after midnight.
Achernar's visual magnitude ( mv) is 0.45, and it is a hot blue-white star of B3 spectral type. The width of the field is 24 arcminutes and the faintest stars are mv 15.
To the left and higher than Achernar, the faint constellation of Phoenix may be seen.
Its brightest star is Ankaa or Alpha Phoenicis, a mv 2.39 star which is halfway between Diphda and Achernar, but slightly above.
One-and-a-half degrees to the right of Ankaa is a fourth magnitude star,
Kappa Phoenicis.
Due south, the Large Magellanic Cloud (LMC) is a faint, diffuse glowing patch low to the horizon, only eight degrees up from the latitude of Nambour. The Small Magellanic Cloud (SMC) is about a handspan above it, and to the right of Achernar. Both of these Clouds appear as faint smudges of light, but in reality they are dwarf galaxies containing millions of stars.
From Nambour's latitude, these two clouds never set. Each day they circle the South Celestial Pole, which is a point in our sky 26.6 degrees above the horizon's due south point. Objects in the sky that never set are called 'circumpolar'. The LMC and SMC are described below.
The Southern Cross (Crux) is setting low in the south-west, with the two Pointers Alpha and Beta Centauri vertically above it. Alternative names for these two Pointers are Rigel Kentaurus and Hadar. The two pointers are eight degrees apart. Alpha is the one further away from Crux. Whereas Alpha Centauri (see below) is the nearest star system to our Sun, only 4.2 light years distant, Beta is eighty times further away. Beta Centauri must have an absolute magnitude much greater than Alpha, in order to appear nearly as bright.
At top - the two Pointers, Alpha and Beta Centauri. Centre - Crux (Southern Cross) with the
dark cloud of dust known as the Coalsack above Alpha Crucis. Bottom - star clusters in the Milky Way with the Eta Carinae nebula near the lower edge.
Just above the second brightest star in the Cross (Beta Crucis) is a brilliant small
star cluster known as 'Herschel's Jewel Box'. In the centre of the cluster is a red supergiant star, which is just passing through.
Beta Crucis (left) and the Jewel Box cluster
If the night is dark and the skies are clear, a black dust cloud known as the Coalsack can be seen just above Acrux, the left-most and brightest star of the
Cross. Surrounding Crux on three sides is the large constellation Centaurus, its two brightest stars being the brilliant Alpha and Beta Centauri. The rest of the
constellation of Centaurus arches over Crux from above it, to its right-hand side, and then underneath it, where it adjoins Carina and Vela.
Adjoining Crux on its left-hand side is a small, fainter quadrilateral of stars, Musca, the Fly. Out of all the 88 constellations, it is the only insect. To the upper left of Alpha Centauri is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern Triangle.
Six of the zodiacal constellations are overhead tonight. The faint constellation of Libra is low in the west. Starting from Libra and looking above it, we see the bright constellation of Scorpius, the Scorpion, with the red-supergiant star Antares marking the Scorpion's heart. This famous zodiacal constellation is like a large letter 'S', and, unlike most constellations, is easy to recognise as the shape of a scorpion. Antares is the brightest star in Scorpius, and is a red type M supergiant of magnitude 0.9. Antares is the fifteenth brightest star.
Another very distant star that is easily seen with the unaided eye is
Zeta Scorpii, which, like Deneb, also lies at a distance of 2600 light years. It
can be found by following the line of the tail of Scorpius, and is the fourth star from Antares, heading south. It lies at the point where the tail takes a
sharp turn east. Actually, there are two stars there, Zeta 1 and Zeta 2. Zeta 1, the distant star, is a blue-white B1 type, while Zeta 2 is an orange K type, and
at a distance of only 151 light years is 17 times closer.
Zeta 1 Scorpii is the upper star in the bright group of three at centre right. Zeta 2 is below it.
Between Scorpius and Centaurus
is an interesting constellation composed of mainly third magnitude stars, Lupus, the Wolf. Midway between Triangulum Australe and Scorpius is an asterism like a
small, elongated triangle. This is Ara, the Altar.
The constellations surrounding the Southern Cross
High in the north-west, between Scorpius and Hercules, are two large but faint constellations,
Serpens, the Snake, and Ophiuchus, the Serpent Bearer. East of Scorpius is Sagittarius the Archer, which is almost directly overhead, having just passed the meridian. The 'Teapot' asterism
in this constellation is illustrated below. This year Sagittarius is favoured by the presence of the ringed planet Saturn, which is brighter than any of the stars in that part of the sky.
The Milky Way passes through Sagittarius, and the centre of our galaxy is close
to the zenith at this time. Sagittarius teems with stars, glowing nebulae and
dust clouds, as it is in line with the centre of our galaxy.
The body of Scorpius is at right, with the two stars in the Sting to the left, at centre right of picture. The red supergiant star Antares appears close to the top right corner. The stars in the left half of the picture are in Sagittarius. The well-known 'Teapot' shape may be seen. Near the lower left margin is a graceful curve of fourth magnitude stars, Corona Australis, the Southern Crown. To see this view, stand facing south and look directly overhead at 7.00 pm at the beginning of the month. Later in the month, the stars will be further to the west at the same time.
Antares, a red supergiant star
The star which we call Antares is a binary system. It is dominated by the great red supergiant Antares A which, if it swapped places with our Sun, would enclose all the planets out to Jupiter inside itself. Antares A is accompanied by the much smaller Antares B at a distance of between 224 and 529 AU - the estimates vary. (One AU or Astronomical Unit is the distance of the Earth from the Sun, or about 150 million kilometres or 8.3 light minutes.) Antares B is a bluish-white companion, which, although it is dwarfed by its huge primary, is actually a main sequence star of type B2.5V, itself substantially larger and hotter than our Sun. Antares B is difficult to observe as it is less than three arcseconds from Antares A and is swamped in the glare of its brilliant neighbour. It can be seen in the picture above, at position angle 277 degrees (almost due west or to the left) of Antares A. Seeing at the time was about IV on the Antoniadi Scale, or in other words below fair. Image acquired at Starfield Observatory in Nambour on July 1, 2017.
The Trifid Nebula, M20, in Sagittarius, is composed of a reflection nebula (blue), an emission nebula (pink), and dark lanes of dust.
The centre of our galaxy is teeming with stars, and would be bright enough to turn night into day, were it not for intervening dust and molecular clouds. This dark cloud is known as 'The Snake'. A satellite passed through the field of view at right.
Between Capricornus and Pisces is another rather faint constellation,
Aquarius, the Water Bearer. There are no stars brighter
than third magnitude in this constellation, but it does contain many interesting objects, including a group of four stars known as the 'Water Jar'. Also, this year
faint Neptune may be found in its centre, about a handspan
east of the star Deneb Algedi. Culminating high in the north, between Capricornus and Albireo, is the constellation of
Aquila, the Eagle (see picture). The
centre of this constellation is marked by a short line of three stars, of which the central star is the brightest. These stars, from north to south, are
Tarazed,
Altair and Alshain, and they indicate the Eagle's body. A handspan to the east of bright, first magnitude Altair is a faint but easily recognised
diamond-shaped group of stars, Delphinus the Dolphin. The line of the ecliptic along which the Sun, Moon and planets travel, passes through the following constellations this month: Libra, Scorpius, Sagittarius,
Capricornus, Aquarius and Pisces. The aborigines had a large constellation which is visible tonight, the
Emu. The Coalsack forms its head, with the faint sixth magnitude star in
the Coalsack, its eye. The Emu's neck is a dark lane of dust running east through the two Pointers, to Scorpius. The whole constellation Scorpius forms the Emu's
body. The Emu is sitting, waiting for its eggs to hatch. The eggs are the large star clouds of Sagittarius. More photographs of the amazing sights visible in our sky this month are found in our
Gallery. If you would like to become familiar with the constellations, we suggest that you access one of the world's best collections of constellation pictures
by clicking
Mira, the Wonderful
The amazing thing about the star Mira or Omicron Ceti is that it
varies dramatically in brightness, rising to magnitude 2 (brighter than any other star in Cetus), and then dropping to magnitude 10 (requiring a telescope
to detect it), over a period of 332 days. This drop of eight magnitudes means that its brightness diminishes over a period of five and a half months to one six-hundredth of what it had been,
and then over the next five and a half months it regains its original brightness. In the mid-17th century, the astronomer Johannes Hevelius watched the star fade away
during the year until it disappeared, and then it slowly reappeared again. Its not surprising that he named it Mira, meaning 'The Wonderful' or 'The Miraculous One'. We
now know that many stars vary in brightness, even our Sun doing so to a small degree, with a period of 11 years. One type of star varies, not because it is
actually becoming less bright in itself, but because another, fainter star moves around it in an orbit roughly in line with the Earth, and obscures it on each
pass. This type of star is called an eclipsing variable and they are very common. The
star Mira though, varies its light output because of processes in its interior. It is what is known as a pulsating variable. Stars of the Mira type are
giant pulsating red stars that vary between 2.5 and 11 magnitudes in brightness. They have long, regular periods of pulsation which lie in the range from 80 to
1000 days.
Last year, Mira reached a maximum brightness of magnitude 3.4 on December 29, 2017 and has now dropped
well below naked-eye visibility (magnitude 6) again. It reached its minimum brightness of magnitude 9.3 on August 18. The next maximum will occur on November 26 next.
Mira near minimum, 26 September 2008
Mira near maximum, 22 December 2008
Astronomers using a NASA space telescope, the Galaxy Evolution Explorer, have spotted an amazingly long comet-like tail behind
Mira as the star streaks through space. Galaxy Evolution Explorer ("GALEX" for short) scanned the well-known star during its ongoing survey of the entire
sky in ultraviolet light. Astronomers then noticed what looked like a comet with a gargantuan tail. In fact, material blowing off Mira is forming a wake 13
light-years long, or about 20,000 times the average distance of Pluto from the sun. Nothing like this has ever been seen before around a star.
More,
including pictures
Double and multiple stars
Estimates vary that between 15% and 50% of stars are single bodies like our Sun, although the latest view is that less than 25% of stars are solitary. At least 30% of stars and possibly as much as 60% of stars are in double systems, where the two stars are gravitationally linked and orbit their mutual centre of gravity. Such double stars are called binaries. The remaining 20%+ of stars are in multiple systems of three stars or more. Binaries and multiple stars are formed when a condensing Bok globule or protostar splits into two or more parts.
Binary stars may have similar components (Alpha Centauri A and
B are both stars like our Sun), or they may be completely dissimilar, as with
Albireo (Beta Cygni, where a bright golden giant star is paired with a smaller bluish main sequence star).
The binary stars Rigil Kentaurus (Foot of the Centaur, or Alpha Centauri) at left, and Beta Cygni (Albireo), at right.
Rigel (Beta Orionis, left) is a binary star which is the seventh brightest star in the night sky. Rigel A is a large white supergiant which is 500 times brighter than its small companion, Rigel B, Yet Rigel B is itself composed or a very close pair of Sun-type stars that orbit each other in less than 10 days. Each of the two stars comprising Rigel B is brighter in absolute terms than Sirius (see above). The Rigel B pair orbit Rigel A at the immense distance of 2200 Astronomical Units, equal to 12 light-days. (An Astronomical Unit or AU is the distance from the Earth to the Sun.) In the centre of the Great Nebula in Orion (M42) is a multiple star known as the Trapezium (right). This star system has four bright white stars, two of which are binary stars with fainter red companions, giving a total of six. The hazy background is caused by the cloud of fluorescing hydrogen comprising the nebula.
Acrux, the brightest star in the Southern Cross, is also known as Alpha Crucis. It is a close binary, circled by a third dwarf companion.
Alpha Centauri (also known as Rigil Kentaurus, Rigil Kent or Toliman) is a binary easily
seen with a small telescope. The components are both solar-type main sequence stars, one of type G and the other, slightly cooler and fainter, of type K. Through a
telescope this star system looks like a pair of distant but bright car headlights. Alpha Centauri A and B take 80 years to complete an orbit, but a tiny third
component, the 11th magnitude red dwarf Proxima Centauri, takes about 1 million years to orbit the other two. It is about one tenth of a light year from the bright
pair and a little closer to us, hence its name. This makes it our nearest interstellar neighbour, with a distance of 4.3 light years. Red dwarfs are by far the most
common type of star, but, being so small and faint, none is visible to the unaided eye. Because they use up so little of their
energy, they are also the longest-lived of stars. The bigger a star is, the shorter its life.
Close-up of the star field around Proxima Centauri
Knowing the orbital period of the two brightest stars A and B, we can apply
Kepler’s Third Law to find the distance they are apart. This tells us that Alpha Centauri A and B are about 2700 million kilometres apart or about 2.5 light hours.
This makes them a little less than the distance apart of the Sun and Uranus (the orbital period of Uranus is 84 years, that of Alpha Centauri A and B is 80 years.)
Albireo (Beta Cygni) is sometimes described poetically as a large topaz with a small blue sapphire. It is one of the sky’s most beautiful objects. The stars are of classes G and B, making a wonderful colour contrast. It lies at a distance of 410 light years, 95 times further away than Alpha Centauri.
Binary stars may be widely spaced, as the two examples just mentioned, or so close that a telescope is struggling to separate them (Acrux, Antares, Sirius). Even closer double stars cannot be split by the telescope, but the spectroscope can disclose their true nature by revealing clues in the absorption lines in their spectra. These examples are called spectroscopic binaries. In a binary system, closer stars will have shorter periods for the stars to complete an orbit. Eta Cassiopeiae takes 480 years for the stars to circle each other. The binary with the shortest period is AM Canum Venaticorum, which takes only 17½ minutes.
Sometimes one star in a binary system will pass in front of the other one, partially blocking off its light. The total light output of the pair will be seen to vary, as regular as clockwork. These are called eclipsing binaries, and are a type of variable star, although the stars themselves usually do not vary.
The Milky Way
A glowing band of
light crossing the sky is especially noticeable during September. This
glow is the light of millions of faint stars combined with that coming from
glowing gas clouds called emission nebulae. It is concentrated along the plane of our
galaxy, and this month it is seen crossing the sky, starting from the
south-south-west and passing through Crux to Centaurus, Scorpius, Sagittarius and Aquila to Cygnus in the north-east. The plane of our galaxy from Scutum (at left) through Sagittarius and Scorpius (centre) to Centaurus
and Crux (right). The Eta Carinae nebula is at the right margin, below centre. The Coalsack is clearly visible, and the dark dust lanes can be seen. Taken with an
ultra-wide-angle lens.
The centre of our galaxy. The constellations partly visible here are Sagittarius (left), Ophiuchus (above centre) and Scorpius (at right). The planet Jupiter is the bright object below centre left. This is a normal unaided-eye view.
The Season of the Scorpion
The spectacular constellation of Scorpius is about a handspan west of the zenith in mid-September. Three bright stars in a gentle curve mark his head, and another three mark his body. Of this second group of three, the centre one is a bright, red supergiant, Antares. It marks the red heart of the scorpion. This star is so large that, if it swapped places with our Sun, it would engulf the Earth and extend to the orbit of Mars. It is 604 light years away and shines at magnitude 1.06. Antares, an M type star, has a faint companion which can be seen in a good amateur telescope if the seeing is excellent.
The rest of the stars run around the scorpion's tail, ending with two blue-white B type stars, Shaula (the brighter of the two) and Lesath,
at the tip of the scorpion's sting. These two stars are at the eastern end of the constellation, and are near the bottom of the picture below. West of
Lesath in the body of the scorpion is an optical double star, which can be seen as two with the unaided eye.
Scorpius, with its head at top left and tail (with sting) at lower right.
Probably the two constellations most easily recognisable (apart from Crux, the Southern Cross) are Orion the Hunter and Scorpius the Scorpion. Both are
large constellations containing numerous bright stars, and are very obvious 'pictures in the sky'. Both also contain a very bright red supergiant
star, Betelgeuse in Orion and Antares in Scorpius.
The Lagoon Nebula, M8, in Sagittarius, adjacent to Scorpius
The centre of the Lagoon Nebula
Why are some constellations bright, while others are faint ?
The Milky Way is a barred spiral galaxy some 100000 – 120000 light-years in diameter which contains 100 – 400 billion stars. It may contain at least as many planets as well. Our galaxy is shaped like a flattened disc with a central bulge. The Solar System is located within the disc, about 27000 light-years from the Galactic Centre, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. When we look along the plane of the galaxy, either in towards the centre or out towards the edge, we are looking along the disc through the teeming hordes of stars, clusters, dust clouds and nebulae. In the sky, the galactic plane gives the appearance which we call the Milky Way, a brighter band of light crossing the sky. This part of the sky is very interesting to observe with binoculars or telescope. The brightest and most spectacular constellations, such as Crux, Canis Major, Orion and Scorpius are located close to the Milky Way.
If we look at ninety degrees to the plane, either straight up and out of the galaxy or straight down, we are looking through comparatively few stars and gas clouds and so can see out into deep space. These are the directions of the north and south galactic poles, and because we have a clear view in these directions to distant galaxies, these parts of the sky are called the intergalactic windows. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It is out of sight this month.
The southern window is in the constellation Sculptor, just south of the star Fomalhaut. This window is low in the south-east in the early evening, but later in the night it will rise high enough for distant galaxies to be observed. Some of the fainter and apparently insignificant constellations are found around these windows, and their lack of bright stars, clusters and gas clouds presents us with the opportunity to look across the millions of light years of space to thousands of distant galaxies.
Finding the South Celestial Pole:
The South Celestial Pole is that point in the southern sky around which the stars rotate in a clockwise direction. The Earth's axis is aimed exactly at this point. For an equatorially-mounted telescope, the polar axis of the mounting also needs to be aligned exactly to this point in the sky for accurate tracking to take place.
To find this point, first locate the Southern Cross. Project a line from the top of the Cross down through its base and continue straight on towards the south for another four Cross lengths. This will locate the approximate spot. There is no bright star to mark the Pole, whereas in the northern hemisphere they have Polaris (the Pole Star) to mark fairly closely the North Celestial Pole.
Another way to locate the South Celestial Pole is to draw an imaginary straight line joining Beta Centauri high in the south-west to Achernar low in the south-east. Both stars will be at about the same elevation above the horizon at 8 pm in the middle of September. Find the midpoint of this line to locate the pole.
Interesting photographs of this area can be taken by using a camera on time exposure. Set the camera on a tripod pointing due south, and open the shutter
for thirty minutes or more. The stars will move during the exposure, being recorded on the film as short arcs of a circle. The arcs will be different
colours, like the stars are. All the arcs will have a common centre of curvature, which is the south celestial pole.
Star trails between the South Celestial Pole and the southern horizon. All stars that do not pass below the horizon are circumpolar.
Star Clusters
The two clusters in Taurus, the Pleiades and the Hyades,
are known as Open Clusters or Galactic Clusters. The name 'open cluster' refers to the fact that the stars in the cluster are grouped together, but not as
tightly as in globular clusters (see below). The stars appear to be loosely arranged, and this is partly due to the fact that the cluster is relatively close to us, i.e.
within the disc of our galaxy, hence the alternate name, 'galactic cluster'. These clusters are generally formed from the condensation of gas in a nebula into stars, and some
are relatively young. The photograph below shows a typical open cluster,
Galactic Cluster M7 in Scorpius
Outside the plane of our galaxy, there is a halo of Globular Clusters. These are very old, dense clusters,
containing perhaps several hundred thousand stars. These stars are closer to each other than is usual, and because of its great distance from us, a globular
cluster gives the impression of almost a solid mass of faint stars. Most other galaxies also have a halo of globular clusters circling around them.
The largest and brightest globular cluster in the sky is
NGC 5139**, also known as Omega Centauri. It has a slightly oval shape. It is an outstanding winter object, and this month it is observable before midnight. Shining at fourth magnitude, it is faintly visible to the unaided eye, but is easily seen with binoculars, like a light in a fog. A telescope of 20 cm aperture or better will reveal its true nature, with millions of faint stars giving the impression of diamond dust on a black satin background. It lies at a distance of 5 kiloparsecs, or 16 300 light years. Omega Centauri is not a typical globular cluster, being much larger than any other in our Galaxy, with millions of stars. It is also slightly oval in shape.
The globular cluster Omega Centauri
The central core of Omega Centauri
There is another remarkable globular, second only to Omega Centauri. Close to the SMC (see below), binoculars can detect a fuzzy star. A
telescope will reveal this faint glow as a magnificent globular cluster, lying at a distance of 5.8 kiloparsecs. Its light has taken almost 19 000 years to reach us. This is
NGC 104, commonly known as 47 Tucanae. Some regard this cluster as being more spectacular than Omega Centauri, as it is more compact, and the faint stars
twinkling in its core are very beautiful. This month, neither Omega Centauri nor 47 Tucanae
is at its best position for viewing, but both are observable.
The globular cluster 47 Tucanae.
Observers aiming their telescopes towards the
SMC generally also look at the nearby 47 Tucanae, but there is another globular cluster nearby which is also worth a visit. This is
NGC 362, which is
less than half as bright as the other globular, but this is because it is more than twice as far away. Its distance is 12.6 kiloparsecs or 41 000 light years,
so it is about one-fifth of the way from our galaxy to the SMC. Both NGC 104 and NGC 362 are always above the horizon for all parts of Australia south of the
Tropic of Capricorn.
* M7: This number means that Ptolemy's Cluster in Scorpius is No. 7 in a list of 103 astronomical objects compiled and published in 1784 by Charles Messier. Charles was interested in the discovery of new comets, and his aim was to provide a list for observers of fuzzy nebulae and clusters which could easily be reported as comets by mistake. Messier's search for comets is now just a footnote to history, but his list of 103 objects is well known to all astronomers today, and has even been extended to 110 objects. As Messier had quite a small telescope, his list is an excellent guide to the brightest and most spectacular objects in the sky, although he did not include any of our excellent southern objects such as 47 Tucanae, Omega Centauri and Eta Carinae, as they were never visible from his home in Paris.
** NGC 5139: This number means that Omega Centauri is No. 5139 in the
New General Catalogue of
Non-stellar Astronomical Objects. This catalogue was first published in 1888 by J. L. E. Dreyer under the auspices of the Royal Astronomical Society, as his
New General Catalogue of Nebulae and Clusters of Stars. It contained only
objects that needed a telescope to be seen. It was an updated version of the
previous General Catalogue
The recent explosion of discovery in astronomy has meant that more and more catalogues are being produced, but they tend to specialise in particular types of objects, rather than being all-encompassing, as the NGC / IC try to be. Some examples are the H
I Parkes All-Sky Survey (HIPASS: HI = clouds of neutral hydrogen in space), Planetary Nebulae Catalogue (PK) which lists 1455 nebulae, the Washington Catalogue of Double Stars (WDS) which lists 12 000 binaries, the General Catalogue of Variable Stars (GCVS) which lists 28 000 variables, and the Principal Galaxies Catalogue (PGC) which lists 73 000 galaxies. The largest modern catalogue is the Hubble Guide Star Catalogue (GSC) which was assembled to support the Hubble Space Telescope's need for guide stars when photographing sky objects. The GSC contains nearly 19 million stars brighter than magnitude 15.
Two close galaxies
Very low in the south, two faint smudges of light may be seen. These are the two
Clouds of Magellan, known to astronomers as the LMC (Large
Magellanic Cloud) and the SMC (Small Magellanic Cloud). The LMC is above the SMC, and is noticeably larger.
They lie at distances of 190 000 light years for the LMC, and 200 000 light years for the SMC. They are about 60 000 light
years apart. These dwarf galaxies circle our own much larger galaxy, the Milky Way. The LMC is slightly closer, but this does
not account for its larger appearance. It really is larger than the SMC, and has developed as an under-sized barred spiral galaxy.
Astronomers have recently reported the largest star yet found, claimed to have 300 times the mass of the Sun, located in a cluster of stars embedded in the Tarantula Nebula (above). Such a huge star would be close to the Eddington Limit, and would have a short lifespan measured in only a couple of million years.
These two Clouds are the closest galaxies to our own, but lie too far south to be seen by the large telescopes in Hawaii, California and
Arizona. They are 15 times closer than the famous Andromeda and Triangulum galaxies, and so can be observed in much
clearer detail. Our great observatories in Australia, both radio and optical, have for many years been engaged in important research involving these, our
nearest inter-galactic
Astronomy Picture of the Day:
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