June
2018
Updated: 1 June 2018
Welcome to the night skies of Winter, featuring Virgo, Carina, Crux, Centaurus, Scorpius, Sagittarius, Jupiter, Saturn and Mars
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The Alluna RC-20 Ritchey Chrétien telescope was installed in March, 2016.
The 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.
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 middle of the constellation of Taurus, the Bull. It leaves Taurus and passes into Gemini, the Twins on June 21.
Moon Phases: Lunations (Brown series): #1180, 1181, 1182
Last Quarter: June
07
04:33 hrs diameter =
30.2'
New Moon:
June
14
05:44 hrs diameter =
33.1'
Lunation #1181 begins
First Quarter:
June
20
20:51 hrs diameter = 31.6'
Full Moon:
June 28
14:54 hrs diameter = 29.5'
Last Quarter: July 06
17:52 hrs diameter =
30.7'
New Moon: July
13
12:48 hrs diameter =
33.4'
Lunation #1182 begins Partial Solar Eclipse
First Quarter: July
20
05:53 hrs diameter =
31.0'
Full Moon:
July 28 06:21 hrs
diameter = 29.4' Lunar Eclipse
Lunar Orbital Elements:
June 03: Moon at apogee (405 335 km) at 02:17 hrs, diameter = 29.5'
June 03: Moon at descending node at 22:35 hrs, diameter = 29.5'
June 15: Moon at perigee (359 508 km) at 09:50 hrs, diameter = 33.2'
June 16: Moon at ascending node at 03:49 hrs, diameter = 33.0'
June 30: Moon at apogee (406 047 km) at 13:15 hrs, diameter = 29.4'
July 13: Moon at perigee (357 439 km) at 18:10 hrs, diameter = 33.4'
July 14; Moon at ascending node at 12:51 hrs, diameter = 33.4'
July 27: Moon at apogee (406 199 km) at 15:35 hrs, diameter = 29.4'
July 28: Moon at descending node at 08:39 hrs, diameter = 29.4'
Moon at 8 days after
New, as on
June 21.
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
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 Archimedes.
Close-up of Archimedes, 85 km in diameter. The smallest craterlets visible are less than 1 km in diameter. The photograph was taken from Nambour at 8:14 pm on August 2, 2017.
This feature is the largest ringed plain on the Mare Imbrium. It has broad, complex walls which average 1.2 kilometres in height above the interior, although some peaks reach a height of 2.2 kilometres. The interior is flat and level, and is about 200 metres below the outside surface of the Mare. Several landslips from the walls intrude upon the floor, which is crossed from east to west by light-coloured streaks which are also found crossing the lava plain surrounding Archimedes. There are well over 50 craterlets on the floor of Archimedes, of which about a dozen are clearly seen. The two largest craterlets are 3 kilometres across - one is close to the western (left) wall, and the other is adjacent to the eastern wall. On the outside of the wall in the south and south-west may be found two chains of tiny 1 kilometre wide craterlets, six in one and five in the other.
South-west of Archimedes is a 14 kilometre impact crater named Bancroft. South-east of Archimedes is a similarly-sized crater, Spurr, but it is much older, and when the
Mare Imbrium was formed, lava flows swept over it, completely demolishing the northern ramparts and flooding the interior. Only the southern wall survives, and it is what
we call a 'ghost crater'. Around Archimedes are some other craters and a few isolated mountain peaks. To its south and north-east are rugged mountainous areas.
Archimedes
Archimedes of Syracuse (287-212 BCE, left) is regarded as the greatest mathematician of antiquity, and one of the greatest of all time. Syracuse was one of the oldest Greek colonies, and was established by the Corinthians in Sicily in the 7th century BCE. The most widely known anecdote about Archimedes tells of how he invented a method for determining the volume of an object with an irregular shape. A new crown in the shape of a laurel wreath had been made for King Hiero who had supplied the gold, but the King was suspicious. He asked Archimedes to determine whether it was of pure gold, or whether a cheaper metal had been surreptitiously added to make an alloy. Archimedes had to solve the problem without damaging the crown, so he could not melt it down into a regularly shaped body in order to calculate its density.
While taking a bath, he noticed that the level of the water in the tub rose as he got in, and realised that this effect could be used to determine the volume
of the crown. For practical purposes water is incompressible, so the submerged crown would displace an amount of water equal to its own volume. By dividing the
weight of the crown by the volume of water displaced, the density of the crown could be found. This density would be lower than that of gold if a cheaper and
less dense metal had been added. Archimedes then took to the streets, so excited by his discovery that he had forgotten to dress, shouting "Eureka! Eureka!"
meaning "I have found it!" Unfortunately for the goldsmith who made the crown, it was found that some of the gold that had been supplied by the King had been ‘cut’ with silver –
the goldsmith had kept the rest.
Plutarch describes how Archimedes designed some pulley systems, allowing sailors to use the principle of leverage to lift objects that would otherwise have been too heavy to move. Archimedes was employed by the Roman Army to improve the power and accuracy of the battle catapults during the First Punic War, and was given the task of inventing a measuring device so that milestones could be placed on every road leading out of Rome. He designed a four-wheeled cart with an odometer – a mechanism geared to the wheels that dropped a small metal ball into a container after each mile travelled. It is believed that this device was the first to use tapered metal gear-teeth as we know them, rather than square-shaped wooden pegs. Some mileposts dating from Archimedes’ time still exist along the Appian Way from Rome to Brindisi, a distance of 350 miles (563 kilometres). Later, when Sicily was at war with Rome, Lucian tells us that Archimedes used fire to burn enemy ships during the Siege of Syracuse in 214-212 BCE, possibly focusing sunlight with mirrors or burning glasses – sometimes referred to as an ‘Archimedes heat ray’. While Archimedes did not invent the lever, he wrote the earliest known rigorous explanation of the principle involved. According to Pappus of Alexandria, his work on levers caused him to remark: “Give me a place to stand, [and a lever long enough], and I will move the Earth.” He also invented the Archimedes screw, a very efficient method of raising water.
The only writing on astronomy by Archimedes that has survived is The Sand Reckoner, in which he discusses contemporary ideas about the size of the Earth and the distance between various celestial bodies. In this treatise, he also estimates the number of grains of sand that would fit inside the universe. By using a system of numbers based on powers of the myriad, Archimedes concludes that the number of grains of sand required to fill the universe would be 8 followed by 63 zeros (8 x 1063 in modern notation. The introduction to the book states that Archimedes’ father was an astronomer named Phidias.
[ Present Day Note: The number 8 × 1063 is incomprehensibly huge. If the Big Bang is thought to have occurred 13.79 billion years ago, there have been ‘only’ about 4.5 × 1017 seconds since the birth of the universe. It is estimated that the Earth is made up of roughly 5.5 × 1050 atoms; the number of atoms in the Milky Way Galaxy is approximately 5 × 1068, and the number of atoms in the observable universe is estimated to be 7.1 × 1079. That would equal about 1.4 × 1060 grains of sand. ]
The ancients knew that if the position of the Moon when in conjunction with a bright planet or star was observed simultaneously by two widely-spaced observers, the distance between the two sky objects showed a slight alteration. They called this variation ‘παραλλασσειν’ (‘parallassein’: the apparent shift in position when viewed from different vantage points) – we call it parallax. They also noted that when Saturn appeared very close to stars in the sky, they looked the same distance apart to all observers no matter where they were – there was no parallax effect observed. This meant that the starry sphere was much farther away than even Saturn, the most distant planet. Most philosophers prior to Archimedes had agreed that the stars were about ten thousand Earth-diameters away.
Archimedes was a contemporary of Aristarchus, but objected to the latter’s heliocentric proposal (that the Earth revolved around the Sun in a year, instead of the belief then held that the Sun revolved around the Earth each day). Archimedes said that if it were true, changes would be observed in the apparent positions of the fixed stars as the Earth moved around the Sun, and they should vary in brightness, too, as the Earth approached them and then receded. Aristarchus countered that argument by claiming that the stars were so far away that the changes in position or stellar parallax would be too small to detect, which was why he didn’t bother to try to do it. He was correct in this opinion, and it was not until the 19th century that telescopes could finally measure the tiny angles. Archimedes, on the other hand, maintained his view that the world was flat and unmoving. However, we are obliged to him for describing Aristarchus’ heliocentric theory of the universe in The Sand Reckoner. None of Aristarchus’ works has survived, so without the accounts of Seleucus and Archimedes, we would never have been aware of his thinking. Archimedes wrote:
“But Aristarchus of Samos brought out a book consisting of some hypotheses, in which
the premises lead to the result that the universe is many times greater than that now so called. His hypotheses are that the fixed stars and the Sun remain
unmoved, that the Earth revolves about the Sun in the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of the fixed
stars, situated about the same centre as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance
of the fixed stars as the centre of the sphere bears to its surface.”
Archimedes died during the Second Punic War, when a Roman army under General Marcus Marcellus captured Syracuse. According to the historian Plutarch writing 300 years later, he was contemplating a geometrical diagram when the Roman forces entered the city. A soldier commanded him to come and meet General Marcellus, but Archimedes refused to accompany him, saying that he needed to finish work on his problem. The soldier flew into a rage, and killed Archimedes with his sword. Plutarch also offers an alternative account, in which the soldier killed Archimedes in order to steal the valuable mathematical instruments he was carrying. Whatever the true version, Marcellus was angered when he heard of this, for he regarded Archimedes as a valuable scientific asset and had ordered that he not be harmed. Archimedes’ last words reportedly were, “Do not disturb my circles.”
The Archimedes area is shown by the yellow rectangle.
Click
Geocentric Events:
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.
June 1: Moon 2.2º north of Saturn at 11:34 hrs
June 2: Limb of Moon 22 arcminutes north of the star Pi Sagittarii (mv= 2.88) at 05:54 hrs
June 2: Moon 1.8º north of Pluto at 13:39 hrs
June 3: Moon 3.5º north of Mars at 18:58 hrs
June 4: Jupiter 52 arcminutes north of the star Zuben Elgenubi (Alpha Librae, mv= 2.75) at 04:17 hrs
June 5: Moon 1.6º north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 09:35 hrs
June 6: Mercury in superior conjunction at 11:49 hrs (diameter = 5.1")
June 6: Mercury at perihelion at 20:13 hrs (diameter = 5.1")
June 7: Moon 2.1º south of Neptune at 04:41 hrs
June 7: Neptune at western quadrature at 15:43 hrs (diameter = 2.2")
June 10: Moon 4º south of Uranus at 17:06 hrs
June 13: Moon 1.6º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 08:10 hrs
June 14: Limb of Moon 27 arcminutes south of the star Zeta Tauri (mv= 2.97) at 08:11 hrs
June 14: Moon 4.4º south of Mercury at 23:31 hrs
June 15: Moon 1.7º south of the star Mu Geminorum (mv= 2.87) at 01:48 hrs
June 15: Mercury 2.6º north of the star Mu Geminorum (mv= 2.87) at 21:50 hrs
June 16: Moon 2.2º south of Venus at 23:29 hrs
June 18: Moon 1.9º north of of the star Regulus (Alpha Leonis, mv= 1.36) at 20:14 hrs
June 19: Neptune at western stationary point at 06:15 hrs (diameter = 2.3")
June 21: Winter solstice at
20:09 hrs
June 23: Saturn 1.9º south of the star 21 Sagittarii (mv= 4.8) at 00:23 hrs
June 24: Moon 4.5º north of Jupiter at 07:38 hrs
June 27: Mars at western stationary point at 05:19 hrs (diameter = 20.1")
June 27: Saturn at opposition at 23:16 hrs (diameter = 18.3")
June 28: Moon 2.4º north of Saturn at 12:56 hrs
June 28: Limb of Moon 43 arcminutes north of the star Pi Sagittarii (mv= 2.88) at 11:27 hrs
June 29: Moon 1.6º north of Pluto at 17:08 hrs
July 01: Moon 5.3º north of Mars at 10:03 hrs
July 02: Moon 1.6º north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 14:38 hrs
July 04: Moon 1.8º south of Neptune at 13:03 hrs
July 05: Earth at aphelion at 17:28 hrs
July 08: Moon 4.6º south of Uranus at 01:10 hrs
July 10: Venus 59 arcminutes north of the star Regulus (Alpha Leonis, mv= 1.36) at 14:22 hrs
July 10: Moon 1.3º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 19:58 hrs
July 11: Jupiter at eastern stationary point at 01:55 hrs (diameter = 40.3")
July 11: Limb of Moon 50 arcminutes south of the star Zeta Tauri (mv= 2.97) at 20:38 hrs
July 12: Limb of Moon 57 arcminutes south of the star Propus (Eta Geminorum (mv= 3.31) at 09:31 hrs
July 12: Limb of Moon 55 arcminutes south of the star Mu Geminorum (mv= 2.87) at 13:53 hrs
July 12: Pluto at opposition at 19:40 hrs (diameter = 0.1")
July 13: Mercury at greatest elongation east (26º 25') at 13:58 hrs (diameter = 8.0")
July 15: Moon 2.8º north of Mercury at 08:01 hrs
July 16: Moon 2º north of the star Regulus (Alpha Leonis, mv= 1.36) at 03:25 hrs
July 16: Moon 2.1º north of Venus at 15:03 hrs
July 20: Mercury at aphelion at 19:50 hrs (diameter = 9.3")
July 21: Moon 4.9º north of Jupiter at 11:18 hrs
July 25: Moon 2.5º north of Saturn at 14:11 hrs
July 25: Uranus at western quadrature at 21:24 hrs (diameter = 3.5")
July 26: Mercury 7.7º west of the star Regulus (Alpha Leonis, mv= 1.36) at 01:58 hrs
July 26: Mercury at eastern stationary point at 15:02 hrs (diameter = 10.2")
July 26: Limb of Moon 30 arcminutes north of the star Pi Sagittarii (mv= 2.88) at 15:32 hrs
July 26: Moon 1.4º north of Pluto at 23:59 hrs
July 27: Mars at opposition at14:35 hrs (diameter = 24.2")
July 27: Venus 1.3º south of the star Sigma Leonis (mv= 4.05) at 20:48 hrs
July 28: Full Moon 7.1º north of Mars at 06:05 hrs
July 29: Moon 1.1º north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 19:08 hrs
July 31: Moon 2º south of Neptune at 16:26 hrs
The Planets for this month:
Mercury: On June 6, Mercury will pass through superior conjunction (on the opposite side of the Sun), and in the second half of the month it will appear in the western twilight sky in the constellation Gemini. At 5:30 pm on June 15 it will be between the thin crescent Moon and the west-north-western horizon. Although shining nearly as bright as Sirius, it may be hard to spot Mercury due to the solar glare. By the end of the month the innermost planet will be easier to find. Mercury will be at its best for observing in July, especially in the second half. Mercury will cross into Cancer on June 28.
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'. On June 1, Venus will be nearly 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 'Gibbous Moon' with a magnitude of -4 and a phase of 80%. On that date, Venus will be in the middle of the constellation Gemini. On June 12 it crosses into Cancer, and on June 29 into Leo. Venus will be just above the waxing crescent Moon on the evening of June 16.
(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 September 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 the first two months of 2017, Venus appeared as an 'Evening Star', but on March 25 it moved to the pre-dawn sky and became a 'Morning Star'. Each of these appearances lasts about eight to nine months. Venus passed on the far side of the Sun (superior conjunction) on January 9, 2018, and has now returned to the evening sky to become an 'Evening Star' once again.
Because Venus was 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.
This is the year of Mars:
The red planet is becoming easier to observe this month as on June 1 it rises at about 9 pm in the constellation Capricornus. As the Earth continues to catch up to Mars, the red planet will brighten and appear a little larger each night. Its movement east will continue until June 27, when it will stop and head back towards Sagittarius, performing its 'retrograde loop'. On June 1, Mars will have a brightness of magnitude -1.2 and an angular diameter of 15 arcseconds. By June 30, Mars will have brightened to magnitude -2.1 (almost as bright as Jupiter), and its angular diameter will have increased to 21 arcseconds. The opposition of Mars will occur on July 27.This will be a very favourable opposition, as Mars will appear bigger (24.2 arcseconds in diameter) and brighter (magnitude -2.8, twice as bright as Jupiter) than it has for many years. It will be particularly favourable for us in the southern hemisphere, as during the month of opposition it will be in the constellation of Capricornus, almost directly overhead each night from the Sunshine Coast. The coming winter will be an excellent time for planet observing, with Mars, Jupiter and Saturn all available each evening and high overhead. As twilight fades in the west, Mercury and Venus will also be available. The next time that Mars will have an opposition in which it reaches a size as favourable as this July will be in September 2035, when Mars will be in the constellation Aquarius.
Just before 10 pm on June 3, the waning gibbous Moon will be just below (east) of Mars.
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.
Jupiter:
This gas giant planet is now visible in the sky all night long, 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. At mid-month Jupiter will be seen well up in the east-south-east as darkness falls, as it passed through opposition on May 9. The waxing gibbous Moon will be seen just to the left of Jupiter soon after sunset on June 23.
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
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.
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.
Saturn:
Uranus:
Neptune:
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.
Meteor Showers:
Arietids
June 8 Waning
crescent Moon,
40% sunlit ZHR = 60 Zeta Perseids
Beta Taurids June 29 Full Moon,
100% sunlit ZHR = 25.
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.
Comets:
Radiant: Near the star Hamal
Radiant: Between the Hyades and the
Pleiades
Radiant: Between the stars Betelgeuse and El Nath
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.
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.
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
View horizon at this observing site
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 May, the Eta Carinae Nebula is ideally placed for viewing in the early evening, as it culminates at 7 pm at mid-month.
The Stars and Constellations for this month:
These descriptions of the night sky are for 8 pm on June 1 and 6 pm on June 30. They start at the western horizon.
Setting in the west are Orion's big and little dogs, Canis Major and Canis Minor. Sirius (Alpha Canis Majoris) is close to the west-south-western horizon, with the large right-angled triangle of the Big Dog's hindquarters above it.
Procyon, the brightest star in Canis Minor, is heading towards the horizon a little to the north of west. Above Canis Major is the constellation, Puppis
, the Stern (of the ship, Argo Navis).
Two handspans to the left of Sirius and about a handspan above the south-western horizon is the second brightest star in the night sky, Canopus
(Alpha Carinae). Although Canopus appears almost as bright as Sirius but a little more yellow, the two stars are entirely dissimilar. Sirius is a
normal-sized star that is bright because it is close to us - only 8.6 light years away. Canopus, on the other hand, is a F0 type supergiant, over 100 times
brighter than Sirius, but 36 times further away (312 light years). Tonight, Canopus
Setting in the west-north-west is the faint constellation of Cancer, the Crab. Though a fairly unremarkable constellation in other ways, Cancer does contain a large star cluster called Praesepe or the Beehive, which is a good sight in binoculars.
Leo the Lion has passed culmination and is beginning to head down towards the north-western horizon. It will have completely disappeared by midnight. We see Leo upside-down from the Southern Hemisphere. Leo's brightest star is Regulus, which means 'the King star'. It is also called Alpha Leonis, and marks the Lion’s heart. It is on the left-hand side of the constellation, in the north-west. A handspan to the right of Regulus is Denebola, a white star marking the tip of the lion's tail. Regulus is the highest star in a pattern called 'The Sickle' (or reaping-hook). It marks the top of the Sickle's handle, with the other end of the handle, the star Eta Leonis, directly underneath. The blade of the Sickle curves around clockwise from Eta Leonis. The Sickle forms the mane and head of the lion, when observed right-way-up.
Between Leo and the northern horizon is a faint grouping of fourth magnitude stars. This is the small and inconspicuous constellation of Leo Minor, the small lion. Leo Minor is halfway between Leo and Ursa Major, the Great Bear, whose stomach, legs and tail are the only parts of him that are visible from Nambour. They are best seen low to the northern horizon as soon as darkness falls.
High in the north-east, we can find the third brightest star in the night sky, Arcturus. It is outshone
only by Sirius and Canopus. Arcturus differs from those just named, for it is an obvious orange colour, a K2 star of zero magnitude. It is a
particularly beautiful star, and, as it is the brightest in the constellation of Boötes, the Herdsman, it has the alternative name of Alpha Boötis.
Boötes is pronounced 'Bo-oh-tees').
Arcturus
East of Boötes and closer to the north-eastern horizon is a fainter circle of fourth magnitude stars, Corona Borealis, the Northern
Crown. The brightest star in the Crown is named Alphecca, and it shines at magnitude 2.3.
Between Arcturus and Denebola is a faint Y-shaped cluster of stars called Coma Berenices, or Berenice's Hair. Most of the stars in this group have a visual magnitude of about 4.5. Originally the tuft at the end of Leo's tail, Coma Berenices was made a separate constellation in 1536 by a mapmaker called Casper Vopel, and adopted by the great Danish naked-eye astronomer of the 16th century, Tycho Brahe. It is the only present-day constellation that represents a real person or part of a person, Queen Berenice II of Egypt who lived in the third century BCE.
To the north of the zenith is the next zodiacal constellation after Leo, Virgo, the Virgin. The brightest star in Virgo is Spica, an ellipsoidal variable star whose brightness averages magnitude 1. This star, also known as Alpha Virginis, is a hot, blue-white star of spectral type B2. It is the sixteenth brightest star, and the rest of the constellation Virgo lies to the north-west of it. Tonight, Spica is nearly overhead, and is midway between Arcturus and the Southern Cross.
The area of sky between Spica and Coma Berenices is called a 'galactic window'. Being well away from the plane of the Milky Way (which we can see passing from Sirius in the west to Crux high in the south and then to Sagittarius in the east), there are fewer stars and dust clouds to obscure our view, and we can see right out of our galaxy into the depths of intergalactic space. A 20 cm (eight inch) telescope can see numerous galaxies in this region, nearly thirty being brighter than twelfth magnitude. A larger amateur telescope can detect hundreds more. Professional telescopes equipped with sensitive cameras can detect millions of galaxies in this part of the sky.
A handspan west of Spica is the constellation of Corvus the Crow. Corvus is a lopsided quadrilateral of four third-magnitude stars, and is almost directly overhead at this time tonight. A large but faint constellation, Hydra the Water-snake, winds its way from near Procyon west of the zenith and around Corvus and Virgo to Libra, which is now about a handspan east of the zenith.
Hydra has one bright star, Alphard, mv=2.2. Alphard is an orange star that was known by Arabs in ancient times as ‘The Solitary One’, as it lies in an area of sky with no bright stars nearby. Tonight it is about 40 degrees above the western horizon.
About a handspan to the south-east of Alphard is a bright planetary nebula, the 'Ghost of Jupiter' NGC 3242. It is the remnant left when the
central star exploded (below).
Scorpius is high above the east-south-eastern horizon (see below). This famous zodiacal constellation is like a large letter 'S', and, unlike most constellations, is easy to recognise as the shape of a scorpion. At this time of year, he has his tail down and claws raised. The brightest star in Scorpius is Antares, a red type M supergiant of magnitude 0.9. Antares is the fifteenth brightest star.
On the eastern horizon, another fainter constellation, Ophiuchus, the Serpent Bearer, is nearly completely risen. This constellation is completely outshone by its brilliant neighbours, Scorpius and Sagittarius.
Beneath Scorpius and well above the east-south-eastern horizon is another zodiacal
constellation, Sagittarius, the Archer, through which the Milky Way passes. Sagittarius teems with stars, glowing nebulae and dust clouds, as it is in line with the
centre of our galaxy.
This year it hosts the ringed planet Saturn. At present the brightest object in
this part of the sky, Saturn is located near the star Kaus Borealis. At the beginning of June Saturn is part-way through
its retrograde loop, heading westwards. It will reach its stationary point on August 25 and then will turn eastwards once again, crossing back into
Sagittarius on November 17. It will take until March 17, 2020 to pass through Sagittarius. Adjoining Sagittarius to the south (right-hand side), there is a beautiful curve of faint stars. This is Corona Australis, the Southern Crown,
and it is very elegant and delicate.
Below Sagittarius is the next zodiacal constellation, Capricornus, where brilliant Mars is shining brightly, but these will not rise until a
little later.
The body of Scorpius is at top, with the two stars in the Sting underneath, just
above the centre of the picture. The red supergiant star Antares appears close to the top left corner. The stars in the lower half of the picture are in
Sagittarius. Near the lower right margin is a graceful curve of fourth magnitude stars, Corona Australis, the Southern Crown.
At left - the two Pointers, Alpha and Beta Centauri. Centre - Crux (Southern Cross)
with the dark cloud of dust known as the Coalsack at its lower left. Right - star clusters in the Milky Way and the Eta Carinae nebula.
High in the south, Crux Australis (Southern Cross,
usually abbreviated to Crux) is nearly vertical. The two Pointers,
Alpha and
Beta Centauri lie to its left. Alpha is the one further away from Crux. Whereas Alpha Centauri 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. Crux will be in a vertical
position in the early evening tonight. If the night is moonless and the skies are clear, a black dust cloud known as the Coalsack can be seen just to
the left of Acrux, the bottom and brightest star of the Cross. Surrounding Crux on three sides is the large constellation
of Centaurus, its
two brightest stars being the brilliant Alpha and Beta Centauri. The rest of the constellation of Centaurus arches over Crux to its
right-hand side, where it adjoins Carina and Vela.
The constellations surrounding the Southern Cross.
Just to the left of 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.
Between Crux and Sirius is a very large area of sky filled with interesting objects. This was once the constellation Argo, named for Jason’s famous ship used by the Argonauts in their quest for the Golden Fleece. The constellation Argo was found to be too large, so modern star atlases divide it into three sections - Carina (the Keel), Vela (the Sails) and Puppis (the Stern).
On the border of Carina and Vela is the False Cross, larger and more lopsided than the Southern Cross. The False Cross is two handspans to the right of Crux, and is also lying tilted to the left at this time of year. It has passed culmination, and is beginning to head for the south-south-western horizon. Both of these Crosses are actually more like kites in shape, for, unlike Cygnus (the Northern Cross) they have no star at the intersection of the two cross arms.
Between the Southern Cross and the False Cross may be seen a glowing patch of light. This is the famous
Eta Carinae Nebula, which is a remarkable
sight through binoculars or a small telescope working at low magnification. It is a turbulent area of dark dust lanes and fluorescing gas. The brightest star in the nebula,
Eta Carinae itself, is a peculiar unstable star which has been known to explode, becoming very bright. It last did this in 1842, and is called a 'cataclysmic
variable star', or 'recurrent nova'. It is a very massive star, and
when it reaches the end of its life, it will explode in a stupendous blast, a 'supernova'
or possibly a 'hypernova'. In these cataclysmic events, all the elements
denser than iron are formed. If you have a gold ring, the gold in it was created
by a supernova in the distant past - that is the only way that gold, silver,
lead and other dense metals can be formed. Like yourself, your wedding ring and
everything else on the Earth is made of 'star stuff' - elements created in the
interiors of stars.
The Homunculus, a tiny planetary nebula ejected by the eruptive variable star, Eta Carinae.
Low in the south-south-west, about 20 degrees above the horizon, the Large Magellanic Cloud (LMC) is faintly visible as a diffuse glowing patch. It is about a handspan to the left (south) of Canopus. About a handspan to the left of the LMC is the Small Magellanic Cloud (SMC), a smaller glowing patch, not far above the southern horizon. From Nambour's latitude, these two clouds never set. Each day they circle the South Celestial Pole, which is a point in the sky 26.5 degrees above the horizon's due-south point. The Earth's axis points to the South Celestial Pole. Objects in the sky that never set are called 'circumpolar'. The LMC and SMC are described below.
The line of the ecliptic along which the Sun, Moon and planets travel passes through the following constellations this month: Cancer, Gemini, Leo, Virgo, Libra, Scorpius and Sagittarius.
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 that runs 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.
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
here . To see some of the best astrophotographs taken with the giant Anglo-Australian Telescope, click here .
Some fainter constellations
Between Regulus and Alphard is the inconspicuous constellation of Sextans, the Sextant. Between Sextans and the quadrilateral of Corvus, the Crow is another faint star group, Crater, the Cup. Between the Milky Way and the southern horizon may be found the lesser-known constellations of Apus the Bird of Paradise, Chamaeleon, Volans the Flying Fish, Pavo the Peacock, Apus the Bird of Paradise, Octans the Octant, Mensa the Table Mountain, Dorado the Goldfish, Indus the Indian, Tucana the Toucan, Hydrus the Southern Water Snake, Pictor the Painter and Telescopium the Telescope.
Why are some constellations bright, while others are faint ?
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.
This window is high above the northern horizon this month soon after darkness falls, so it's an ideal time for observing galaxies in the Virgo cluster. The southern window is in the constellation Sculptor, not far from the star Fomalhaut. This window will not rise until just before midnight. 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.
The Milky Way
A glowing band of light crossing the sky is especially noticeable during the winter months. This glow is the light of millions of faint stars combined with that coming from glowing gas clouds called nebulae. It is concentrated along the plane of our galaxy, and this month it is seen crossing the sky, starting from almost due west (between Sirius and Procyon), and passing through Carina to Crux, Centaurus and Scorpius to Sagittarius in the east.
It is rewarding to scan along this band with a pair of binoculars or small telescope, looking for star clusters and emission nebulae. By midnight in June, the Milky Way crosses the sky through the zenith, almost dividing it in two. It runs from south-west to north-east, and the very centre of our galaxy passes directly overhead.
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 southern horizon for another 4 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 find the South Celestial Pole from southern Queensland is to choose a time when the Southern Cross is vertical (8.00 pm on June 1, 6.00 pm on June 30), and simply locate that spot in the sky which is midway between the bottom star of the Cross (Alpha Crucis), and the actual (not apparent) southern horizon.
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 Earth's rotation will cause the stars to appear to 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.
A wide-angle view of trails around the South Celestial Pole, with Scorpius and Sagittarius at left, Crux and Centaurus at top, and Carina and False Cross at right.
Star trails between the South Celestial Pole and the southern horizon. All stars that do not pass below the horizon are circumpolar.
The Season of the Scorpion
The spectacular constellation of Scorpius rises at sundown in June. 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.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 closest to the eastern horizon tonight, and are near the bottom of the picture below. Above
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 (below).
The red supergiant star Antares.
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.) 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
III on the Antoniadi Scale, or in other words about fair. Image acquired at Starfield Observatory in Nambour on July 1, 2017.
The ancient Greeks believed that Orion avoided the Scorpion, hurrying to leave the sky in the west when he saw Scorpius rising in the east.
This was the basis for their stories of the goddess Artemis, who had Actaeon torn to pieces by dogs for accidentally seeing her unclothed, and who had her
paramour Orion stung to death by scorpions for touching her.
From Australia though, we can (at certain times of the year) observe both constellations in the sky at the one time, even though they are on the opposite sides of the celestial sphere. In the early evenings of the first week of June, the stars in Scorpius can be seen rising above the east-south-eastern horizon, while Orion is preparing to set in the west.
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 Kent (Alpha Centauri) at left, and Beta Cygni (Albireo), at righ
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
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 the
smallest 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 small
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 , 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 separated them (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.
Star Clusters
The photograph below shows a typical open cluster, M7
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 or in some cases over a million 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 a solid mass of faint stars. Many 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 all night. 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 hundreds 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.
The globular cluster
The central core of Omega Centauri
There is another remarkable globular, second only to Omega Centauri. About two degrees below 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
Globular Cluster NGC 104 in Tucana
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
* M42:This number means that the Great Nebula in Orion is No. 42 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. ** 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. As larger telescopes built early in the 20th century discovered fainter objects in space,
and also dark, obscuring nebulae and dust clouds, the NGC was supplemented with the addition of the Index Catalogue (IC). Many non-stellar objects in the
sky have therefore NGC numbers or IC numbers. For example, the famous Horsehead Nebula in Orion is catalogued as IC 434. The NGC was revised in 1973, and lists
7840 objects. 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 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 Galaxy 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
A little more than a handspan above the south-south-western horizon, and below and to the left of Canopus, 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 SMC is due south and quite low to the horizon (about ten degrees up), and the LMC is above it and to its right. The LMC is noticeably larger and brighter. They lie at a distance of 160 000 light years, and are about 60 000 light years apart. They are dwarf galaxies, and they 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.From our latitude both Magellanic Clouds are circumpolar. This means that they are closer to the South Celestial Pole than that Pole's altitude above the horizon, so they never dip below the horizon. They never rise nor set, but are always in our sky. Of course, they are not visible in daylight, but they are there, all the same.
The Large Magellanic Cloud - the bright knot of gas to left of centre is the famous Tarantula Nebula (below).
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 intergalactic neighbours.
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