July
2018
Updated: 14 July 2018
Welcome to the night skies of Winter, featuring Virgo, Carina, Crux, Centaurus, Scorpius, Sagittarius, 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.
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 Gemini, the Twins. It leaves Gemini and passes into Cancer, the Crab on July 21.
Partial Solar Eclipse, July 13:
There will be a partial solar eclipse visible from south-eastern Australia soon after midday on Friday, July 13 next. No aspect of the eclipse will be visible from Queensland or New South Wales. Only parts of South Australia, the southern half of Victoria and all of Tasmania will be able to see it. Mount Gambier in South Australia will see only 1.15% of the Sun covered, while Melbourne will only see 0.45%, barely noticeable. Hobart will see 3.5% of the Sun eclipsed, but the best views will be from further south. Antarctica is too far south, as the Sun will be below the northern horizon at midday. From Hobart, the eclipse will begin at12:52 pm, and will end at 1:24 pm. Observers should take the recommended precautions when observing solar eclipses, as looking at the Sun at any time without the correct filters is very dangerous and can cause permanent damage to your eyesight.
Total Lunar Eclipse, July 28:
There will be a total lunar eclipse visible from the eastern states of Australia on Saturday, July 28 next. The timing is not favourable, as it will begin soon after 3 am and will continue until dawn breaks. The Moon will set before the eclipse is over, but Western Australia will see more of the eclipse. The Full Moon will enter the Earth's penumbra at 3:13 am when it is about 42 degrees above the west-south-western horizon. This phase is hardly noticeable. The eclipse proper will begin at 4:24 am, when the Moon begins to enter the Earth's main shadow, which is called the umbra. Even the most casual observer will see a bite appearing out of the edge of the Moon. The Full Moon will gradually lose its brightness as more of it disappears into our shadow. By 5:30 am the bright Moon will be completely immersed in the shadow, but it will still be faintly visible as a dull, reddish disc. This is the total phase of the eclipse, and mid-eclipse occurs at 6:22 am. After that the Moon will very slowly brighten, and totality will end at 7:14 am. Then the western edge of the Moon will begin to come out of the umbra, and the whole Moon will have emerged from the Earth's shadow by 8:19 am. The Moon will still take another hour to leave the penumbra, and the eclipse will be over by 9:30 am. Unfortunately, these final aspects of the eclipse will not be visible from south-east Queensland, as the Sun will rise at 6:31 am and the Moon will set at 6:34 am. Before and after a lunar eclipse, the Full Moon looks brighter than normal, as the Sun, Moon and Earth are so perfectly aligned.It is quite safe to watch lunar eclipses as they occur at night. Solar eclipses are the dangerous ones. Total lunar eclipses are not rare, but quite uncommon. The last one visible from Starfield Observatory occurred on January 31 last. If you miss the July 28 event for any reason, you will need to wait until May 26, 2021 for the next. However, there will be a partial lunar eclipse on July 17 next year.
Moon Phases: Lunations (Brown series): #1181, 1182, 1183
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' Total Lunar Eclipse
New Moon:
August
11
19:58 hrs diameter =
33.3' Lunation #1183 begins
First Quarter:
Full Moon:
Lunar Orbital Elements:
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
July 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 Rupes Altai or Altai Mountains.
This photograph was taken from Nambour at 7:04 pm on July 30, 2017. The crater Catharina is at the top of the image. The Rupes Altai or Altai Mountains
is a 450 km long cliff or fault escarpment which runs in a huge curve from west of Catharina to the 90 km crater Piccolomini at the lower-right corner. The cliff
averages 700 to 1000 metres in height, with some summits approaching 2 km high. One peak is 4 km high.
The curve of the Rupes Altai is concentric with the large impact basin called Mare Nectaris (Sea of Nectar), which is 420 kilometres to the north-east. The
fault escarpment is obviously contemporaneous with and caused by the Nectaris event. Mare Nectaris is off the image above to the upper right, but is visible
on the chart below and on the smaller-scale image of the Moon at 8 days, shown above this section.
This feature, like some other lunar mountain ranges such as the Alps, Apennines
and Caucasus mountains, was named after a terrestrial range. The Altai Range on
Earth is located in central Asia, north of the Himalayas and the Tibetan
Plateau, and between the large lakes of Balkhash and Baikal. Russia, China,
Mongolia and Kazakhstan come together there. Four of its peaks are over 4 km
high. The name 'Altai' means 'Gold Mountain' in Mongolian.
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.
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
August 1: Saturn 54 arcminutes south of the star 14 Sagittarii (mv= 5.49) at 20:36 hrs
August 4: Moon 4.1º south of Uranus at 11:24 hrs
August 7: Moon 1.5º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 03:53 hrs
August 7: Jupiter at eastern quadrature at 09:12 hrs (diameter = 37.2")
August 7: Uranus at western stationary point at 22:43 hrs (diameter = 3.6")
August 8: Limb of Moon 33 arcminutes south of the star Zeta Tauri (mv= 2.97) at 04:20 hrs
August 8: Moon 1.6º south of the star Mu Geminorum (mv= 2.87) at 22:34 hrs
August 9: Mercury in inferior conjunction at 11:59 hrs (diameter = 11.1")
August 11: Moon 5.8º north of Mercury at 15:02 hrs
August 12: Moon 2.1º north of the star Regulus (Alpha Leonis, mv= 1.36) at 15:23 hrs
August 15: Moon 6.3º north of Venus at 03:57 hrs
August 15: Saturn 1º north of the star 11 Sagittarii (mv= 4.96) at 23:40 hrs
August 17: Jupiter 14 arcminutes north of the star Zuben Elgenubi (Alpha Librae, mv= 2.75) at 8:11 hrs
August 17: Venus at at greatest elongation east (45º 54') at 16:59 hrs (diameter = 24.2")
August 18: Moon 4.5º of Jupiter at 00:50 hrs
August 19: Mercury at western stationary point at 14:12 hrs (diameter = 9.2")
August 21: Moon 2.2º north of Saturn at 19:14 hrs
August 23: Limb of Moon 18 arcminutes north of the star Pi Sagittarii (mv= 2.88) at 00:49 hrs
August 23: Moon 2º north of Pluto at 05:49 hrs
August 24: Moon 7º north of Mars at 01:24 hrs
August 26: Moon 1.5º north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 04:21 hrs
August 27: Mercury at greatest elongation west (18º 19') at 07:46 hrs (diameter = 7.3")
August 27: Moon 2.2º south of of Neptune at 20:28 hrs
August 28: Mars at eastern stationary point at 00:33 hrs (diameter = 21.6")
August 31: Moon 4.4º south of Uranus at 15:34 hrs
The Planets for this month:
Mercury: On July 1, Mercury will be visible in the north-western twilight sky in the faint constellation of Cancer. It 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. Although it will be visible all month, the first three weeks of July will be best for observing it. We are fortunate in that brilliant Venus is in the same part of the sky, and can be used as an aid to locate the innermost planet. Simply look for Mercury between Venus and the place on the horizon where the Sun has set. Look just as twilight starts to fade.
Mercury reaches its greatest angular distance east of the Sun (26º 25') on July 13, and on July 15 the waxing crescent Moon will be just above and to its right. Mercury crosses into Leo that night just before midnight, on July 15. In the last week of July Mercury's orbit will make it start to swing back towards the Sun, and from the beginning of August it will be harder to find. Mercury will pass between the Earth and the Sun on August 9, and the following week it will appear in the north-eastern sky just before sunrise.
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 July 1, Venus will be 40º or more than 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 -4.1 and a phase of 70%. On that date, Venus will be in the constellation Leo. Venus will be just to the left of the waxing crescent Moon on the evening of July 16. 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 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 perfectly placed to observe this month as on July 8 it rose at 6:43 pm in the constellation Capricornus. Mars entered Capricornus on May 14 last, and was at that time being rapidly overtaken by the speeding Earth. Just as a car that we are overtaking appears to move backwards relative to us, so Mars appears to move backwards in the sky for a short period, in this case the two months from June 27 to August 28. On the first of these two dates, Mars' eastwards movement through the background stars of Sagittarius slowed down and stopped as Mars swung around to start its retrograde motion. Since then it has been moving westwards with respect to the stars. On August 28 it will halt once more, swing around and head eastwards again. This movement, when projected on the sky, appears as a loop (called a 'retrograde loop'), and is a perspective effect caused by the fact that we ourselves are not motionless in space but riding on a planet moving at 107 200 kilometres per hour, while Mars is moving at only 86 430 kilometres per hour, or only 80% of the Earth's speed. Mars does not actually stop and reverse direction as it appears to us.Midway through its retrograde loop, Mars will reach Opposition (directly opposite the Sun in our sky) on July 27. Four days later, Mars will reach its closest approach to Earth, when it will appear to us at its biggest and brightest. This event, which happens close to Opposition, will occur on July 31. On that day, Mars will have a brightness of magnitude -2.8 (twice as bright as Jupiter) and will have an angular diameter of 24 arcseconds.
This will be a very favourable opposition, as Mars will appear bigger 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 almost directly overhead each midnight 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.
The only problem with the current appearance of Mars is that on May 31 last, a huge dust storm developed in its atmosphere. By mid-June the dust storm had enveloped half of the planet, completely obscuring the normally easily visible surface features. 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. It therefore went into 'sleep' mode, periodically waking up to check on conditions. By July 14 there was still not enough sunlight reaching the rover to enable it to resume normal operation.
By July 1 the martian dust storm had intensified into a Planet-Encircling Dust Event, or PEDE, that would cover both North America and Russia completely if it were on Earth. Very few features on Mars are visible through the murk, those that can be discerned are very hazy. Though its location is far from Opportunity's, the Curiosity rover, currently studying Gale Crater, captured the growing impacts of the storm in a selfie snapped on June 15, and now is itself swamped by the dust cloud.
Unlike Opportunity, Curiosity relies on a nuclear-powered battery rather than solar power, and continues to work, albeit under a thickening haze of dust already measured at greater opacity than ever before. When its most recent selfie was taken, the haze around Curiosity was already six to eight times thicker than normal for this time of the martian year. The rover’s engineers estimate minimal impact to the rover’s hardware from the storm, though the rover is currently pointing its camera downward after each use to minimise the effects of blowing dust on the optics.
The dust storm couldn't have come at a worse time — Mars is quickly approaching opposition
on July 27, with its closest approach to Earth occurring shortly after. This approach normally affords excellent views of the planet’s surface features, which certainly
won’t be the case if it’s hidden under a thick cloud of dust.
Some PEDEs persist for many months.
Regional dust storms are common, but scientists aren’t entirely sure what causes them to occasionally grow into PEDEs. The planet’s dust storms are typically prompted by
warming as the planet approaches its closest point to the Sun; temperature contrasts generate winds that pick up and spread fine surface dust grains. As the polar ice caps
melt, the additional carbon dioxide in the atmosphere increases surface pressure and suspends the particles in the air, sometimes in clouds reaching up to 60 kilometres
high. We hope that the storm dissipates soon, so that we can clearly see details on Mars again, and enabling Opportunity to 'wake up' and resume
its work.
Visit the July 2 and subsequent editions
of
Spaceweather
for more information and animations.
On
July 1 and 28, the almost Full Moon will be close to Mars in the 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.
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
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.
Jupiter:
This gas giant planet is now visible in the sky for most of the night, 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. In mid-July at 7 pm, Jupiter will be found almost directly overhead, about 12º or two-thirds of a handspan north of the zenith. The waxing gibbous Moon will be seen just to the left of Jupiter soon after sunset on July 21.
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:
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.Pegasids
July 10
Waning crescent Moon, 16%
sunlit ZHR = 8
Radiant: Near the star Markab
S Delta Aquarids
July 29
Almost Full Moon, 98%
sunlit ZHR = 20
Radiant:
Between the stars Skat and Deneb Algedi
Alpha Capricornids
July 30
Almost Full Moon, 96% sunlit ZHR =
8
Radiant: Near the star Algedi
Use
this
Fluxtimator
to calculate the number
of meteors predicted per hour for any meteor swarm on any date, for any place in
the world. 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:
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
PANSTARRS (C/2017 S3)
Approaching green comet explodes
A comet that could become visible to the naked eye in August has exploded in brightness, suddenly increasing its luminosity 16-fold. Whatever happened on Comet PANSTARRS (C/2017 S3) on July 1 has given it an expanding green atmosphere almost twice the size of the planet Jupiter. Visit the July 4 edition of Spaceweather and subsequent news releases for pictures and more information about this approaching comet.
Comet Lulin
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 July, the Eta Carinae Nebula is ideally placed for viewing in the early evening, to the west of the Southern Cross.
The Stars and Constellations for this month:
These descriptions of the night sky are for 8 pm on July 1 and 6 pm on July 31. They start at the western horizon.
In the north-west, Leo the Lion is preparing to set. It will have completely disappeared by 10.30 pm. The bright star
Regulus (Alpha Leonis) marks the Lion’s heart. Mercury will be
about a degree from Regulus as darkness falls on July 26. A handspan to the right and above Regulus is
Denebola, a white star marking the lion's tail. It is about 30 degrees
above the north-western horizon. We see the lion upside-down from the Southern Hemisphere. Regulus is the western-most star in a pattern called 'The Sickle' (or
reaping-hook). It marks the end of the Sickle's handle, with the other end of the handle, the star
Eta Leonis, below and to the right. The blade of the Sickle
curves around clockwise from Eta Leonis to the horizon. The Sickle forms the mane and head of the lion, when observed right-way-up. The Sickle is just touching the
theoretical horizon at this time tonight.
The constellation Leo, as we see it from Australia. Regulus is above centre left, and Denebola above centre right. The Sickle curves down from Regulus.
High in the north, (about 43 degrees above the horizon, and about 10 degrees west of the meridian or north-south line) 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). Boötes is due north (culminating) at this time of night.
Arcturus
East of Boötes and above the north-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 shines at magnitude 2.3.
East of the Northern Crown is Hercules, stretching from the north-north-eastern horizon upwards. Rising in the north-east is a bright white A0 star,
Vega, which is the fifth brightest star, after Sirius, Canopus, Arcturus and Alpha Centauri. Vega is the main star in the small constellation of
Lyra the Lyre, which contains the famous Ring Nebula, M 57.
The Ring Nebula was driven away from the central star by powerful stellar winds.
About fifteen degrees (a little less than a handspan) to the right of Vega can be seen Albireo, a beautiful double star with contrasting colours. It is the highest
star of the Northern Cross, Cygnus.
Rising above the eastern horizon is the great main-sequence star Altair. This A7 white star is the eleventh brightest in the heavens. Altair is also known as Alpha Aquilae, as it is the brightest star in the constellation of Aquila, the Eagle. It marks the heart of the eagle, and is flanked by two lesser stars marking each wing-tip, Gamma Aquilae and Beta Aquilae. This threesome, making a short horizontal line in the east, is easy to find.
Just to the west of the zenith is the next zodiacal constellation after Leo, Virgo, the Virgin. It is a large but fairly inconspicuous constellation, but it does have one bright star, Spica, which is 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 at an altitude of 65 degrees, between the zenith and Corvus. It is roughly halfway between Arcturus and the Southern Cross.
Directly overhead is the faint constellation of Libra, the Scales, the brightest stars of which are two of magnitude 2.7 with exotic names, Zuben Elgenubi and Zuben Eschamali. This year, Libra is dominated by the presence of the brilliant planet Jupiter - it is by far the brightest object in that part of the sky. On November 20 Jupiter will cross into Scorpius. Two handspans west of Jupiter is the constellation of Corvus the Crow. Corvus is a lopsided quadrilateral of four third magnitude stars. It is about three handspans above the western horizon.
Approaching the zenith is the spectacular constellation of Scorpius, the Scorpion (see below), which is very rich in objects to
find with a small telescope or binoculars. 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, and will be almost exactly overhead at 9:40 pm on July 1 (4 minutes earlier per night for succeeding nights).
Below or east of Scorpius is 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 and the next 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 July Saturn is part-way through its retrograde loop, heading westwards. It will
reach its stationary point on September 6 and then will turn eastwards once again. It will take until March 17, 2020 to complete its passage through Sagittarius and then
enter Capricornus.
Adjoining Sagittarius to the south (right), there is a beautiful curve of faint stars This is Corona Australis, the Southern Crown, and it is very elegant
and delicate. The brightest star in this constellation has a magnitude of only 4.1. Below Sagittarius and above the eastern horizon is a large constellation known as
Capricornus, the Sea-Goat. This constellation is lacking in any bright stars, and is fairly unremarkable, except for one thing - this year the planet
Mars is passing through. This will be a spectacular event, Mars becoming so close to us this month that it will be brighter even than Jupiter. For more information,
see
The Planets for this Month: Mars
(above).
The Trifid Nebula, M20, in Sagittarius, is composed of a reflection nebula (blue), an emission nebula (pink), and dark lanes of dust.
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.
Between Scorpius and Corona Borealis are two large but faint constellations, Serpens, the Serpent, and Ophiuchus, the Serpent Bearer.
Neither is as spectacular as Scorpius or Sagittarius. High in the south-south-west, Crux (Southern Cross) is at an altitude of 50 degrees. Crux was in a vertical position about two hours ago (6.00
pm on July 1), but now it is has tilted over to the west so that it leans at an angle of 30 degrees from the vertical. The two Pointers, Alpha and Beta
Centauri, lie to its left and form a horizontal line. The two pointers are 8 degrees apart. Alpha is the one further away from Crux. Whereas Alpha Centauri is the
nearest star system to our Sun, only 4.37 light-years distant, Beta is 390 light-years away. Alpha is composed of two Sun-like stars, but Beta Centauri is
a supergiant, which accounts for its appearing almost as bright despite being nearly 90 times further away. If the night is dark 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 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.
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.
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
Herschel's Jewel Box
Below Crux and to its left is a small, fainter quadrilateral of stars, Musca, the Fly. Out of all the 88 constellations, it is the only insect.
Below and to the left of Alpha Centauri is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern
Triangle. 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
Very close to the south-south-western horizon, the star Canopus may be glimpsed soon after darkness falls.
It will have set by 8:30 pm on July 1. You will need a flat horizon in this direction. Canopus is the second-brightest star in the sky after Sirius, the Dog Star. The path of the Milky Way between Aquila and Canopus is filled with clusters, dark clouds, glowing nebulae, multiple stars and other
interesting objects. Check it out with binoculars or a telescope. Halfway between Crux and the southern horizon is a white star of magnitude 1.7, Miaplacidus. It is the second-brightest star in the constellation
Carina, after Canopus, so it has the alternative name of Beta Carinae. Half a handspan to the right of Miaplacidus is the False Cross, larger and more
lopsided than the Southern Cross. The False Cross is two handspans below Crux, and is also tilted in the same way. It is about a handspan above the south-western
horizon, and will have completely set by midnight. Both of these Crosses are actually more like kites in shape, for, unlike Cygnus (the Northern Cross, rising in
the north-north-east just before midnight on July 1), 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. A photograph of this emission
nebula with dark lanes appears below. 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 also an extremely large and massive star, and is a possible
candidate for the next supernova in our Galaxy.
The central part of the Eta Carinae nebula, showing dark lanes, molecular clouds, and glowing clouds of fluorescing hydrogen.
The Keyhole, a dark cloud obscuring part of the Eta Carinae Nebula.
The Homunculus, a tiny planetary nebula ejected by the eruptive variable star, Eta Carinae.
Low in the south-south-west, about 10 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, also close to 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 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. Between Arcturus and Denebola, and 30 degrees above the north-western horizon, 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. 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 the west-south-western horizon through the star clusters and nebulae of Carina to Crux and Centaurus high in the south, and then through Scorpius and
Sagittarius high in the east to Aquila on the east-north-eastern horizon), there are fewer stars and dust clouds to obscure our view, and we can see right out of our galaxy
into the depths of inter-galactic 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. Large telescopes equipped with sensitive cameras can detect millions of galaxies in this part of the sky. The line of the ecliptic along which the Sun, Moon and planets travel, passes through the following constellations this month: Leo, Virgo, Libra,
Scorpius, Sagittarius and Capricornus. 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 of Scorpius forms the Emu's
body. The Emu is sitting, waiting for its eggs to hatch. The eggs are the large and bright 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
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 the south-west in the
constellation Carina, and passing through Crux, Centaurus,
Lupus, Scorpius, Sagittarius and Scutum to Aquila and
Lyra 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. It is rewarding to scan along this band with a pair of binoculars, looking for star clusters and
emission nebulae. Dust lanes along the plane of the Milky Way appear to split it in two in some parts of the sky. One of these lanes can be easily seen, starting
near Alpha Centauri and heading towards Antares. At 10.00 pm in mid-July, the Milky Way crosses the zenith, almost dividing the sky in two. It runs from
south-west to north-east, and the very centre of our galaxy passes directly overhead.
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, except that it reaches
fainter stars than the eye can see.
The Season of the Scorpion
Between Crux and the south-western horizon is a very large area of sky filled with interesting objects. This was once the constellation Argo, named by ancient
Greeks 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).
The spectacular constellation of Scorpius is about 45 degrees above the eastern horizon at sundown in July. 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 root 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.
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.
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, Pavo the Peacock, Octans the Octant, Mensa the Table Mountain, Dorado the Goldfish, Indus the Indian, Hydrus the Southern Water Snake, Pictor the Painter's Easel 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 50 degrees above the north-western horizon early in the evenings this month, so it's a good 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 be rising well 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.
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 (6.00 pm on July 1), and simply locate that spot in the sky which is midway between the bottom star of the Cross (Alpha Crucis), and the theoretical 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.
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 (Alpha Centauri) at left, and Albireo (Beta Cygni) 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 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 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 separated them (Acrux, Castor, 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 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 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, M7
*. It lies in the constellation Scorpius, just below the scorpion's sting. It lies in the direction of our galaxy's centre. The cluster itself is the group of white stars in the centre of the field. Its distance is about 380 parsecs or 1240 light years. M7 is visible tonight.
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 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 naked-eye galaxies
Close to the southern horizon, 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 to the right of 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.
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 inter-galactic neighbours.
Astronomy Picture of the Day:
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