May  2018

Updated:   20 May 2018

 

Welcome to the night skies of Autumn, featuring Canis Major, Leo, Carina, Crux, Scorpius, 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:  

 

Times for transient sky phenomena are given using a 24 hour clock, i.e. 20:30 hrs = 8.30 pm. Times are in Australian Eastern Standard Time (AEST), which equals Universal Time (UT) + 10 hours. Daylight saving is not observed in Queensland. Observers in other time zones will need to make their own corrections where appropriate. With conjunctions of the Moon, planets and stars, timings indicate the closest approach. Directions (north or south) are approximate. The Moon’s diameter is given in arcminutes ( ’ ). The Moon is usually about 30’ or half a degree across. The 'limb' of the Moon is its edge as projected against the sky background.

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 Aries, the Ram. It leaves Aries and passes into Taurus, the Bull, on May 14.   

 

 

Moon Phases:  Lunations (Brown series):  #1179, 1180, 1181 

 

Last Quarter:           May 08                      12:10 hrs          diameter = 29.7' 
New Moon:              May 15                       21:49 hrs          diameter = 32.5'     Lunation #1180 begins
First Quarter:           May 22                      13:50 hrs          diameter = 32.0'
Full Moon:                May 30                      00:21 hrs          diameter = 30.0' 

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'   


 
 

Lunar Orbital Elements:



May 06:                Moon at apogee (404 457 km) at 10:26 hrs, diameter = 29.5'
 May 07:                Moon at descending node at 20:20 hrs, diameter = 29.6'
 May 18:                Moon at perigee (363 777 km) at 07:07 hrs, diameter = 32.8'
 May 20:                Moon at ascending node at 23:13 hrs, diameter = 32.5'

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'
 

Moon at 8 days after New, as on May 23.

The photograph above shows the Moon when approximately eight days after New, just after First Quarter.  A detailed map of the Moon's near side is available here.  A rotatable view of the Moon, with ability to zoom in close to the surface (including the far side), and giving detailed information on each feature, may be downloaded  here.

Click here for a photographic animation showing the lunar phases. It also shows the Moon's wobble or libration, and how its apparent size changes as it moves from perigee to apogee each month. It takes a little while to load, but once running is very cool !  All these downloads are freeware, although the authors do accept donations if the user feels inclined to support their work.

 

 

Lunar Feature for this Month:

 

Each month we describe a lunar crater, cluster of craters, valley, mountain range or other object, chosen at random, but one with interesting attributes. A recent photograph from our Alluna RC20 telescope will illustrate the object. As all large lunar objects are named, the origin of the name will be given if it is important. This month we will look at the craters Triesnecker and Hyginus, which are associated with a remarkable system of clefts.

At top centre is the 6 km wide crater Hyginus. Its northern wall is deformed by a 2 km wide smaller crater. A long rille passes through it. This area of the Moon shows numerous clefts, extending south past the crater Triesnecker at lower centre. North is at the top, east to the right. The photograph was taken at 7:21 pm on July 31, 2017.

 

This area is just above the centre of the Moon's visible disc. The largest crater in this area is 27 kilometre Triesnecker, below centre. It sits on a lava plain, and its fame is due to the complex series of clefts or rilles that are found on the plain to its east. These rilles stretch for over 100 kilometres to the north and a similar distance to the south of Triesnecker, and in many places they cross over each other.

At top centre is the 6 km wide crater Hyginus. Its northern wall is deformed by a 2 km wide smaller crater. Passing through Hyginus is a notable valley or rille, 226 kilometres long, rather disjointed in outline, appearing in places to be made up of chains of collapse craterlets.  


 Triesnecker

Franz Triesnecker (1745-1817) was an Austrian Jesuit mathematician and astronomer who published a number of treatises on astronomy and geography. Much of his work was in making measurements of celestial bodies at the Vienna Observatory, which were published from 1787 until 1806.

 

Hyginus

Gaius Julius Hyginus (64 BCE – AD 17), a Roman historian, wrote the Poeticon astronomicon, also known as the Astronomica, 160 years before Ptolemy’s Mathematike Syntaxis or Almagest. The book has drawings of numerous star constellations, and tells the myths and legends associated with them. Some astronomical historians think that a later Hyginus was responsible for the text in the 2nd century AD, claiming that because the Poeticon astronomicon describes 47 out of 48 constellations listed in Ptolemy’s Syntaxis of AD 150, then it must have used the Syntaxis as a starting point. This ignores the fact that the 48 constellations predate Hyginus, Ptolemy and Hipparchus by centuries, and in fact most of them were described by Eudoxus in his Phaenomena in the 4th century BCE.

During the medieval period in Europe, interest grew in the stories of Greek mythology, and the Poeticon astronomicon was rediscovered. The illustration at right is from a hand-written copy that was translated into Latin in Italy in AD 1475. It shows the obsolete constellation Argo Navis (the name ‘Argo’ can be seen in the upper-right-hand corner). The engraver has depicted a stylised fifteenth century ship of his own time, not one of Hyginus’ time. The star positions are fanciful.           

The Poeticon astronomicon was not formally published using the movable-type printing technology invented by Johannes Gutenberg until 1482 by Erhard Ratdolt in Venice. This edition carried the full title Clarissimi uiri Hyginii Poeticon astronomicon opus utilissimum. Ratdolt commissioned a series of woodcuts depicting the constellations to accompany Hyginus’ text.

As with many other star atlases that would follow it, the positions of various stars were indicated overlaid on the image of each constellation. Also available as hand-coloured copies, Ratdolt’s version was quite popular and was re-issued a number of times. Unfortunately, the relative positions of the stars in the woodcuts bear little resemblance to the descriptions given by Hyginus in the text or the actual positions of the stars in the sky. Though this approach was typical at the time, it would soon end, as scholars and sailors demanded more accurate star maps and constellation pictures.

The northern constellation of Cassiopeia in the Ratdolt edition is shown at left.

As a result of the inaccuracy of the depicted star positions and the fact that the constellations are not shown within any context (such as neighbouring or adjoining constellations, the celestial equator, poles or ecliptic), the Poeticon astronomicon is not particularly useful as a guide to the night sky. However, the book is of historic value as the illustrations commissioned by Ratdolt served as patterns for future sky atlas renderings of the constellation figures and pictures. Even more significant, along with the writings of Eratosthenes, the text is an important source, and occasionally the only source, for some of the more obscure Greek myths..

 



The Triesnecker - Hyginus area is shown by the yellow rectangle.

 

Click  here  for the  Lunar Features of the Month Archive.




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.

 

May 1:            Moon 3.8º north of Jupiter at 06:54hrs
May 5:            Moon 1.9º north of Saturn at 07:19 hrs
May 5:            Moon 1º north of the star Pi Sagittarii (mv= 2.88) at 19:53 hrs
May 6:            Moon 1.6º north of Pluto at 08:16 hrs
May 6:            Moon 3.3º north of Mars at 16:20 hrs
May 8:            Moon 1.6º north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 23:09 hrs
May 9:            Jupiter at opposition at 10:21 hrs (diameter = 44.7")
May 10:          Moon 1.6º south of Neptune at 20:02 hrs
May 13:          Mercury 2.2º south of Uranus at 20:57 hrs
May 14:          Moon 4.3º south of Uranus at 02:35 hrs
May 14:          Moon 2.2º south of Mercury at 03:11 hrs
May 16:          Venus at perihelion at 11:19 hrs (diameter = 12.2")
May 16:          Limb of Moon 1.1º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 23:40 hrs
May 18:          Limb of Moon 51 arcminutes south of the star Zeta Tauri (mv= 2.97) at 00:14 hrs
May 18:          Moon 4.7º south of Venus at 3:32 hrs
May 18:          Moon 1.2º south of the star Propus (Eta Geminorum, mv= 3.31) at 13:37 hrs
May 18:          Moon 1.3º south of the star Mu Geminorum (mv= 2.87) at 18:06 hrs
May 22:          Moon 2º north of the star Regulus (Alpha Leonis, mv= 1.36) at 11:09 hrs
May 23:          Venus 2.6º north of the star Propus (Eta Geminorum, mv= 3.31) at 13:37 hrs
May 24:          Venus 2.5º north of the star Mu Geminorum (mv= 2.87) at 15:32 hrs
May 28:          Moon 4.1º north of Jupiter at 07:25 hrs
May 28:          Venus 15 arcminutes south of the star Mebsuta (Epsilon Geminorum, mv= 3.06) at 12:25 hrs

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 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

  

 

The Planets for this month:   

 

Mercury:   On April 2, Mercury passed through inferior conjunction (between us and the Sun), and this month will be a pre-dawn object, rising in the east before the Sun. On May 1 it will be visible a little north of due east at 5 am, when it will be in the constellation Pisces. Mercury was at its maximum angular distance from the Sun (about one and a half handspans) on April 30, and during May its angular distance from the Sun will decrease, as it heads for superior conjunction (on the far side of the Sun)  on June 6. The innermost planet will become an easy object in the western twilight sky in late July and August. The thin crescent Moon will be just to the right of Mercury on the morning of May 14.

 

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. It is now visible as a so-called 'evening star'. On May 1, Venus will be a little over a handspan east of the Sun, low in the west-north-western sky as darkness falls, and very bright.  It will appear in a small telescope as a tiny 'Gibbous Moon' with a magnitude of -3.9 and a phase of 88%. On that date, Venus will be just underneath the Hyades star cluster in the constellation Taurus. Between May 2 and May 4, Venus will pass by that cluster. It will cross into Gemini on May 20. Venus will be near the waxing crescent Moon on the evenings of May 17 and 18.

(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:   Having passed through conjunction with the Sun on July 27, the red planet is becoming easier to observe this month as an early morning object, rising at about 10 pm in the constellation Sagittarius on May 1. It will shine at magnitude -0.4, will have a diameter of 11 arcseconds, and be east of the Sagittarius 'Teapot'. Before midnight on May 14, Mars will be less than 20 arcminutes south-south-east of the globular cluster M75, which is 6.8 arcminutes across. 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, and it will cross into Capricornus on May 15. Its eastward drift will slow down and stop on June 27, when it will begin its retrograde loop. On that date it will head back the way it came, heading towards opposition on July 27. It will cross back into Sagittarius on August 24.

The change its its appearance will be slow this month, but during June and July Mars will become markedly bigger and brighter from week to week, so that at opposition it will be nearly twice as bright as Jupiter.

This will be a very favourable opposition, as Mars will appear bigger (24.2 arcseconds in diameter) and brighter (magnitude -2.8) 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 midnight on May 6, 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 night sky well before midnight, in the constellation of  Libra, the Scales. It spends the month below the star Zuben Elgenubi, which is the brightest star in Libra. At mid-month Jupiter will be seen rising in the east-south-east as darkness falls. It will reach opposition on May 9. The almost Full Moon will rise just to the left of Jupiter soon after sunset on May 27.

       

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 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 at opposition, May 9, 2018

     

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:   The ringed planet was in western quadrature on March 30 (rising at midnight), and is now a late evening object. On May 1 it rises soon after 9 pm above the east-south-eastern horizon, and the red planet Mars follows it one and a half hours later. These two planets are brighter than any of the stars in the vicinity. Saturn will be 1.7 degrees north of the bright globular cluster M22, and during May its retrograde loop will take it west against the background stars, passing by the cluster on May 14. Saturn will remain in the constellation Sagittarius all year. The waning gibbous Moon will be close to Saturn on May 4. Saturn will be in the vicinity of the Trifid Nebula (M20) and the Lagoon nebula (M8) in September.

 

Left: Saturn showing the Rings when edge-on.    Right: Over-exposed Saturn surrounded by its satellites Rhea, Enceladus, Dione, Tethys and Titan - February 23/24, 2009.
Saturn with its Rings wide open on July 2, 2017. The shadow of its globe can just be seen on the far side of the Ring system. There are three main concentric rings: Ring A is the outermost, and is separated from the brighter Ring B by a dark gap known as the Cassini Division, which is 4800 kilometres wide, enough to drop Australia through. Ring A also has a gap inside it, but it is much thinner. Called the 'Encke Gap', it is only 325 kilometres wide and can be seen in the image above.The innermost parts of Ring B are not as bright as its outermost parts. Inside Ring B is the faint Ring C, almost invisible but noticeable where it passes in front of the bright planet as a dusky band. Spacecraft visiting Saturn have shown that there are at least four more Rings, too faint and tenuous to be observable from Earth, and some Ringlets. Some of these extend from the inner edge of Ring C to Saturn's cloudtops. The Rings are not solid, but are made up of countless small particles, 99.9% water ice with some rocky material, all orbiting Saturn at different distances and speeds. The bulk of the particles range in size from dust grains to car-sized chunks. At bottom centre, the southern hemisphere of the planet can be seen showing through the gap of the Cassini Division. The ring system extends from 7000 to 80 000 kilometres above Saturn's equator, but its thickness varies from only 10 metres to 1 kilometre. The globe of Saturn has a diameter at its equator of 120 536 kilometres. Being made up of 96% hydrogen and 3% helium, it is a gas giant, although it has a small, rocky core. There are numerous cloud bands visible.

The photograph above was taken when Saturn was close to opposition, with the Earth between Saturn and the Sun. At that time, the shadow of Saturn's globe upon the Ring system was directly behind the planet and hardly visible. The photograph below was taken on September 18, 2017, when Saturn was near eastern quadrature. At such a time, the angle from the Sun to Saturn and back to the Earth is near its maximum, making the shadow fall at an angle across the Rings as seen from Earth. It may be seen falling across the far side of the Ring to the left side of the globe. The dark hexagonal atmospheric feature located at Saturn's North Pole is visible.

 

Uranus:  This ice giant planet is not observable this month, as it reached conjunction with the Sun on April 18. When it is away from the Sun, Uranus shines at about magnitude 5.8, so a pair of binoculars or a small telescope is required to observe it. It is currently in the constellation Pisces, near the south-west corner of Aries. The waning crescent Moon will be in the vicinity of Uranus in the early hours of May 14.

 

Neptune:   The icy blue planet is an early morning object this month, as it reached conjunction with the Sun on March 4. In mid-May it will be nearly 70 degrees from the Sun, rising a little after 1 am. The waning crescent Moon will be just below and to the right of Neptune in the early hours of May 11.

Neptune, photographed from Nambour on October 31, 2008


Pluto:   The erstwhile ninth and most distant planet is a late evening object this month, as it was in conjunction with the Sun on January 9. It rises in mid-May at about 9 pm. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth that of Neptune. Located just east of the 'Teaspoon' which is north-east of the Sagittarius 'Teapot', it is a faint 14.1 magnitude object near the centre of Sagittarius. A telescope with an aperture of 25 cm or more is necessary to observe Pluto. On May 1, Pluto and Mars are less than 2.7 degrees apart, but this distance will increase rapidly during the month until speedy Mars is over 14.4 degrees from Pluto on May 31.

 

  

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:

Alpha Scorpids               May 4                         Waxing gibbous Moon, 58% sunlit                      ZHR = 10
                                        Radiant:  Near the star Antares.

Eta Aquarids                   May 5-6                     Waxing gibbous Moon, 70% sunlit                      ZHR = 60 in Southern Hemisphere
                                        Radiant: Near the boundary between Aquarius and Pegasus.   Associated with Comet Halley. 


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.


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.

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:



Comet Lulin

This comet, (C/2007 N3), discovered in 2007 at Lulin Observatory by a collaborative team of Taiwanese and Chinese astronomers, is now in the outer Solar System, and has faded below magnitude 15.

Comet Lulin at 11:25 pm on February 28, 2009, in Leo. The brightest star is Nu Leonis, magnitude 5.26.

 

The LINEARrobotic telescope operated by Lincoln Near Earth Asteroid Research is used to photograph the night skies, searching for asteroids which may be on a collision course with Earth. It has also proved very successful in discovering comets, all of which are named ‘Comet LINEAR’ after the centre's initials. This name is followed by further identifying letters and numbers. Generally though, comets are named after their discoverer, or joint discoverers. There are a number of other comet and near-Earth asteroid search programs using robotic telescopes and observatory telescopes, such as:
Catalina Sky Survey, a consortium of three co-operating surveys, one of which is the Australian Siding Springs Survey (below),
Siding Spring Survey, using the 0.5 metre Uppsala Schmidt telescope at Siding Spring Observatory, N.S.W., to search the southern skies,
LONEOS, (Lowell Observatory Near-Earth Object Search), concentrating on finding near-Earth objects which could collide with our planet,
Spacewatch, run by the Lunar and Planetary Laboratory of the University of Arizona,
Ondrejov, run by Ondrejov Observatory of the Academy of Sciences in the Czech Republic, 
Xinglong, run by Beijing Astronomical Observatory 

Nearly 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 May 1 and 6 pm on May 31. They start at Orion, which is due west. 

 

This month, Orion (see below) will be setting on the western horizon. It will not be visible in June. Canis Major (the Large Dog) is above him, with the brilliant white star Sirius (Alpha Canis Majoris) showing the Dog's heart. Sirius, the Dog Star, is the brightest star in the night sky and is about a handspan above the western horizon.

Sirius (Alpha Canis Majoris) is the brightest star in the night sky. It has been known for centuries as the Dog Star. It is a very hot A0 type star, larger than our Sun. It is bright because it is one of our nearest neighbours, being only 8.6 light years away. The four spikes are caused by the secondary mirror supports in the telescope's top end. The faintest stars on this image are of magnitude 15. To reveal the companion Sirius B, which is currently 10.4 arcseconds from its brilliant primary, the photograph below was taken with a magnification of 375x, although the atmospheric seeing conditions in the current heatwave were more turbulent. The exposure was much shorter to reduce the overpowering glare from the primary star.



 Sirius is a binary, or double star. Whereas Sirius A is a main sequence star like our Sun, only larger, hotter and brighter, its companion Sirius B is very tiny, a white dwarf star nearing the end of its life. Although small, Sirius B is very dense, having a mass about equal to the Sun's packed into a volume about the size of the Earth. In other words, a cubic centimetre of Sirius B would weigh over a tonne. Sirius B was once as bright as Sirius A, but reached the end of its lifespan on the main sequence much earlier, whereupon it swelled into a red giant. Its outer layers were blown away, revealing the incandescent core as a white dwarf. All thermonuclear reactions ended, and no fusion reactions have been taking place on Sirius B for many millions of years. Over time it will radiate its heat away into space, becoming a black dwarf, dead and cold. Sirius B is 63000 times fainter than Sirius A. Sirius B is seen at position angle 62º from Sirius A (roughly east-north-east, north is at the top), in the photograph above which was taken at Nambour on January 31, 2017.  That date is exactly 155 years after Alvan Graham Clark discovered Sirius B in 1862 with a brand new 18.5 inch (47 cm) telescope made by his father, which was the largest refractor existing at the time.


Above the Dog is the constellation, Puppis, the Stern (of the ship, Argo).

Approaching the north-western horizon is the constellation of Gemini, the Twins. The two twin stars at its north-eastern end, Pollux and Castor, are very distinctive, Pollux being brighter than Castor. Both of these stars will have set by 9.30 pm on May 1. 

A handspan above and to the left of Pollux and Castor, is the first magnitude star Procyon, which is the brightest star in the constellation Canis Minor (the Small Dog).

High in the north is another zodiacal constellation, Leo, the Lion. The bright star Regulus (Alpha Leonis) marks the Lion’s heart, and is on the left-hand side of the constellation, in the north-north-west. Denebola, the star marking the root of the lion's tail, is approaching culmination.

Between Gemini and Leo 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.

Skimming the northern horizon is the constellation of Ursa Major, the Great Bear. Known in the northern hemisphere as the 'Big Dipper' or 'The Plough', it always appears to us upside-down. We only see Ursa Major at this time of year, and it is always very low in the north, and only partially visible. It can never be seen from the southern states of Australia. The further north an observer goes, the higher Ursa Major will appear above the northern horizon. If the observer travels to Europe or North America, the Great Bear will always be seen in the night sky, circling the Pole Star, Polaris, as it is circumpolar from those latitudes.

High in the north-east is a particularly beautiful orange star with a fine name: Arcturus, meaning 'the follower of the Bear'. This is the third brightest star (after Sirius and Canopus). It is a K2 star of magnitude -0.06, and lies at a distance of 36 light years. It is the brightest star in the constellation Boötes the Herdsman, and therefore has the alternative name of Alpha Boötis.

Just appearing above the horizon, slightly to the east of Boötes, is a circle of stars called Corona Borealis, the Northern Crown. The brightest star in the crown is called Alphecca, and it shines at magnitude 2.3.

Between Leo and Arcturus may be seen a large Y-shaped cluster of faint stars. This is Coma Berenices, the Hair of Berenice. Its chief claim to fame is that it is near the northern galactic window (see below), and a small telescope can detect dozens of galaxies in this area. Large telescopes equipped with sensitive cameras can detect millions of galaxies in this part of the sky.

About 70 degrees above the eastern horizon 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 makes it the sixteenth brightest star, and its colour is blue-white.

Above Spica and almost directly overhead is the constellation Corvus the Crow, shaped like a small quadrilateral of magnitude 3 stars. 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 above the eastern horizon. 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 25 degrees north-west of the zenith. 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).

 

The planetary nebula NGC 3242

 

Nearly a handspan above the eastern horizon will be seen a very bright object, brighter than anything else in the evening sky apart from the Moon. This is the giant planet Jupiter, which is three times brighter than even the brightest star, Sirius. Jupiter is this month located in the faint constellation Libra, the Scales, east of the star Zubenelgenubi (below it as it rises), and is brighter than anything else in this evening's sky apart from the Moon. Jupiter will reach opposition on May 9. 'Opposition' means that it is opposite the Sun in the sky - as the Sun sets in the west, the planet rises in the east. At opposition the Earth and the planet are at their closest, so the planet appears as its largest and brightest from our vantage point on Earth.

Just above the east-south-eastern horizon is Scorpius, the Scorpion. This famous zodiacal constellation is like a large reclining 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 object in Scorpius is the star Antares (Alpha Scorpii, mv= 1.05)

East of Scorpius, or underneath it tonight as we see it from Australia, is the bright constellation Sagittarius, the Archer. This year it is host to the ringed planet Saturn, which will come into view above the theoretical horizon at 8 pm in mid-May. Saturn is brighter than the surrounding stars of Sagittarius, but is fainter than Jupiter. An hour and a half after Saturn rises, the red planet Mars will come into view above the east-south-eastern horizon, about 9:35 pm in mid-May. At that time Mars and Saturn will be a little more than a handspan apart, Mars being twice as bright as Saturn but not as bright as Jupiter. On May 15 Mars will cross into neighbouring Capricornus. These three planets will provide a brilliant display for the winter months.

Close to the eastern horizon, just to the left of Scorpius, another fainter constellation, Ophiuchus, the Serpent Bearer, is nearly completely risen. High in the south-south-east, Crux (Southern Cross) is at an angle of about sixty degrees. It will be vertical at 9:15 pm in mid-May, and 7:15 pm in mid-June.

Close by 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

 

The two Pointers Alpha and Beta Centauri lie below Crux and to the left. Crux will have rotated clockwise to a vertical position by 9.00 pm at mid-month. Surrounding Crux on three sides is the large constellation Centaurus, its two brightest stars being the Pointers of the Southern Cross, brilliant Alpha and Beta Centauri. Beta is the one nearer to Crux.

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.

 

Slightly to the right and below Crux is a small, fainter quadrilateral of stars, Musca, the Fly. Out of all the 88 constellations, it is the only insect. Below Alpha Centauri is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern Triangle. It is well above the south-south-eastern horizon.

Between Crux and Sirius is a very large area of sky filled with interesting objects. This was once the constellation Argo Navis, 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).

Two handspans south of Sirius is the second brightest star in the night sky, Canopus (Alpha Carinae). Although appearing 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).

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 star in its centre, Eta Carinae itself, is an eruptive variable star called a recurrent nova.

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

 

Extremely close to the south-south-western horizon and soon to set is Achernar, Alpha Eridani. It is the brightest star in Eridanus the River, which winds its way with faint stars from Achernar around the south-western horizon to Cursa, a mv= 2.9 star close to brilliant Rigel in Orion. At magnitude 0.49, Achernar is the ninth brightest star.

High in the south-south-west, about 25 degrees above the horizon, the Large Magellanic Cloud (LMC) is faintly visible as a diffuse glowing patch. It is about a handspan south of Canopus. About a handspan below and to the left of the LMC is the Small Magellanic Cloud (SMC), a smaller glowing patch, not far above the 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 26.6 degrees above the horizon's due south point. Objects in the sky that never set are called 'circumpolar'. The LMC and SMC are in actual fact nearby dwarf galaxies and are described below.

The line of the ecliptic along which the Sun, Moon and planets travel passes through the following constellations this month: Gemini, Cancer, Leo, Virgo, Libra, Scorpius and 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 .

 

 

The Season of the Lion

 

We see Leo the Lion upside-down from the Southern Hemisphere. Its brightest star is Regulus, which means 'the King star'. 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. The star Denebola, a handspan east of Regulus, marks the tip of the lion's tail.
 

 

About four degrees to the right and below Eta Leonis is a beautiful double star, Algieba or Gamma Leonis. With a total magnitude of 2.61, the two stars are only 4.3 arcseconds apart, and may be distinguished with a small telescope. Both are orange in colour.

There are also numerous galaxies in this area of the sky. On one of Leo's back legs, the three bright galaxies M65, M66 and NGC 3628 can be viewed together in the same low-power telescopic field.

Between Leo and the northern horizon is a faint grouping of three 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 Hunter and his Dogs

 

Two of the most spectacular constellations in the sky may be seen low in the western sky as soon as darkness falls. These are Orion the Hunter, and his large dog, Canis Major.

Orion:

This is one of the most easily recognised constellations, as it really does give a very good impression of a human figure. From the northern hemisphere he appears to stand upright when he is high in the sky, but from our location ‘down under’ he appears lying down when rising and setting, and upside down when high in the sky.

Orion is quite a symmetrical constellation, with the three similar stars in his Belt at its centre and the two shoulder stars off to the north and the two knee stars to the south. It is quite a large star group, the Hunter being over twenty degrees (a little more than a handspan) tall.

Orion straddles the celestial equator, midway between the south celestial pole and its northern equivalent. This means that the centre of the constellation, the three stars known as Orion's Belt, rises due east and sets due west. 

The star Mintaka is actually less than 18 arcminutes south of the celestial equator.

At the beginning of May he is close to the horizon. The central part of Orion, popularly called 'The Saucepan', is very easy to recognise and is due west tonight.

Orion has two bright stars marking his shoulders, the red supergiant Betelgeuse and blue-white Bellatrix. A little north of a line joining these stars is a tiny triangle of stars marking Orion’s head. The three stars forming his Belt are, from top to bottom, Alnitak, Alnilam and Mintaka. These three stars are related, and all lie at a distance of 1300 light years. They are members of a group of hot blue-white stars called the Orion Association.

The red supergiant star Betelgeuse

 

To the south of the Belt, at a distance of about one Belt-length, we see another faint group of stars in a line, fainter and closer together than those in the Belt. This is the Sword of Orion. Orion’s two feet are marked by the hot, blue-white stars, brilliant Rigel and fainter Saiph. Both of these stars are also members of the Orion Association.

The Saucepan, with Belt at right, M42 at upper left.

 

The stars forming the Belt and Sword are popularly known in Australia as ‘The Saucepan’, with the Sword forming the Saucepan’s handle. Tonight this asterism appears right-side up, as in the photographs above. The faint, fuzzy star in the centre of the Sword, or the Saucepan's handle, is a great gas cloud or nebula where stars are being created. It is called the ‘Great Nebula in Orion’ or ‘M42’ (number 42 in Messier’s list of nebulae). Photographs of it appears below:

The Sword of Orion, with the Great Nebula, M42, at centre

The central section of the Great Nebula in Orion.

New stars are forming in the nebula. At the brightest spot is a famous multiple star system, the Trapezium, illustrated below.

 Canis Major:

 

To the upper left of Orion as twilight ends, a brilliant white star will be seen about one handspan away. This is Sirius, or Alpha Canis Majoris, and it is the brightest star in the night sky with a visual magnitude of -1.43. It marks the heart of the hunter's dog, and has been known for centuries as the Dog Star. As he rises, the dog is on his back with his front foot in the air. The star at the end of this foot is called Mirzam. It is also known as Beta Canis Majoris, which tells us that it is the second-brightest star in the constellation. Mirzam is about one-third of a handspan below Sirius.

The hindquarters of the Dog are indicated by a large right-angled triangle of stars located to the upper left of Sirius and tilted. The end of his tail is the upper left corner of the triangle, about one handspan south (to the upper left) of Sirius. It is marked by the star Aludra (Eta Canis Majoris).

Both Sirius and Rigel are bright white stars and each has a tiny, faint white companion. Whereas a small telescope can reveal the companion to Rigel quite easily, the companion to Sirius the Dog Star, (called ‘the Pup’), can only be observed by using a powerful telescope with excellent optics, as it is very close to brilliant Sirius and is usually lost in the glare (see above).

 Canis Minor:

At the onset of darkness, this small constellation is about 45 degrees (about two handspans) above the northern horizon. It contains only two main stars, the brighter of which is Procyon (Alpha Canis Minoris). This yellow-white star of mv= 0.5 forms one corner of a large equilateral triangle, the other two corners being the red Betelgeuse and white Sirius. Beta Canis Minoris is also known as Gomeisa, a blue-white star of mv= 3.1.

 

 

 

Some fainter constellations

Between the two Dogs is the constellation Monoceros the Unicorn, undistinguished except for the presence of the remarkable Rosette Nebula. To the left of Orion is a small constellation, Lepus the Hare. Between Lepus and the star Canopus is the star group Columba the Dove. Between Corvus and the star Regulus are two faint constellations, Crater the Cup and Sextans the Sextant. Between the zenith and the south-western horizon are a number of small, faint constellations: above the Milky Way are Antlia and Pyxis, while Volans and Mensa are below it. The LMC lies in the constellation Dorado, and the South Celestial Pole is in the very faint constellation Octans.

 

 

 

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 (the red star Gacrux) down through its base (the star Acrux) and continue straight on towards the south for another four Cross lengths. This will locate the approximate spot. There is no bright star to mark the Pole, whereas in the northern hemisphere they have Polaris (the Pole Star) to mark fairly closely the North Celestial Pole.

Interesting photographs of this area can be taken by using a camera on time exposure. Set the camera on a tripod pointing due south, and open the shutter for thirty minutes or more. The stars will move during the exposure, being recorded on the film as short arcs of a circle. The arcs will be different colours, like the stars are. All the arcs will have a common centre of curvature, which is the south celestial pole.

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, 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 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.

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 readily observable.

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 Omega Centauri

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 NGC 104, commonly known as 47 Tucanae. Some regard this cluster as being more spectacular than Omega Centauri, as it is more compact, and the faint stars twinkling in its core are very beautiful. This month, 47 Tucanae is low in the south-south-west, and not clearly visible. By 9 pm Omega Centauri is high enough for detailed viewing.
 

Globular Cluster NGC 104 in Tucana

The globular cluster NGC 6752 in the constellation Pavo.


Observers aiming their telescopes towards the SMC generally also look at the nearby 47 Tucanae, but there is another globular cluster nearby which is also worth a visit. This is NGC 362, which appears to lie above 47 Tucanae as we see it in mid-evening this month. It is less than half as bright as the other globular, but this is because it is more than twice as far away. Its distance is 12.6 kiloparsecs or 41 000 light years, so it is about one-fifth of the way from our galaxy to the SMC. Both NGC 104 and NGC 362 are always above the horizon for all parts of Australia south of the Tropic of Capricorn.

*     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

High in the south-south-west, below and to the left of Canopus, two large 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 and above the SMC, and is noticeably larger. 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 inter-galactic neighbours. 

 

 

 

Why are some constellations bright, while others are faint ?

The Milky Way is a barred spiral galaxy some 100000 – 120000 light-years in diameter which contains 100 – 400 billion stars. It may contain at least as many planets as well. Our galaxy is shaped like a flattened disc with a central bulge. The Solar System is located within the disc, about 27000 light-years from the Galactic Centre, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. When we look along the plane of the galaxy, either in towards the centre or out towards the edge, we are looking along the disc through the teeming hordes of stars, clusters, dust clouds and nebulae. In the sky, the galactic plane gives the appearance which we call the Milky Way, a brighter band of light crossing the sky. This part of the sky is very interesting to observe with binoculars or telescope. The brightest and most spectacular constellations, such as Crux, Canis Major, Orion and Scorpius are located close to the Milky Way.

If we look at ninety degrees to the plane, either straight up and out of the galaxy or straight down, we are looking through comparatively few stars and gas clouds and so can see out into deep space. These are the directions of the north and south galactic poles, and because we have a clear view in these directions to distant galaxies, these parts of the sky are called the intergalactic windows. The southern window is in the constellation Sculptor, not far from the star Fomalhaut. This window is below the horizon in the early evenings this month. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It begins to rise in the east-north-east at sunset at mid-month, and is well placed for viewing at 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 out across the millions of light years of space to thousands of distant galaxies. 

 

 

 

 

Astronomy Picture of the Day:

Click here to access a new spectacular picture every day - this link will also provide you with access to a wonderful library of astronomical photographs from telescopes, spacecraft and manned lunar missions.

 

Virtual Moon freeware

Study the Moon in close-up, spin it around to see the far side, find the names and physical attributes of craters, seas, ranges and other features, by clicking  here.

 

 

Calsky software

Check out where the planets and their moons are, as well as most other sky objects, by clicking  here.


 

 

Stellarium freeware

New version. Check out where the stars and constellations are, as well as most other sky objects, by clicking  here.

 

 

Observatory Home Page and Index