May  2017

Updated:   26 May 2017

 

 

Welcome to the night skies of Autumn, featuring Orion, Canis Major, Gemini, Leo, Carina, Crux, Scorpius, Jupiter and Saturn 

 

Note:  Some parts of this webpage may be formatted incorrectly by older browsers.

 

The new 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 covers an angle of approximately 18 - 20 degrees.

mv = visual magnitude or brightness. Magnitude 1 stars are very bright, magnitude 2 less so, and magnitude 6 stars are so faint that the unaided eye can only just detect them under good, dark conditions. Binoculars will allow us to see down to magnitude 8, and the Observatory telescope can reach visual magnitude 17 or 22 photographically. The world's biggest telescopes have detected stars and galaxies as faint as magnitude 30. The sixteen very brightest stars are assigned magnitudes of 0 or even -1. The brightest star, Sirius, has a magnitude of -1.44. Jupiter can reach -2.4, and Venus can be more than 6 times brighter at magnitude -4.7, bright enough to cast shadows. The Full Moon can reach magnitude -12 and the overhead Sun is magnitude -26.5. Each magnitude step is 2.51 times brighter or fainter than the next one, i.e. a magnitude 3.0 star is 2.51 times brighter than a magnitude 4.0. Magnitude 1.0 stars are exactly 100 times brighter than magnitude 6.0 (5 steps each of 2.51 times, 2.51x2.51x2.51x2.51x2.51 = 2.515 = 100).

 

The Four Minute Rule:   

How long does it take the Earth to complete one rotation? No, it's not 24 hours - that is the time taken for the Sun to cross the meridian on successive days. (The meridian is an imaginary semicircular line running from the due south point on the horizon and arching overhead through the zenith, and coming down to the horizon again at its due north point.) This 24 hours is a little longer than one complete rotation, as the curve in the Earth's orbit means that it needs to turn a fraction more (~1 degree of angle) in order for the Sun to cross the meridian again. It is called a 'solar day'. The stars, clusters, nebulae and galaxies are so distant that most appear to have fixed positions in the night sky on a human time-scale, and for a star to return to the same point in the sky relative to a fixed observer takes 23 hours 56 minutes 4.0916 seconds. This is the time taken for the Earth to complete exactly one rotation, and is called a 'sidereal day'.

As our clocks and lives are organised to run on solar days of 24 hours, and the stars circulate in 23 hours 56 minutes approximately, there is a four minute difference between the movement of the Sun and the movement of the stars. This causes the following phenomena:

    1.    The Sun slowly moves in the sky relative to the stars by four minutes of time or one degree of angle per day. Over the course of a year it moves ~4 minutes X 365 days = 24 hours, and ~1 degree X 365 = 360 degrees or a complete circle. Together, both these facts mean that after the course of a year the Sun returns to exactly the same position relative to the stars, ready for the whole process to begin again.

    2.    For a given clock time, say 8:00 pm, the stars on consecutive evenings are ~4 minutes or ~1 degree further on than they were the previous night. This means that the stars, as well as their nightly movement caused by the Earth's rotation, also drift further west for a given time as the weeks pass. The stars of autumn, such as Orion are lost below the western horizon by mid-June, and new constellations, such as Sagittarius, have appeared in the east.  The stars change with the seasons, and after a year, they are all back where they started, thanks to the Earth's having completed a revolution of the Sun and returned to its theoretical starting point.

We can therefore say that the star patterns we see in the sky at 11:00 pm tonight will be identical to those we see at 10:32 pm this day next week (4 minutes X 7 = 28 minutes earlier), and will be identical to those of 9:00 pm this date next month or 7:00 pm the month after. All the above also includes the Moon and planets, but their movements are made more complicated, for as well as the Four Minute Drift  with the stars, they also drift at different rates against the starry background, the closest ones drifting the fastest (such as the Moon or Venus), and the most distant ones (such as Saturn or Neptune) moving the slowest.

 

 

 Solar System

 

Sun:   The Sun begins the month in the constellation of Aries, the Ram. It leaves Aries and passes into Taurus, the Bull on May 14.   

 

 

Moon Phases:  Lunations (Brown series):  #1167, 1168 

 

First Quarter:        May 03          12:48 hrs          diameter = 31.6'
Full Moon:             May 11          07:44 hrs          diameter = 29.5'
Last Quarter:        May 19          10:34 hrs          diameter = 30.7' 
New Moon:            May 26          05:45 hrs          diameter = 33.4'

First Quarter:        June 01         22:43 hrs          diameter = 31.0'
Full Moon:             June 09         23:11 hrs          diameter = 29.4'         
Last Quarter:        June 17         21:34 hrs          diameter = 31.3' 
New Moon:           June 24          12:31 hrs         diameter = 33.3'  

       

 

 

Lunar Orbital Elements:



May 04        Moon at ascending node at 20:45 hrs, diameter = 31.1'
May 13:          Moon at apogee (406 216 km) at 05:33 hrs, diameter = 29.4'
May 19:          Moon at descending node at 11:27 hrs, diameter = 30.7'
May 26
:          Moon at perigee (357 227 km) at 11:02 hrs, diameter = 33.4'
May 31        Moon at ascending node at 21:59 hrs, diameter = 31.5'

June 09:         Moon at apogee (406 372 km) at 08:28 hrs, diameter = 29.4'
June 15:         Moon at descending node at 12:37 hrs, diameter = 30.4'
June 23:         Moon at perigee (357 956 km) at 20:53 hrs, diameter = 33.4'
June 28:         Moon at ascending node at 02:26 hrs, diameter = 32.1'


Moon at 8 days after New, as on May 04

Moon at 9 days after New, as on May 05

The two photographs above show the Mare Imbrium area in the Moon's northern hemisphere. They were taken a day apart, just after First Quarter. Mare Imbrium (the Sea of Rains) is a large lava flow caused by the Imbrium Event - a cataclysmic collision of an asteroid with the Moon many millions of years ago. A comparison of the two photographs will show how the appearance of lunar features changes with the angle of the Sun. 

In the first photograph, Mare Imbrium (left) is separated from Mare Serenitatis (right) by two ranges of mountains, the Alps to the north and the Apennines to the south. Two large craters at upper right are Aristoteles and Eudoxus. The straight Alpine Valley may be seen cutting through the Alps. Mt Piton (height 2000 metres) is visible as a bright spot with a shadow, due south of the southern end of the Alpine Valley. Archimedes is the large crater at left. It is a walled plain 80 kilometres in diameter with a flat floor. To its right are two bowl-shaped craters, Aristillus and Autolycus.  These craters are all formed by impact with large meteors. Apollo 15 landed close by the Apennines, in a small enclosed area to the right and below Archimedes, on the picture's central vertical axis.

In the second photograph, the sunrise line (called the 'terminator') has moved to the left, revealing a large walled plain in the Alps, known as Plato. South of Plato, an isolated mountain protruding through the lava flow is called Mt Pico. Ripples in the lava, called 'wrinkle ridges', are visible. The crater at lower left is Timocharis, 42 kilometres in diameter.

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, 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 !<

 

 

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. This month we will describe the craters Aristarchus and Herodotus, and the unusual Schröter's Valley.


This area was photographed from Starfield Observatory, Nambour on September 14, 2016. East (where the Sun is rising) is to the right, north is to the top.
Aristarchus is the bright crater at right, Herodotus is to its left.
Schröter's Valley begins at a small crater in the highlands between Aristarchus and Herodotus, widens out and then narrows again heading north before zig-zagging to the north-west and then turning to the south-west, where it narrows and peters out.  It has been described as like a snake, in particular as a cobra, and its widest area near its starting point has become known as the 'Cobra Head'. A tiny rille like a dry river valley meanders along most of its length, best seen above as it crosses the Cobra Head.

           
 
 

The ellipse shows the location of the Aristarchus - Herodotus area.


This area is in the north-western quadrant of the Moon, and is considerably fore-shortened. The brightest crater is Aristarchus, 41 kilometres in diameter and being less than 500 million years old is fairly recent. It is one of the brightest lunar craters, and can be seen in the dark part of the waxing crescent Moon, being faintly illuminated by Earthshine (sunlight reflected off the Earth, which would be nearly Full if the Moon is New). It stands on a rocky plateau, and its walls rise about 600 metres above the surrounding lava plain of the Oceanus Procellarum (Ocean of Storms). The interior walls have rugged terraces, and there is a small central mountain which is only about 350 metres high. The western interior slopes have a number of dark, radial bands running up them, two of which can be seen in small telescopes. The interior of Aristarchus is bowl-shaped, except for a small flat section at its centre, where lava has welled up and pooled.

Herodotus is a much older crater, being over three billion years old. It is 36 kilometres in diameter and its dark interior is in striking contract to that of Aristarchus. The narrow walls enclose a flat floor, which was once a lake of molten lava. The northern part of the crater wall has been struck by another meteor, leaving a 4 kilometre crater called Herodotus N. Between Aristarchus and Herodotus is an upland region, probably volcanic in origin.

Schröter's Valley begins in this upland area, in a deep grater  8 kilometres across (the Cobra Head), which adjoins a larger crater about 10 kilometres across (sometimes called the Cobra's Hood). There is a large landslip on the western side of this larger crater. The Valley heads north, then north-west, then south-west, for a length of about 165 kilometres. Its width varies between 6 and 10 kilometres, tapering down to about 500 metres at its western end. The fine rille which runs the length of the floor is 200 metres wide. Schröter's Valley is without doubt one of the strangest and interesting formations on the Moon.

 
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Aristarchus

Aristarchus of Samos (ca. 310-230 BCE), born ten years after the death of Aristotle, spent time conducting research at the new Great Library of Alexandria. He was one of the first to apply mathematical principles to astronomy, and taught that all the seven planets were spherical as was the Earth; what is more, the Earth rotated on its axis daily. Interested in improving mapping, he worked towards the concepts of latitude and longitude, which were developed further by Eratosthenses.

Aristarchus obtained reasonable estimates of the Moon’s size and distance by logical reasoning and sound application of Euclid’s geometry, after observing a lunar eclipse, thus:  A full shadow of the Earth on the Moon has an apparent radius of curvature equal to the difference between the apparent radii of the Earth and the Sun as seen from the Moon. This radius can be seen to equal 0.75 degree, from which (with the solar apparent radius of 0.25 degree) we get an apparent radius of the Earth of 1 degree. This gives an Earth-Moon distance of 60 Earth radii or 384 000 km. This result can be compared with the Earth-Moon distances this month under Lunar Orbital Elements above.

Aristarchus expressed the belief that the Sun was twenty times further away than was the Moon. He also deduced that the Sun was far bigger than the Earth, and this may have led him to the conclusion that the Earth should revolve around the Sun, rather than the reverse. Developing this idea further, he proposed a true heliocentric system, the first person to do so. He taught that the Earth rotated on its axis each day, and travelled around the Sun once in a year. In this he anticipated Copernicus by nearly two millennia.

Aristarchus pictured a universe many times larger than any that had previously been imagined. The geocentric theories of his predecessors cosseted the Earth snugly inside the planets’ crystalline shells like a cosmic egg, but Aristarchus placed the Sun at the centre of a huge void, with the Moon close to Earth, the planets much farther away, and the stars at such great distances as to be almost beyond comprehension. We do not know for sure if he realised that the stars were at different distances, or if he agreed with the accepted wisdom of the time (harking back three centuries to Anaximander) that they were all located on a remote black ‘sphere of fixed stars’.

The idea of a spinning Earth was ridiculous to the geocentrists, and indeed to practically everyone. Stand outside at night, they said, and watch the stars slowly wheeling overhead. The Earth feels solid and motionless under your feet. If it were spinning, you would feel it. Also, they knew that the Earth was large, and if it rotated once in a day, then mountains, rivers and cities would be careering around at many hundreds of kilometres per hour. Gale-force winds would rake the landscapes. Houses, animals and people would be flung off and whirled away up into the sky and out of sight. Surely, common sense dictated that the Earth doesn’t move, but the objects in the sky do. The Sun rises and sets, does it not? Geocentrists also claimed that, if the Earth were truly moving, a ball thrown vertically upward would not fall back into the hand of the thrower, but would be left behind as the Earth carried the thrower away. The concept of inertia was not known at the time.

The heliocentric theory of Aristarchus was rejected by all major thinkers, with one exception, the mathematician Seleucus of Seliucia (190-150 BCE). Living near Babylon, Seleucus only heard about it 100 years after Aristarchus had proposed it. According to Plutarch, Seleucus believed the Sun-centred, revolving Earth world view was a fact, and was the first to prove it so through reasoning. We do not know what arguments he used. The world was not yet ready for a heliocentric solar system, and the Earth-centred universe would be the accepted model for another 1800 years.   


Herodotus

Herodotus (484-420 BDE) was a Greek historian in the fifth century before Christ. He was a contemporary of Philolaus and Socrates, in the generation before Plato. Cicero has called him "the Father of History", as he was the first writer to record historical events critically as a factual narration, rather than following the tradition of Homer in romanticising and embellishing the stories, as in the legend of the Odyssey. His work is the earliest Greek prose that has survived intact.


Schröter

Johann Hieronymus Schröter (1745-1816)was a wealthy German lawyer in Hanover who became interested in astronomy. In 1779 he acquired a three feet long (almost one metre) achromatic refractor with a 2¼ inch (57 mm) lens by Dollond to observe the Sun, Moon and Venus. Herschel’s discovery of Uranus in 1781 inspired Schröter to pursue amateur astronomy more seriously. He resigned his post and moved out of Hanover to the darker and clearer skies of the countryside, becoming chief magistrate and district governor of Lilienthal.

In 1786 he paid 600 Reichstaler (equivalent to six months earnings) for a 2.14 metre focal length, 16.5 cm aperture reflector by Schrader, with eyepieces allowing up to 1200 magnification. He paid another 26 Thaler for a screw-micrometer.

Soon he had become an accomplished selenographer, and in 1791 he published an important early study on the topography of the Moon entitled Selenotopographische Fragmente zur genauern Kenntniss der Mondfläche, (Fragments of the Moon’s Topography for a more accurate Knowledge of the Lunar Surface). By this time, Latin had been phased out as the language of science, and such books were being written in the common language of their authors. This book introduced the Latin word ‘crater’ (cup), and the German word ‘Rille’ (groove) to the lunar terminology.

Schröter knew his history of astronomy and named 76 new features, including the craters Alhazen, Bernoulli, Bradley, Cassini, Euler, LaCaille, Tobias Mayer, Mercator, Picard and Rømer. He named two craters Hooke and Newton, but, knowing of the bitter enmity between these two men in their lifetimes, he placed them as far apart on the Moon as he could, Hooke away up in the northern hemisphere  and Newton near the South Pole.

Also named were Mount Huygens, Mount Hadley, Mount Pico, Mont Blanc and the Leibniz Mountains. Like most astronomers, he preferred Riccioli’s names over those of Hevelius and Langrenus, and this fact influenced the official acceptance in the 20th century of most of Riccioli’s names by the International Astronomical Union. He discovered Schröter's Valley in 1787, which was named after him by later astronomers.

 

We are currently conducting a course called "Understanding the Universe". These eight-week courses, which are limited to eight participants, involve using the robotic 0.51 metre telescope seen above as a vital part of the astronomical experience. On May 4 the current participants were present when the image below was acquired as a video stream at 6:44 pm. The 1700 frames were aligned automatically, sorted and stacked by Registax 6 software, and then processed using the wavelet technique. After about five minutes the people present saw the image below. The video camera was then removed from the telescope and other areas on the Moon, such as the Straight Wall and ash volcanoes in the crater Alphonsus, were viewed through eyepieces. Persons interested in the next round of courses should contact Starfield Observatory by  email to secure a place.

 

The eastern end of Mare Imbrium (Sea of Rains). The crater at the centre of the top margin is Callippus. The largest crater in the image, with two smaller craters inside it, is Cassini. Below Cassini and above the bottom margin is Aristillus, a fine crater with a cluster of small mountains at its centre. A tiny craterlet only 3 kilometres across is on the right margin about 30% of the way up from the bottom. This is Linné, a very recent impact which is surrounded by a light-coloured halo of ejecta. The centre of the image is mostly filled with mountain massifs, which separate the Mare Imbrium in the west from the Mare Serenitatis (Sea of Serenity) in the east.

 

On Thursday, May 25 the skies were clear and seeing was above average. Good views were had of Jupiter, Saturn, the binary star Algieba, the globular cluster Omega Centauri, the spiral galaxies Messier 65 and Messier 66 in Leo, the largest star known VY Canis Majoris, and the quasi-stellar-object (QSO or quasar) 3C-273. At the conclusion of the evening, Saturn was photographed. The image may be found below in the 'Saturn' section.


 

 

Geocentric Events:

May 4:           Mercury at western stationary point at 02:24 hours  (diameter = 10.7")
May 4:           Moon occults the star Regulus (Alpha Leonis, mv=1.36) between 20:15 and 21:38 hrs
May 6:           Mercury at aphelion at 23:32 hrs  (diameter = 10.2")
May 8:           Moon 2.3º north of Jupiter at 09:10 hrs
May 14:         Moon 3.5º north of Saturn at 09:57 hrs
May 16:         Moon 2.6º north of Pluto at 07:36 hrs
May 18:         Mercury at Greatest Elongation West (25º 35') at 09:11 hrs  (diameter = 8.2")
May 19:         Saturn 19 arcminutes south of the star 58 Ophiuchi (mv= 4.86) at 14:43 hrs
May 20:         Moon occults Neptune between 16:12 and 16:41 hrs
May 22:         Moon 2º south of Venus at 23:01 hrs
May 23:         Venus 1.7º south of Uranus at 03:55 hrs
May 23:         Moon 3.1º south of Uranus at 17:49 hrs
May 24:         Limb of Moon 35 arcminutes south of Mercury at 12:59 hrs
May 26:         Moon 1.3º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 14:39 hrs
May 27:         Moon 4.6º south of Mars at 12:48 hrs

June 3:          Venus at greatest elongation west (45º 49') at 14:59 hrs  (diameter = 23.9")
June 3:          Venus 1.7º south of Uranus at 18:32 hrs
June 4:          Moon 2.7º north of Jupiter at 10:43 hrs
June 4:          Limb of Moon 43 arcminutes north of the star Porrima (Gamma Virginis, mv= 2.74) at 06:49 hrs
June 5:          Neptune at western quadrature at 02:02 hrs  (diameter = 2.2")
June 9:          Jupiter at eastern stationary point at 23:39 hrs  (diameter = 39.7")
June 10:        Moon 3.7º north of Saturn at 11:41 hrs
June 12:        Moon 2.2º north of the star Pi Sagittarii (mv= 2.88) at 08:21 hrs
June 12:        Moon 2.9º north of Pluto at 11:59 hrs
June 13:        Venus at aphelion at 10:04 hrs  (diameter = 21.5")
June 13:        Mars 1.8º north of the star Mu Geminorum (mv= 2.87) at 10:08 hrs
June 15:        Saturn at opposition at 20:00 hrs  (diameter = 18.3")
June 16:        Neptune at western stationary point at 16:55 hrs  (diameter = 2.3")
June 16:        Limb of Moon 14 arcminutes south of Neptune at 21:32 hrs
June 19:        Mercury 2.9º north of the star Zeta Tauri (mv= 2.97) at 13:28 hrs
June 19:        Mercury at perihelion at 23:08 hrs  (diameter = 5.1")
June 20:        Moon 3.7º south of Uranus at 02:41 hrs
June 20:        Mars 1.1º south of the star Mebsuta (Epsilon Geminorum, mv= 3.06) at 10:45 hrs
June 21:        Winter solstice at 14:23 hrs
June 21:        Moon 1.7º south of Venus at 07:48 hrs
June 22:        Mercury at superior conjunction at 00:05 hrs  (diameter = 5.1")
June 23:        Limb of Moon 24 arcminutes north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 00:02 hrs
June 24:        Mercury 
2.2º north of the star Mu Geminorum (mv= 2.87) at 08:47 hrs
June 24:        Moon 
5º south of Mercury at 19:43 hrs
June 25:        Moon 
4.2º south of Mars at 03:48 hrs
June 26:        Mercury
28 arcminutes south of the star Mebsuta (Epsilon Geminorum, mv= 3.06) at 12:18 hrs
June 28:        Uranus 
1º north of the star Omicron Piscium (mv=4.26) at 05:21 hrs
June 28:        Limb of Moon 22 arcminutes north of the
star Regulus (Alpha Leonis, mv=1.36) at 09:42 hrs
June 29:        Mercury 46 arcminutes north of Mars at 05:51 hrs








The Planets for this month:

 

Mercury:    On May 1, Mercury will be in the eastern pre-dawn sky, rising 72 minutes before the Sun. It will be a pre-dawn object for the whole month. It will be best seen towards the middle of the month, as it reaches Greatest Elongation West on May 18, when it will have an angular separation of 25.6 degrees from the Sun and will shine at magnitude 0.6. The best way to find it is to look about midway between brilliant Venus and the estimated position of the Sun below the horizon.  The waning crescent Moon will be seen just above Mercury on May 24.

 

Venus:  This, the brightest planet, as the famous 'evening star' dominated the western twilight sky earlier in the year, but on March 25 it passed between us and the Sun (inferior conjunction), and left the evening sky. It is now dominating the pre-dawn sky as a 'morning star', and for the next six months will be noticeable to even the most casual observer, rising in the east before the Sun. At the beginning of May the crescent shape of Venus will be visible in even the smallest telescope, as its angular diameter will be 38 arcseconds and its phase will be 26.8%, very similar to its appearance when the first photograph below was taken. By the end of May its phase will have increased to 47.6% (like a tiny half-Moon) but its diameter will decrease to 25 arcseconds as it moves further away from us. Its brightness will therefore remain constant at about magnitude -4.4. 

On the morning of May 23, the waning crescent Moon will be just below and to the right of Venus.

(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).

                           April 2017                              June 2017                         December 2017                      

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 will pass on the far side of the Sun (superior conjunction) on January 9, 2018, when it will return to the evening sky and 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.

 

Mars:   Having passed through opposition on May 22 last year, the red planet continues to shrink and fade as the speeding Earth leaves it behind. At opposition it reached magnitude -2, rivalling Jupiter in brightness, but by May 1 it has faded to 1.6 (one thirtieth as bright). In the same period, its apparent size has shrunk from 18.4 arcseconds to 4 arcseconds. It is a faint orange object low in the west as soon as twilight fades, in the constellation Taurus. At the beginning of the month it will be just below the Hyades star cluster. On May 27 it will be 2.1 degrees north of the Crab Nebula with the waxing crescent Moon close by.

Mars is now approaching the far side of its orbit, about as far away as it can get, and is very difficult to observe. It will cross into Gemini on June 5 and will reach conjunction with the Sun on July 27.

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 a spectacular evening object as it passed through opposition (directly opposite the Sun in the sky) on April 8. This month it may be easily seen in the eastern sky, being about two handspans above the horizon as darkness falls, at mid-month. It will be about one-and-a-half handspans north of the zenith at 9 pm at that time. It is in the constellation Virgo, north of the first magnitude star Spica. The gibbous Moon will be close to Jupiter as night falls on May 7 and 8.


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.

 

Saturn:   The ringed planet is a midnight object this month, as it was in conjunction with the Sun on December 10. It passed through western quadrature on March 18, when it rose at midnight. In mid-May it will be visible low in the east just before 8:30 pm. The waning gibbous Moon will be to the left of Saturn at 8:30 pm on May 13.

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 May 25, 2017. The shadow of its globe can 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.

 

Uranus:  This ice giant planet is not in a good position for viewing in early May, as it reached conjunction with the Sun on April 14 and is still involved with the solar glare. Uranus is now a  morning sky object, and shines at about magnitude 5.8, so a pair of binoculars or a small telescope is required to observe it.

 

Neptune:   The icy blue planet was in conjunction with the Sun on March 2, so will be a pre-dawn object to observe this month. Neptune is located in the constellation of Aquarius, between the magnitude 3 star Skat (Delta Aquarii) and the four-star asterism known as the Water-Jar. As it shines at about magnitude 8, a small telescope is required to observe Neptune.

Neptune, photographed from Nambour on October 31, 2008

Pluto:   The erstwhile ninth and most distant planet is a midnight object this month, as it reached conjunction with the Sun on January 7. Located inside the 'Teaspoon' which is above the Sagittarius 'Teapot', it is a faint 14.1 magnitude object near the centre of Sagittarius. Pluto will reach opposition on July 10, when it rises at sunset. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth that of Neptune.

 

  

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.

The Eta Carinae Nebula is nearly a handspan to the right of the Southern Cross at 10 pm in mid-April.

 


 

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. For most of this year, Virgo is dominated by the presence of the brilliant planet Jupiter - it is much brighter than any of Virgo's stars. It is currently near the star Spica, heading towards Leo on its retrograde loop, but will reverse direction and head eastwards again towards Libra on June 9. On November 14 it will cross into Libra.

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

 

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 for most of this month is the planet Mars, presently located just to the north-east (left and below) of the star Antares (Alpha Scorpii, mv= 1.05)

Between Scorpius and Virgo is the fainter zodiacal constellation of Libra, the Scales. On the eastern horizon, another fainter constellation, Ophiuchus, the Serpent Bearer, is nearly completely risen. Libra and Ophiuchus are completely outshone by brilliant Scorpius and, later in the night, Sagittarius, which is below the Scorpion. At present the brightest object in this part of the sky is the planet Saturn, located near the boundary between Ophiuchus and Sagittarius. At the beginning of May Saturn is at the western end of Sagittarius, but is part-way through its retrograde loop. Heading westwards, it will cross into Ophiuchus on May 22, and will reach its stationary point on August 25. Moving eastwards once again, it will cross back into Sagittarius on November 17. It will take until March 17, 2020 to pass through Sagittarius.

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 dwarf 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.

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 are about 145 globular cluster in a halo surrounding our galaxy. Many other galaxies are also surrounded by a large family of globular clusters.



*     M42:This number means that the Great Nebula in Orion is No. 42 in a list of 103 astronomical objects compiled and published in 1784 by Charles Messier. Charles was interested in the discovery of new comets, and his aim was to provide a list for observers of fuzzy nebulae and clusters which could easily be reported as comets by mistake. Messier's search for comets is now just a footnote to history, but his list of 103 objects is well known to all astronomers today, and has even been extended to 110 objects.

**    NGC 5139: This number means that Omega Centauri is No. 5139 in the New General Catalogue of Non-stellar Astronomical Objects. This catalogue was first published in 1888 by J. L. E. Dreyer under the auspices of the Royal Astronomical Society, as his New General Catalogue of Nebulae and Clusters of Stars. As larger telescopes built early in the 20th century discovered fainter objects in space, and also dark, obscuring nebulae and dust clouds, the NGC was supplemented with the addition of the Index Catalogue (IC). Many non-stellar objects in the sky have therefore NGC numbers or IC numbers. For example, the famous Horsehead Nebula in Orion is catalogued as IC 434. The NGC was revised in 1973, and lists 7840 objects. 

The recent explosion of discovery in astronomy has meant that more and more catalogues are being produced, but they tend to specialise in particular types of objects, rather than being all-encompassing, as the NGC / IC try to be. Some examples are the Planetary Nebulae Catalogue (PK) which lists 1455 nebulae, the Washington Catalogue of Double Stars (WDS) which lists 12 000 binaries, the General Catalogue of Variable Stars (GCVS) which lists 28 000 variables, and the Principal Galaxy Catalogue (PGC) which lists 73 000 galaxies. The largest modern catalogue is the Hubble Guide Star Catalogue (GSC) which was assembled to support the Hubble Space Telescope's need for guide stars when photographing sky objects. The GSC contains nearly 19 million stars brighter than magnitude 15.

 

 

 

Two close galaxies

A little more than a handspan above the south-south-western horizon, and below and to the left of Canopus, a large smudge of light may be seen. This is the Large Cloud of Magellan, and there is also a Small Cloud close to the horizon. They are known to astronomers as the LMC (Large Magellanic Cloud) and the SMC (Small Magellanic Cloud). The SMC is due south and quite low to the horizon (about ten degrees up), and the LMC is above it and to its right. The LMC is noticeably larger and brighter. They lie at a distance of 200 000 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 not well placed for viewing this month, being below the horizon until well after midnight, but when it is higher many distant galaxies can be observed in this area of the sky. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It is ideally placed for viewing in May, culminating at 8.45 pm at mid-month. The winter months are best for observing galaxies in this window, particularly those in the Virgo cluster.

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.  

 

 

 

 

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