January  2018

Updated:   12 January 2018

 

Welcome to the night skies of Summer, featuring Andromeda, Perseus, Auriga, Taurus, Gemini, Orion, Canis Major and Carina

 

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 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 Sagittarius, the Archer. It leaves Sagittarius and passes into Capricornus, the Sea-Goat on January 20.   

 

 

Total Lunar Eclipse, January 31:

There will be a total lunar eclipse visible from south-east Queensland on Wednesday, January 31 next. All aspects of the eclipse will be visible. The Full Moon will enter the Earth's penumbra at 8:49 pm when it is about 26 degrees above the east-north-eastern horizon. This phase is hardly noticeable. The eclipse proper will begin at 9:50 pm, when the Moon begins to enter the Earth's main shadow, which is called the umbra. Even the most casual observer will see a bite appearing out of the edge of the Moon. The Full Moon will gradually lose its brightness as more of it disappears into our shadow. By 10:54 pm the bright Moon will be completely immersed in the shadow, but it will still be faintly visible as a reddish disc. This is the total phase of the eclipse, and mid-eclipse occurs at 11:30 pm. After that the Moon will very slowly brighten, and totality will end at 7 minutes after midnight. Then the western edge of the Moon will begin to come out of the umbra, and the whole Moon will again be visible by 1:10 am. The Moon will still take another hour to leave the penumbra, and the eclipse will be over by 2:14 am. Before and after a lunar eclipse, the Full Moon looks brighter than normal, as the Sun, Moon and Earth are so perfectly aligned. Before and after a lunar eclipse, the Full Moon looks brighter than normal, as the Sun, Moon and Earth are so perfectly aligned.

 It is perfectly safe to watch lunar eclipses as they occur at night. Solar eclipses are the dangerous ones, for looking at the Sun without special eye protection can ruin your eyesight. Total lunar eclipses are not rare, but quite uncommon. The last one visible from Starfield Observatory occurred on April 4, 2015. If you miss this one for any reason, you will need to wait until May 26, 2021 for the next.

 

Moon Phases:  Lunations (Brown series):  #1175, 1176, 1177 

 

Full Moon:               January 02               12:24 hrs          diameter = 33.5'      Supermoon
Last Quarter:          
January 09               08:26 hrs          diameter = 30.7' 
New Moon:              
January 17               12:17 hrs          diameter = 29.5'      Lunation #1176 begins
First Quarter:           January 25               08:20 hrs          diameter = 31.6'
Full Moon:               January 31               23:27 hrs          diameter = 33.2'      Total lunar eclipse

Last Quarter:           February 08             01:55 hrs          diameter = 30.1' 
New Moon:              February 16             07:06 hrs          diameter = 30.0'     
Partial solar eclipse  (not visible from Queensland).        Lunation #1177 begins   
First Quarter:           February 23             18:09 hrs          diameter = 32.0'


'Supermoons' are Full Moons which occur when the Moon is at its closest to Earth, or perigee. This makes the Full Moon fractionally larger and brighter than normal. This happens occasionally, but is not a rare occurrence. It happened last month and will happen again this month, the Full Moon being almost 8% wider and 16% brighter than average.

 
 

Lunar Orbital Elements:



January 02:           Moon at perigee (356 584 km) at 07:36 hrs, diameter = 33.5'
January 04          Moon at ascending node at 17:52 hrs, diameter = 33.0'
January 15:           Moon at apogee (406 466 km) at 12:31 hrs, diameter = 29.4'
January 19:           Moon at descending node at 00:27 hrs, diameter = 29.7'
January 30:          
Moon at perigee (358 986 km) at 19:30 hrs, diameter = 33.3'

February 12:         Moon at apogee (405 713 km) at 00:25 hrs, diameter = 29.5'
February 15:         Moon at descending node at 07:08 hrs, diameter = 29.8'

February 28:         Moon at perigee (363 913 km) at 00:50 hrs, diameter = 32.8'
February 28:         Moon at ascending node at 15:02 hrs, diameter = 32.8'

Moon at 8 days after New, as on January 26.

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 spectacular crater, Posidonius.

 

The photograph  of Posidonius above was taken on November 24, 2017. North is to the left and east to the top.

Posidonius is a large impact crater 99 km in diameter in the Moon's northern hemisphere. It is circular in shape, but its position at latitude 32º north means that, from the Earth, it appears elliptical due to foreshortening. The level floor is heavily fractured due to tectonic uplift. There is a secondary rim  of mountains in the eastern interior.   The largest crater inside Posidonius has a diameter of 11 km and near it is a fresh 3 km crater and some smaller craterlets. There are also five volcanic domes inside Posidonius and three more outside. Of interest is the very sinuous and narrow rille which starts just inside the northern rim (at left above), and follows the rim to its north-western curve, where the rille suddenly turns towards the interior and heads south, aiming for the south-western wall. After reaching a line of hills it turns abruptly to the right and heads directly to the western wall (lower margin in the image above), and hugs the base of this wall until if finds a gap where it exits the crater. Where the wall is breached near the bottom margin, shadow reveals that the floor of Posidonius is higher than the level of Mare Serenitatis outside the wall.

 

Posidonius

During the 150 years before the Christian era, the city of Rhodes on the island of the same name (off the south-western tip of what is now Turkey) was to some extent a rival of Alexandria as a centre of literary and intellectual life. Posidonius of Rhodes (135-51 BCE) believed that the tides are caused by the Moon, but was, however, wrong about the physics. Thinking that the Moon was a combination of the elements air and fire, he attributed the tides to the heat of the Moon, hot enough to cause the water to swell but not hot enough to evaporate it. He tried to improve on the measurement of the Earth’s circumference by using the second-brightest star, Canopus.

As explained by Cleomedes, Posidonius used the elevation of Canopus to determine the difference in latitude between Rhodes and Alexandria, and used the estimated distance between the two cities to determine his result. Due to observational inaccuracies, possibly caused by atmospheric refraction (Canopus culminated only little more than one degree above the southern horizon at Rhodes), his answer was 240 000 stadia (38 400 kilometrs), not quite as good as Eratosthenes’ result.

We can also thank Cleomedes for handing down to us the story of Eratosthenes’ procedure for determining the size of the Earth by measuring noon-time shadows at widely separated sites, which was described last November and can be read near the bottom of this page.  Cleomedes repeated Eratosthenes' experiment by measuring the shadows of vertical gnomons at Alexandria and Syene during a winter solstice, achieving a difference in the incidence of solar rays of 7.2 degrees, the same as Eratosthenes.

 
 Posidonius 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.

 

January 1:            Moon 1.6º south of the star Zeta Tauri (mv= 2.97) at 10:31 hrs
January 2:            Mercury at Greatest Elongation West (22º 35') at 10:36 hrs (diameter = 6.6")
January 2:            Uranus at eastern stationary point ar 22:45 hrs
January 3:            Mars 33 arcminutes north of the star Zuben Elgenubi (Alpha Librae, mv= 2.75) at 06:34 hrs
January 4:            Earth at perihelion at 02:13 hrs
January 4:            Venus 2.9º north of the star Nunki (Sigma Sagittarii, mv= 2.04) at 22:29 hrs
January 5:            Moon 1.2º north of the star Regulus (Alpha Leonis, mv= 1.36) at 17:21 hrs
January 7:            Mars 12 arcminutes south of Jupiter at 10:58 hrs
January 7:            Venus 2.1º south of the star Pi Sagittarii (mv= 2.88) at 17:46 hrs
January 9:            Venus in superior conjunction at 06:27 hrs  (diameter = 9.7")
January 9:            Venus 1.2º south of Pluto at 19:07 hrs
January 9:            Pluto in conjunction at 19:30 hrs  (diameter = 0.1")
January 11:          Moon 4.6º north of Jupiter at 18:59 hrs
January 11:          Moon 5º north of Mars at 22:04 hrs
January 13:          Mercury 38 arcminutes south of Saturn at 14:52 hrs
January 14:          Saturn 1.2º north of the star 11 Sagittarii (mv= 4.96) at 14:56 hrs
January 15:          Uranus at eastern quadrature at 06:38 hrs  (diameter = 3.5")
January 15:          Moon 2.7º north of Saturn at 12:44hrs
January 15:          Moon 3.8º north of Mercury at 18:37 hrs
January 16:          Mercury 2º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 09:47 hrs
January 16:          Moon 2.5º north of Pluto at 21:32 hrs
January 16:          Moon 1.4º north of the star Pi Sagittarii (mv= 2.88) at 16:45 hrs
January 17:          Moon 2.9º north of Venus at 18:03 hrs
January 19:          Moon 2.4º north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 19:25 hrs
January 20:          Mercury 2.8º north of the star Nunki (Sigma Sagittarii, mv= 2.04) at 14:18 hrs
January 21:          Moon 1.2º south of Neptune at 05:28 hrs
January 23:          Mercury 2.4º south of the star Pi Sagittarii (mv= 2.88) at 07:13 hrs
January 23:          Venus at aphelion at 16:55 hrs  (diameter = 9.8")
January 24:          Moon 4.2º south of Uranus at 12:36 hrs
January 25:          Mercury 1.5º south of Pluto at 07:31 hrs
January 25:          Mercury at aphelion at 21:21 hrs  (diameter = 5.0")
January 27:          Moon 1.3º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 20:17 hrs
January 28:          Moon 1.1º south of the star Zeta Tauri (mv= 2.97) at 20:53 hrs
January 31:          Mars 2.6º north of the star Delta Scorpii (mv= 2.29) at 12:31 hrs
January 31:          Total Lunar eclipse at 23:30 hrs  (totality from 22:52 to 00:09 hrs - see above)


February 1:          Mars 22 arcminutes south of the star Graffias (Beta Scorpii, mv= 2.56) at 12:59 hrs
February 2:          Moon 1.1º north of the star Regulus (Alpha Leonis, mv= 1.36) at 05:10 hrs
February 6:          Venus 1.2º north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 07:25 hrs
February 8:          Moon 4.2º north of Jupiter at 09:29 hrs
February 9:          Moon 4.8º north of Mars at 17:48 hrs
February 11:        Jupiter at western quadrature at 09:08 hrs
February 11:        Moon 4.2º north of of Saturn at 23:16 hrs
February 12:        Moon 1.7º north of the star Pi Sagittarii (mv= 2.88) at 21:40 hrs
February 13:        Moon 2.1º north of Pluto at 03:37 hrs
February 15:        Mercury 32 arcminutes north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 03:26 hrs
February 15:        Saturn 1.9º south of the star 21 Sagittarii (mv= 4.8) at 12:29 hrs
February 16:        Moon 2.4º north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 12:03 hrs
February 16:        Moon 1.5º north of Mercury at 02:33 hrs
February 17:        Moon occults Venus between 01:21 and 02:05 hrs
February 17:        Moon 1.2º south of Neptune at 15:01 hrs
February 17:        Mercury in superior conjunction at 22:11 hrs (diameter = 4.9")
February 20:        Moon 3.7º south of Uranus at 20:17 hrs
February 21:        Neptune (mv= 8.0) 1 arcminute south of the star SAO 146431 (mv= 8.98) at 18:34 hrs (both will set at 19:01 hrs)
February 22:        Venus 32 arcminutes south of Neptune at 04:53 hrs
February 24:        Moon 1.3º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 03:12 hrs
February 25:        Moon 1.6º south of the star Zeta Tauri (mv= 2.97) at 05:26 hrs
February 25:        Mercury 26 arcminutes south of Neptune at 22:37 hrs

 

 

 

The Planets for this month:   

 

Mercury:   Mercury overtook the Earth, passing between us and the Sun on December 13. On January 1, Mercury will be in the morning sky, being about a handspan above the Sun before it rises, and quite easy to observe. It will be underneath the spectacular S-shaped constellation of Scorpius. In the first two weeks of 2018, Mercury will move into Sagittarius where Saturn is located. On January 13 and 14, these two planets will be close together in the sky, being less than a degree apart on both days. The waning crescent Moon will join them the following day, January 15. 

 

Venus:  This, the brightest planet, is now so close to the Sun that it is swamped by the glare. No attempt should be made to observe it unless special solar filters are employed. However, while a telescope may reveal it, looking so close to the Sun is extremely dangerous, as an accidental flash of sunlight through a telescope can instantaneously ruin your eyesight in that eye.  Venus will pass through superior conjunction (on the far side of the Sun) on January 9. It will not be behind the Sun, but will be 30 arcminutes (about one Sun-diameter) south of the Sun's edge or 'limb'. After that, it will return to the western twilight sky as an 'evening star', becoming readily visible in March 2018. In mid-March, Venus will be about a handspan east of the Sun, and will appear in a small telescope as a tiny 'Full Moon' with a magnitude of -3.9 and a phase of 96%.

(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 will pass on the far side of the Sun (superior conjunction) on the 9th of this month, 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.

 

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. On January 1 it will be an early morning object, rising at 1:21 am in the constellation Libra. On that date it will be about three degrees above the bright planet Jupiter, with the magnitude 2.75 star Zuben Elgenubi (Alpha Librae) between the two planets. In the first week of 2018, Mars will move closer to Jupiter, heading east, and in the hours before dawn on January 7 they will be less than 15 arcminutes apart. Later that morning, at 10:58 am, Mars will pass by Jupiter at a separation of only 12 arcminutes. Mars will continue to head east in the coming weeks, and will cross into Scorpius on January 31.

When Mars overtakes Jupiter on January 7, it will be only one fifteenth as bright as Jupiter and its angular size will be only one seventh that of its huge neighbour. As 2018 progresses, the Earth will catch up to Mars, and every week it will appear brighter and larger until we overtake it at closest approach (opposition) on July 27. The change its its appearance will be slow at first, 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. Next winter will be an excellent time for planet observing, with Mars, Jupiter and Saturn all available each evening and high overhead.

On January 12 the waning crescent Moon will be just to the left (north) of Mars and Jupiter.

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 early morning sky, in the constellation Libra, the Scales. It begins the month near the star Zuben Elgenubi, which is the brightest star in Libra. Mars is close by (see above). At mid-month Jupiter will rise in the east-south-east at about 12:30 am. The waning crescent Moon will be just to the left (north) of Jupiter and Mars on the morning of January 12.


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. 

 

Saturn:   The ringed planet was in conjunction with the Sun on December 22, and is involved in the solar glare all through January. It begins the month near the Trifid nebula, M 20, and as January progresses it will move steadily away from that spectacular object. Saturn will return to the vicinity of M 20 and the Lagoon nebula , M 8, 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 reasonably placed for viewing this month, as it reached opposition (being in the opposite direction to the Sun, rising in the east as the Sun sets in the west) on October 20. As darkness falls on January 1, Uranus will be about two handspans north-north-west of the zenith. 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 First Quarter Moon will be in the vicinity of Uranus on January 24.

 

Neptune:   The icy blue planet is an early evening object this month. It reached opposition on September 5, and at 7:30 pm on January 1 it is about two handspans above the western horizon. 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. The waxing crescent Moon will be close to Neptune on the evenings of January 20 and 21.

Neptune, photographed from Nambour on October 31, 2008


Pluto
: 
 The erstwhile ninth and most distant planet cannot be observed this month, as it will be in conjunction with the Sun on January 9. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth that of Neptun
e. 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.

 

  

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.

    

 

Planetary Alignments:

On January 7 Mars and Jupiter will have a close encounter in the sky, being only 15 arcminutes apart in the hours before dawn. On January 13 and 14, Mercury and Saturn will be less than a degree apart. By early April, Mars will have caught up to Saturn.

 

 

 

Meteor Showers:

Quadrantids                    January 4                       Waxing crescent Moon, 27% sunlit                                     ZHR = 95
                                        Radiant: Near the star Vega.  Named after the obsolete constellation Quadrans Muralis (Mural Quadrant)  

Alpha Centaurids            February 8                      Waxing gibbous Moon, Moon, 85% sunlit                                              ZHR = 10
                                        Radiant:  Near the star Alpha Centauri.


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.

January is not a good month to observe the Eta Carinae Nebula, as it is very low in the south-east in the evening. It becomes easier to observe closer to midnight and will be at its highest around 3:30 am.

 

 

 

The Stars and Constellations for this month:

 

 

These descriptions of the night sky are for 10 pm on January 1 and 8 pm on January 31. Broadly speaking, the following description starts low in the south-west and follows the horizon to the right, heading round to the east, then overhead, then south.

 

Low in the south-west, the flattened triangle of Grus is setting. It is nearly upside-down at this time. To its right, the star Fomalhaut is curving down to the west-south-western horizon, and in the west-south-west, Aquarius is about to set. A little to the north of due west, Pisces is setting.

In the north-west, the Great Square of Pegasus and Andromeda are also dipping below the horizon. These constellations contain the well-known spiral galaxies M31 (in Andromeda) and M33 (in Triangulum - see below). These large spirals are members of the Local Group of galaxies (our Milky Way is a third member), and can be easily seen with binoculars. They are the nearest galaxies that can be seen from the large observatories in the Northern Hemisphere. This month, they are best seen as soon as darkness falls, for they soon head towards the horizon.
 

The Great Spiral M33 in Triangulum.

The Great Galaxy in Andromeda, M31, photographed at Starfield Observatory with an off-the-shelf digital camera on 16 November 2007.
 

The three main stars in Andromeda are, from left to right, Alpheratz, Mirach, and Almach, and above them is the faint constellation of Triangulum, a narrow triangle of stars. M31 lies about eight degrees (one third of a handspan) below Mirach tonight, while M33 is a similar distance above Mirach.

Above Triangulum is the zodiacal constellation of Aries, now past the meridian in the north-north-west. The two main stars of Aries (from the left, Beta and Alpha Arietis, also known as Sheratan and Hamal), form a short line parallel with the horizon.  Hamal is the brighter, being a second magnitude orange star. A third star nearby is called Mesarthim. M33 lies midway between Hamal and Mirach.

Cetus, the Whale, lies a little north-west of the zenith. Though this part of the sky has no really bright stars, a little more than a handspan west of the zenith is a mv 2.2 star. This is Beta Ceti, the brightest ordinary star in the constellation Cetus, the Whale. Its common name is Diphda, and it has a yellowish-orange colour. It appears all by itself in a large area of sky deficient in bright stars. By rights, we would expect the star Menkar or Alpha Ceti to be brighter, but Menkar is actually more than half a magnitude fainter than Diphda. Menkar may be seen high in the north-east, halfway between Diphda and Aldebaran.

Cetus is a large constellation, and to the unaided eye it appears unremarkable. But it does contain a most interesting star, which even medieval people noticed. Hevelius named it Mira, the Wonderful (see below). Between Cetus and Pegasus is the zodiacal constellation of Pisces, the Fishes. Pisces is found just to the left of Aries, and this year hosts the planet Uranus.

The spectacular constellations are in the eastern half of the sky tonight. Taurus, with its two star clusters the Pleiades (or Subaru) and the Hyades, is high in the north-north-east (see below). The brightest star in Taurus is an orange star dominating the Hyades cluster, but not a member of it. This is Aldebaran, a K5 star with a visual magnitude of 0.87. The Pleiades is a small group like a question mark, and is often called the Seven Sisters, although excellent eyes are needed to detect the seventh star without optical aid. The group is also known as ‘Santa’s Sleigh’, as it appears around Christmas time. All the stars in this cluster are hot and blue. They are also the same age, as they formed as a group out of a gas cloud or nebula. There are actually more than 250 stars in the Pleiades.

The Pleiades is the small cluster at centre left, while the Hyades is the much larger grouping at centre right.

Wisps of the nebula which formed the Pleiades can be seen around the brighter stars in the cluster.

 

The Hyades cluster appears larger, with the appearance of a capital A or inverted V. At the foot of its right leg is a bright orange star called Aldebaran (Alpha Tauri). The V shape looked to the ancients like the face of a bull, with Aldebaran as his angry orange eye. Being in the southern hemisphere, we see it upside down. The Pleiades form the bull’s shoulder. 

Below the Pleiades we can see part of the far-northern constellation of Perseus. The two brightest stars are Mirphak (Alpha Persei) and Algol (Beta Persei). Algol is the higher above the northern horizon of the two. Since ancient times, Algol has shown regular variations in brightness. It usually shines at magnitude 2.1, but every 2 days 20 hours 49 minutes it dims to magnitude 3.4 for 10 hours before recovering its original brightness. Because of this clock-like change, early astronomers called it the 'Demon Star'. It marks the head of Medusa, the Gorgon, which is carried in the hand of Perseus.

The Dutch-born Englishman John Goodricke (1764-1786) was a young amateur astronomer. He was profoundly deaf and also dumb after suffering from scarlet fever as an infant. Goodricke is best known for his explanation of the variations in the star Algol in 1782. Although several stars were already known to vary in apparent magnitude, Goodricke was the first to propose a mechanism to account for this. He suggested that Algol is actually a pair of stars circling each other, and the variation in brightness is caused when one passes behind the other. Such a star system is now known as an eclipsing binary. He presented his findings to the Royal Society in May 1783, and for this work, the Society awarded him the Copley Medal for that year.

Goodricke is also credited with discovering the periodic variation of the namesake, Delta Cephei, of the type of variable stars called Cepheids, in 1784. It was the second Cepheid found, the first, Eta Aquilae, being found by Goodricke’s friend Edward Pigott earlier the same year. Goodricke proposed that Cepheids were unstable stars that regularly swell up and then fall back - 'pulsating variables' (see Mira, the Wonderful  below).

He was elected a Fellow of the Royal Society on April 16, 1786, but never learned of this honour, as he was very ill and died four days later, probably from pneumonia, aged only twenty-one years and seven months.

Between the Hyades and the northern horizon is a large constellation shaped roughly like a tall pentagon. This is Auriga the Charioteer, its brightest star being Capella, at the left side of the base of the pentagon. Above Capella and slightly to the left is a small triangle of stars known as 'The Kids'.

To the left of Capella lies the northern constellation of Perseus. Above Auriga, and to the right of Aries, lies the interesting zodiacal constellation of Taurus, the Bull, with its famous star clusters the Pleiades and Hyades. Taurus is due north at 9:30 pm at the beginning of the month.

To the east of Auriga, Gemini is quite high, with its brightest two twin stars at its eastern end, Pollux and Castor.  Rising in the north-east is the head of Leo, its brightest star Regulus (Alpha Leonis) marking the Lion’s heart. The whole constellation will have risen by midnight on January 1.

Between Gemini and Leo is a fainter constellation, 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 splendid sight in binoculars.

In January, the constellation of Orion is as high as he can ever appear from our latitude. He is about a handspan north of the zenith, and is about to cross the meridian (the line that runs from due south to due north and passing through the zenith, directly overhead). When a sky object crosses the meridian, it is said to be culminating. At that point, it ceases rising and begins setting. The brilliant white star Rigel (Beta Orionis) is approaching the zenith at this time. He appears upside-down to us, but to an observer in the northern hemisphere, e.g. London or New York, he appears right-way-up, striding along the southern horizon this month.

About a hand-span to the south-east of Rigel is the brightest star in the night sky, Sirius, also known as Alpha Canis Majoris. Sirius will culminate at 11.00 pm in mid-month.
 

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 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.

 

Skirting the horizon from north-east to south-east is a long, faint constellation, Hydra, the Water Snake. It is the constellation with the largest area and is very long, stretching half-way across the sky. Well up in the east is Hydra’s brightest star, Alphard, mv= 2.2. This orange star was known by Arabs in ancient times as ‘The Solitary One’, as it lies in an area of sky with no bright stars nearby.

Coming up in the south-south-east, Crux (Southern Cross) is lying at an angle. Crux is the smallest of the 88 constellations. If you have a low, clear horizon in that direction, you will be able to see below it and to the right the two bright Pointers, Alpha and Beta Centauri. To the right of Crux is a small, fainter quadrilateral of stars, Musca, the Fly. In mid-month Crux will be horizontal by midnight, and vertical just before sunrise.

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 search for the Golden Fleece. The constellation Argo Navis was found to be too large, so in 1755 Nicholas Louis de la Caille divided it into three sections - Carina (the Keel), Vela (the Sails) and Puppis (the Stern).

A handspan south of the zenith and two handspans south of Sirius is the second brightest star in the night sky, Canopus (Alpha Carinae). On the border of Carina and Vela is the False Cross, larger and more lopsided than the Southern Cross. 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.

Two handspans south-west of Canopus is Achernar, Alpha Eridani. It is the brightest star in Eridanus the River, which winds its way with faint stars from Achernar in a northerly direction to Cursa, a mv = 2.9 star close to brilliant Rigel in Orion. Achernar is midway between Canopus and Fomalhaut, which is setting low in the south-west.

High in the south, about 50 degrees above the horizon, the Large Magellanic Cloud (LMC) is faintly visible as a diffuse glowing patch. To its right and below is the Small Magellanic Cloud (SMC), a smaller glowing patch. The LMC and SMC are described below.

The zodiacal constellations visible tonight, starting from the south-western horizon and heading overhead to the north-east horizon, are Pisces, Aries, Taurus, Gemini, Cancer and Leo.

 

 

Mira, the Wonderful:

The amazing thing about the star Mira or Omicron Ceti is that it varies dramatically in brightness, rising to magnitude 2 (brighter than any other star in Cetus), and then dropping to magnitude 10 (requiring a telescope to detect it), over a period of 332 days. 

This drop of eight magnitudes means that its brightness diminishes over a period of five and a half months to one six-hundredth of what it had been, and then over the next five and a half months it regains its original brightness. To the ancients, they saw the familiar star fade away during the year until it disappeared, and then it slowly reappeared again. Its not surprising that it became known as Mira, meaning 'The Wonderful' or 'The Miraculous One'.

We now know that many stars vary in brightness, even our Sun doing so to a small degree, with a period of 11 years. One type of star varies, not because it is actually becoming less bright in itself, but because another, fainter star moves around it in an orbit roughly in line with the Earth, and obscures it on each pass. This type of star is called an eclipsing variable and they are very common.

The star Mira though, varies its light output because of processes in its interior. It is what is known as a pulsating variable. Stars of the Mira type are giant pulsating red stars that vary between 2.5 and 11 magnitudes in brightness. They have long, regular periods of pulsation which lie in the range from 80 to 1000 days.

In early 2017 Mira reached a maximum brightness of magnitude 3.4 on February 23 and then dropped well below naked-eye visibility (magnitude 6) again. It reached its minimum brightness of magnitude 9.3 on September 22. In the first three weeks of this month it will brighten rapidly until it reaches its peak on January 19, 2018.  Then it will slowly fade back to its minimum, which will take until August.

    

Mira near minimum, 26 September 2008                Mira near maximum, 22 December 2008

Astronomers using a NASA space telescope, the Galaxy Evolution Explorer, have spotted an amazingly long comet-like tail behind Mira as the star streaks through space. Galaxy Evolution Explorer ("GALEX" for short) scanned the well-known star during its ongoing survey of the entire sky in ultraviolet light. Astronomers then noticed what looked like a comet with a gargantuan tail. In fact, material blowing off Mira is forming a wake 13 light-years long, or about 20,000 times the average distance of Pluto from the sun. Nothing like this has ever been seen before around a star.           More about Mira      Mira's light curves and finder charts 

 

 

 

The Hunter and his Dogs:

Two of the most spectacular constellations in the sky may be seen high above the eastern horizon soon after sunset in January. These are Orion the Hunter, and his greater dog, Canis Major. 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, rise due east and set due west. A smaller constellation, the lesser dog Canis Minor, accompanies them. 


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. You can, though, make him appear upright when high in the sky (near the meridian), by observing him from a reclining chair, with your feet pointing to the south and your head tilted back. 

Orion rising as darkness falls in January

Orion has two bright stars marking his shoulders, the red supergiant Betelgeuse and 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 west to east, Mintaka, Alnilam and Alnitak. 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 knees are marked by brilliant Rigel and fainter Saiph. Both of these stars are also members of the Orion Association.

The Saucepan, with Belt at left, M42 at lower right

Orion is quite a symmetrical constellation, with the 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. 

The stars forming the Belt and Sword are popularly known in Australia as ‘The Saucepan’, with the Sword forming the Saucepan’s handle. This asterism appears upside-down tonight, 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). A photograph 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 right 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 feet in the air. The star at the end of his front 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 above Sirius.

The hindquarters of the Dog are indicated by a large right-angled triangle of stars located to the right of Sirius and tilted. The end of his tail is the lower-right corner of the triangle, about one handspan south (to the right) of Sirius.

Both Sirius and Rigel are bright white stars and each has a tiny, faint 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 Major as it appears high in the east soon after sunset in January

Canis Minor:

By 10.00 pm this small constellation is about 40 degrees up in the east-north-east. 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 orange 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. South of Orion is a small constellation, Lepus the Hare. Between Lepus and the star Canopus is the star group Columba the Dove. Eridanus the River winds its way from near Orion west of the zenith to Achernar, high in the south-west. Between Achernar and the western horizon is the star Fomalhaut, a white star of first magnitude in the small constellation of Piscis Austrinus (the Southern Fish). To the left of Fomalhaut is the triangular constellation of Grus, the Crane. Between the zenith and the south-western horizon are a number of small, faint constellations, Horologium, Pictor, Caelum, Mensa, Tucana, Phoenix, Hydrus and Reticulum. The LMC lies in the constellation Dorado, and the South Celestial Pole is in the very faint constellation Octans.

 

 

Double 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 Beta Cygni (Albireo), at right.  

     

Rigel (Beta Orionis, left) is a binary star which is the seventh brightest star in the night sky. Rigel A is a large white supergiant which is 500 times brighter than its small companion, Rigel B, Yet Rigel B is itself composed or a very close pair of Sun-type stars that orbit each other in less than 10 days. Each of the two stars comprising Rigel B is brighter in absolute terms than Sirius (see above). The Rigel B pair orbit Rigel A at the immense distance of 2200 Astronomical Units, equal to 12 light-days. (An Astronomical Unit or AU is the distance from the Earth to the Sun.) In the centre of the Great Nebula in Orion (M42) is a multiple star known as the Trapezium (right). This star system has four bright white stars, two of which are binary stars with fainter red companions, giving a total of six. The hazy background is caused by the cloud of fluorescing hydrogen comprising the nebula.

Acrux, the brightest star in the Southern Cross, is also known as Alpha Crucis.  It is a close binary, circled by a third dwarf companion.

Alpha Centauri (also known as Rigil Kentaurus, Rigil Kent or Toliman) is a binary easily seen with 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 one 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, at 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.

Alpha Centauri, with Proxima

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 golden topaz with a small blue sapphire. It is one of the sky’s most beautiful objects. The stars are of classes G and B, making a wonderful colour contrast. It lies at a distance of 410 light years, 95 times further away than Alpha Centauri.

Binary stars may be widely spaced, as the two examples just mentioned, or so close that a telescope is struggling to separated them (Acrux, 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; the two stars take only 17½ minutes to complete an orbit.

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.

 

 

Finding the South Celestial Pole:

The South Celestial Pole is that point in the southern sky around which the stars appear to 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 at 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 star Gacrux) through its base (the star Acrux) and continue straight on for another four Cross lengths. This will locate the approximate spot. There is no bright star to mark the Pole, whereas in the northern hemisphere they have Polaris (the Pole Star) to mark fairly closely the North Celestial Pole.

Another way to locate the South Celestial Pole is to draw an imaginary straight line joining Beta Centauri (low in the south-south-east) to Achernar (a little less than two handspans above the south-western horizon at 10 pm at mid-month). Both of these stars will be at about the same altitude, on either side of the South Celestial Pole, at about 1 am. Bisect this line to find the pole.

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

 

   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.

 

 

 

 

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 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, sometimes containing over a million stars. These stars are closer to each other than is usual, and because of its great distance from us, a globular cluster gives the impression of a solid mass of faint stars. Many other galaxies also have a halo of globular clusters circling around them.

The largest and brightest globular cluster in the sky is NGC 5139**, also known as Omega Centauri. It has a slightly oval shape. It is an outstanding winter object, but is either below or close to the horizon during summer nights. 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 thousands 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

Globular Cluster NGC 104 in Tucana

Another remarkable globular, second only to Omega Centauri, is well positioned for viewing in the evenings this month. About two degrees to the right of the SMC (see "Two close galaxies" 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, seen above. 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.

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.
 

The globular cluster NGC 6752 in the constellation Pavo.

 

*     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 two Index Catalogues (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, to the left of Achernar, 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 left and slightly above the SMC, and is noticeably larger. They lie at a distance of about 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. They are linked to our Galaxy by a long arc of hydrogen, the Magellanic Stream. The LMC is a little 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


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 referred to above, 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, Centaurus, Canis Major, Orion, Scorpius and Sagittarius are located within or 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 reasonably well-placed for viewing this month, and many distant galaxies can be observed in this area of the sky. The Hubble Ultra-Deep Field, a photograph with an exposure of one million seconds to detect faint objects at the edge of the observable universe, was taken in this direction. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It is below the horizon in the evenings this month but will rise after midnight.

Some of the fainter and apparently insignificant constellations are found around these windows, and their lack of bright stars, clusters and gas clouds presents us with the opportunity to look across billions of light years of space to untold millions of distant galaxies. 

  


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