December
2018
Updated: 17 December 2018
Best wishes for the coming festive season and New Year to all readers !
Welcome to the night skies of Summer, featuring Mars, Aquarius, Pisces, Andromeda, Cetus, Aries, Taurus and Orion
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 20-inch telescope is able to locate and track any sky object (including Earth satellites and the International Space Station) with software called TheSkyX Professional, into which is embedded a unique T-Point model developed for our site with our equipment over the past year.
Explanatory Notes:
Rise and set times are given for the theoretical horizon, which is a flat horizon all the
way round the compass, with no mountains, hills, trees or buildings to obscure the view. Observers will have to make allowance for their own actual horizon.
Transient phenomena are provided for the current month and the next. Geocentric phenomena are calculated as if the Earth were fixed in space
as the ancient Greeks believed. This viewpoint is useful, as otherwise rising and setting times would be meaningless. In the list of geocentric events, the nearer object is
given first.
When a planet is referred to as ‘stationary’, it means that its movement across the stellar background appears to have ceased, not that
the planet itself has stopped. With inferior planets (those inside the Earth’s orbit, Mercury and Venus), this is caused by the planet heading either directly
towards or directly away from the Earth. With superior planets (Mars out to Pluto), this phenomenon is caused by the planet either beginning or ending its
retrograde loop due to the Earth’s overtaking it.
Apogee and perigee: Maximum and minimum distances of the Moon or artificial satellite from the Earth.
Aphelion and perihelion: Maximum and minimum distances of a planet, asteroid or comet from the Sun.
The zenith is the point in the sky directly overhead
from the observer. The
meridian is a semicircle starting from a point on the horizon that is exactly
due north from the observer, and arching up into the sky to the zenith and
continuing down to a point on the horizon that is exactly due south. On the way
down it passes through the South Celestial Pole which is 26.6 degrees above the
horizon at Nambour. The elevation of the South Celestial Pole is exactly the
same as the observer's latitude, e.g. from Cairns it is 16.9 degrees above the
horizon, and from Melbourne it is 37.8 degrees. The Earth's axis points to this
point in the sky in the southern hemisphere, and to an equivalent point in the
northern hemisphere, near the star Polaris, which from Australia is always below
the northern horizon.
All astronomical objects rise until they reach the meridian, then they begin to
set. The act of crossing or 'transitting' the meridian is called 'culmination'.
Objects closer to the South Celestial Pole than its altitude above the southern
horizon do not rise or set, but are always above the horizon, constantly
circling once each sidereal day. They are called 'circumpolar'. A handspan at arm's length with fingers spread covers an angle of approximately 18 - 20 degrees.
mv = visual magnitude or brightness. Magnitude 1 stars are very bright, magnitude 2 less so, and
magnitude 6 stars are so faint that the unaided eye can only just detect them under good, dark conditions. Binoculars will allow us to see down to magnitude
8, and the Observatory telescope can reach visual magnitude 17 or 22 photographically. The world's biggest telescopes have detected stars and galaxies as faint as
magnitude 30. The sixteen very brightest stars are assigned magnitudes of 0 or even -1. The brightest star, Sirius, has a magnitude of -1.44. Jupiter can reach -2.4, and
Venus can be more than 6 times brighter at magnitude -4.7, bright enough to cast shadows. The Full Moon can reach magnitude -12 and the
overhead Sun is magnitude -26.5. Each
magnitude step is 2.51 times brighter or fainter than the next one, i.e. a magnitude 3.0 star is 2.51 times brighter than a magnitude 4.0. Magnitude 1.0
stars are exactly 100 times brighter than magnitude 6.0 (5 steps each of 2.51 times, 2.51x2.51x2.51x2.51x2.51 = 2.515 = 100).
The Four Minute Rule:
How long does it take the Earth to complete one rotation? No, it's not 24 hours - that is the time taken for the Sun to cross the meridian on successive days. 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 non-zodiacal constellation of Ophiuchus, the Serpent Bearer. It leaves Ophiuchus and passes into Sagittarius, the Archer on December 18.
Moon Phases:
New Moon: January 06 11:28 hrs
diameter = 29.7' Lunation #1188 begins
First Quarter: January 14 16:45 hrs
diameter = 30.4'
Full Moon: January 21
15:16 hrs diameter = 33.4'
Last Quarter: January 28 07:11 hrs
diameter = 31.3'
January 07: Moon at descending node at 10:10 hrs, diameter = 29.6'
January 09: Moon at apogee (406 112 km) at 14:15 hrs, diameter = 29.4'
January 21: Moon at ascending node at 08:44 hrs, diameter = 33.4'
January 22: Moon at perigee (357 354 km) at 05:52 hrs, diameter = 33.4'
Moon at 8 days after New, as on December 16.
The photograph above shows the
Moon when approximately eight days after New, just after First Quarter.
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. A professional version of this freeware
with excellent pictures from the Lunar Reconnaissance Orbiter and the Chang orbiter (giving a resolution of 50 metres on the
Moon's surface) and many other useful features
is available on a DVD from the same website for 20 Euros (about AU $ 33) plus postage.
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 an interesting crater named after a famous Italian - French astronomer, J. D. Cassini.
The largest crater in this picture is Cassini (60 km diameter), and it
lies left of centre. This image of Cassini and its surroundings was taken at 7:12 pm on 18 September 2018.
North is to the top, east to the right. This crater Cassini lies on the easternmost margin of the Mare Imbrium (the Sea of Rains). To its north are isolated mountains
connected to a large group known as the Montes Alpes (Alps), and to its east are the Montes Caucasus (Caucasus Mountains). The creation of the
Mare Imbrium occurred between 3.8 and 3.85 million years ago, which makes it one of the youngest of the lunar 'seas'. From its appearance, we can see that the
Cassini impact came a little later than that. Cassini is an interesting crater in that it has two bowl-shaped newer craters on its floor. These are Cassini A
(17 km) and Cassini B (9 km). There is a craterlet on the northern edge of the floor which has a diameter of 2.5 km. Another craterlet which is only 900 metres across
straddles the crest of Cassini's southern rim.
J. D. Cassini
Cassini's walls reach a height of 1200 metres on the east, but in the west they only reach 500 metres. Lava has flooded the western half of the floor, but the
eastern half has three curved ridges with deep valleys in between. This part of Cassini is a very rugged pyroclastic area. Outside of Cassini's walls are large
deposits of rock melt blown outward from the impact, and fairly smooth in texture. All around Cassini are crater-chains and gouges running in a rough
north-south direction. These were caused by material ejected from another impact that created the Aristillus crater, 165 km to the south-south-west of Cassini (see
the image of the Moon at eight days, above).
In 1665, Giovanni Domenico Cassini (1625-1712, right, note that he is pointing at the long refracting telescope on the roof of the building in the background),
a professor of mathematics and astronomy at the University of Bologna in Italy, quietly carried on the work of Galileo. He shared the discovery of Jupiter’s Great Red Spot
with Robert Hooke, and in 1666 he reported the polar caps on Mars. He also measured the rotation period of Mars to be 24 hours 40 minutes. In 1668 he published the best set
of predictions for eclipses of the Jovian satellites yet seen, based on a multitude of carefully con¬ducted observations. Cassini’s accurate ephemerides brought him to the
attention of people in Paris, at the royal court.
King Louis
XIV of France (called the 'Sun King') had a soft spot for science. He had given his blessing to the founding, in 1666, of the French Académie Royale des Sciences, the brain¬child of his talented Minister for Finance, Jean-Baptiste Colbert, who knew England’s Christopher Wren. The Royal Academy of Sciences would be France’s answer to England’s six-year-old Royal Society, and still exists as one of five Academies forming the Institut de France. At Colbert’s urging, and under the ever- increasing pressure to solve the longitude problem and find the true shape and size of the Earth, King Louis approved the building of an astronomical observatory in Paris, and the Observatoire de Paris was commissioned in 1671. Subsequently, England’s answer to that would be the Royal Observatory at Greenwich, founded in 1675.Colbert enticed famous foreign scientists to France to fill the ranks of the Academy, and invited G. D. Cassini to come to Paris in 1669 to be the Director of the proposed Observatoire de Paris. Knowing of the problems that had beset Galileo, Cassini felt that he had more freedom in France to pursue his scientific work and so moved to Paris. He put down roots there and never returned home to Italy. He changed his first names to their French form, Jean-Dominique. Becoming a French citizen in 1673, he is remembered in history as the French astronomer J. D. Cassini. From his post supervising the construction of the new Paris Observatory in 1670, Cassini arranged for Jean-Felix Picard (1620-1682) and a few assistants to travel to the island of Hven in Denmark, to the ruins of Uraniborg, Tycho Brahe’s ‘castle of the stars’. Using observations of eclipses of Jupiter’s satellites taken at these two sites, Paris and Uraniborg, Cassini and Picard confirmed the latitude and longitude of both places. Cassini also called on observers in Poland and Germany to co-operate with France in an international effort devoted to the finding and measuring of longitude, as gauged by the motions of Jupiter’s moons.
Cassini was the first to make successful measurements of longitude by the method suggested by Galileo, using eclipses of the satellites of Jupiter as a clock. Cassini’s work led to Galileo’s method for finding longitude becoming generally accepted after 1675 – but only on land, as Jupiter's satellites could not be observed from the unsteady deck of a ship at sea. Surveyors and cartographers used Galileo’s technique to redraw the world, and it was in the field of mapmaking that the ability to determine longitude proved its worth. Earlier maps had underestimated the distances to other continents and exaggerated the outlines of individual nations. Now global dimensions could be set, with authority, by the Jovian moons. Indeed, King Louis
XIV, confronted with a revised map of his domain based on accurate longitude measurements, ruefully quipped to Cassini that he was losing more territory to his astronomers than he had gained in all his wars. The success of Galileo’s method had mapmakers demanding further refinements in predicting eclipses of the Jovian satellites. Greater precision in the timing of these events would permit greater exactitude in mapping. With the borders of kingdoms hanging in the balance, numerous astronomers found useful employment in observing the moons and improving the printed tables.In 1672 Cassini sent Jean Richer to Cayenne in French Guiana while he himself stayed in Paris. The two made simultaneous observations of Mars, and thus found its parallax as seen from Earth. This was the first measurement of a planet’s parallax, and the result gave Cassini a relationship between the orbit of Mars and that of the Earth. He then used geometry to make a reasonable estimate of the radius of the Earth’s orbit in French miles. From that he applied Kepler’s Third Law to extrapolate the distances from the Sun of Mercury, Venus, Mars, Jupiter and Saturn. In future years we would find that Cassini’s figure for the Earth-Sun distance was excellent for his time but 7% too small, and therefore all his other planet-Sun distances were similarly 7% below the true figures. Yet his work was revolutionary and the astronomers of his time were amazed, for suddenly the immense size of the Solar System was revealed. Cassini discovered four satellites of Saturn, and the gap between its Ring A and Ring B which is now known as the Cassini Division. He also timed the rotation of Jupiter's clouds and made a detailed map of the Moon.
A spacecraft named Cassini (above) was launched in 1997, and arrived at Saturn in 2004, where it went into orbit around the ringed planet. This was a very successful mission, returning thousands of photographs of Saturn and its satellites, and a host of physical data. The mission was planned to end on 30 July 2008, but the spacecraft performed so well that its mission was extended three times. In 2017, an encounter with Titan changed Cassini’s orbit in such a way that, at closest approach to Saturn, it would be only 3000 kilometres above the planet’s cloudtops, below the inner edge of the D ring. On 26 April 2017, Cassini conducted the first of 22 plunges through the gap between Saturn and its innermost Ring, transmitting many spectacular images to Earth. This sequence of new orbits was the ‘grand finale’ of the highly successful Cassini mission. These ‘proximal orbits’ ended when another encounter with Titan sent the orbiter into Saturn’s atmosphere on 15 September 2017.
Click
Geocentric Events:
It should be remembered that close approaches of Moon, planets and stars are only perspective effects as seen from the Earth - that is why they are called
'geocentric or Earth-centred phenomena'. The Moon, planets and stars do not really approach and dance around each other as it appears to us from the vantage
point of our speeding planet.
December 3: Mars at eastern quadrature at 09:34 hrs (diameter = 9.1")
December 4: Moon 3.9º north of Venus at 06:25 hrs
December 6: Moon 2.3º north of Mercury at 06:44 hrs
December 6: Neptune at eastern quadrature at 08:19 hrs (diameter = 2.2")
December 7: Moon 4º north of Jupiter at 00:24 hrs
December 7: Mercury at western stationary point at 07:18 hrs (diameter = 8.e")
December 8: Mars 2.2 arcseconds north of Neptune at 00:14 hrs
December 9: Moon 1.2º north of Saturn at 16:09 hrs
December 10: Moon occults the star Pi Sagittarii (mv= 2.88) between 04:51 and 5:36 hrs
December 10: Moon 33 arcminutes north of Pluto at 12:48 hrs
December 13: Moon 17 arcminutes north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 08:26 hrs
December 14: Jupiter 54 arcminutes south of the star Psi Ophiuchi, mv= 4.48) at 00:29 hrs
December 15: Moon 2.2º south of Neptune at 03:07 hrs
December 15: Moon 3.2º south of Mars at 10:35 hrs
December 16: Mercury at Greatest Elongation West (21º 09') at 01:08 hrs (diameter = 6.6")
December 17: Mercury 1.1º north of the star Graffias (Beta1 Scorpii, mv= 2.56) at 00:51 hrs
December 18: Moon 4.5º south of Uranus at 15:32 hrs
December 21: Moon 1.8º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 16:02 hrs
December 22: Mercury 50 arcminutes north of Jupiter at 03:56 hrs
December 22: Limb of Moon 2 arcminutes south of the star Zeta Tauri (mv= 2.97) at 16:54 hrs
December 23: Moon 1º south of the star Mu Geminorum (mv= 2.87) at 11:46 hrs
December 23: Jupiter 13 arcnibures north of the star Omega Ophiuchi (mv= 4.45) at 13:50 hrs
December 23: Venus 2.9º north of the star Zuben Elgenubi (Alpha Librae, mv= 2.75) at 17:52 hrs
December 27: Venus at perihelion at 04:07 hrs (diameter = 28.0")
2019
January 2: Moon 1.4º north of Venus at 08:34 hrs
January 2: Saturn in conjunction with the Sun at 15:55 hrs (diameter = 15.0")
January 3: Moon 3.5º north of Jupiter at 19:23 hrs
January 4: Earth at perihelion at 08:28 hrs
January 5: Moon 3.3º north of Mercury at 02:18 hrs
January 6: Moon 1.4º north of Saturn at 03:07 hrs
January 6: North limb of Moon 3 arcminutes south of the star Pi Sagittarii (mv= 2.88) at 14:38 hrs (Sun within 1.5º)
January 6: Venus at Greatest Elongation West (46º 52') at 15:44 hrs (diameter = 24.7")
January 6: Moon 1.3º north of Pluto at 22:38 hrs
January 7: Uranus at eastern stationary point at 04:19 hrs (diameter = 3.6")
January 9: South limb of Moon 22 arcminutes north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 18:15 hrs
January 9: Mercury 1.3º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 21:58 hrs
January 11: Venus 2.4º north of the star Graffias (Beta-1 Scorpii, mv= 2.56) at 03:51 hrs
January 11: Moon 2.8º south of Neptune at 09:03 hrs
January 11: Pluto in conjunction with the Sun at 21:41 hrs (diameter = 0.1")
January 12: Mercury at aphelion at 18:22 hrs (diameter = 4.8")
January 12: Saturn 22 arcminutes north of the stat Nu-1 Sagittarii (mv= 4.86) at 17:05 hrs
January 13: Moon 4.8º south of Mars at 08:41 hrs
January 13: Mercury 2.2º north of the star Nunki (Sigma Sagittarii, mv= 2.04) at 20:13 hrs
January 14: Mercury 1.7º south of Saturn at 00:19 hrs
January 14: Saturn 19 arcminutes north of the stat Nu-2 Sagittarii (mv= 5.00) at 17:08 hrs
January 15: Moon 4.2º south of Uranus at 02:41 hrs
January 16: Mercury 2.9º south of the star Pi Sagittarii (mv= 2.88) at 08:43 hrs
January 18: Moon 1.9º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 05:23 hrs
January 19: Mercury 1.5º south of Pluto at 06:45 hrs
January 19: North limb of Moon 4 arcminutes south of the star Zeta Tauri (mv= 2.97) at 06:37 hrs
January 19: Uranus at eastern quadrature at 11:26 hrs (diameter = 3.5")
January 19: North limb of Moon 21 arcminutes south of the star Propus (Eta Geminorum (mv= 3.31) at 18:39 hrs
January 19: North limb of Moon 4 arcminutes south of the star Mu Geminorum (mv= 2.87) at 23:15 hrs
January 21: Saturn 1.6º south of the star Xi-1 Sagittarii (mv= 5.01) at 01:58 hrs
January 21: Saturn 1.2º south of the star Xi-2 Sagittarii (mv= 3.52) at 09:56 hrs
January 23: Venus 2.4º north of Jupiter at 22:03 hrs
January 30: Mercury at superior conjunction at 12:33 hrs (diameter = 4.8")
January 31: Moon 2.8º north of Jupiter at 11:28 hrs
The Planets for this month:
Mercury:
Mercury passed through inferior conjunction on November 27 and is now in the eastern pre-dawn sky. It shines nearly as bright as Sirius, but is only visible when it is at a large angular distance from the glare of the Sun. As Mercury lies well inside the Earth's orbit and close to the Sun, it can never move more than 27.8º from the Sun. In mid-December, it will be about 15 degrees (a little less than a handspan) above the east-south-eastern horizon prior to sunrise. The thin crescent Moon will be to the left of Mercury on the morning of December 6. Both will be near the horizon, in the constellation of Libra. Mercury's maximum angular distance from the Sun (greatest elongation west) will occur on December 16, when it will equal 21º 09'.Mercury will pass through superior conjunction (on the far side of the Sun) on January 30, 2019, and will then return to our western twilight sky. It will become easily visible in mid-February.
Venus:
This, the brightest planet, passed through inferior conjunction (between the Earth and the Sun) on October 27, and has now disappeared from the western twilight sky.
It has reappeared in the eastern pre-dawn sky where it is very prominent in the early mornings as a so-called 'morning star'. Currently in the constellation Virgo, it will
move into Libra at 9:11 pm on December 13. The waning crescent Moon will be just to the left of Venus on the mornings of December 4 and January 2. Venus will remain a
pre-dawn object until next August.
(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).
December 2018 January 2019 August 2019 March 2020
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 most of 2018, Venus appeared as an 'Evening Star' in the western twilight sky, where it stayed for about nine months. Venus passed between us and the Sun (inferior conjunction) on October 27 last, and is now in the morning sky as a 'Morning Star'. It will return to the evening sky to be an 'Evening Star' once again on August 14 next year, although it won't be away from the Sun's glare to be easily visible until next October.
Because Venus is 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.
Venus at 6.55 pm on September 7, 2018. The phase is 36 % and the angular diameter is 32 arcseconds.
Mars: The red planet is still reasonably well positioned for viewing, but the Earth has now left it far behind, and Mars is much reduced in size, being only 9 arcseconds in diameter on December 1. By December 31 its diameter will have fallen to 7 arcseconds, and during the month its phase will increase slightly from 86% to 87%. On December 1 at 7:30 pm (end of twilight), Mars was about 30º or nearly two handspans north-west of the zenith, in the constellation of Aquarius. By December 31 at 7:40 pm (end of twilight), Mars will be about 45º or two-and-a-half handspans above the west-north-west horizon, in the constellation Pisces. On December 3, Mars reached eastern quadrature (one handspan due north of the zenith at sunset). On December 15, the First Quarter Moon was close to the left of Mars in the early evening sky.
In this image, the south polar cap of Mars is easily seen. Above it is a dark triangular area known as Syrtis Major. Dark Sinus Sabaeus runs off to the left, just south of the equator. Between the south polar cap and the equator is a large desert called Hellas. The desert to upper left is known as Aeria, and that to the north-east of Syrtis Major is called Isidis Regio. Photograph taken in 1971.
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.
Mars near opposition, July 24, 2018
Mars, called the red planet but usually coloured orange, has now taken on a yellowish tint and has brightened by 0.4 magnitude, making it twice as bright as previous predictions for the July 27 opposition. These phenomena have been caused by a great dust storm which has completely encircled the planet, obscuring the surface features so that they are only seen faintly through the thick curtain of dust. Although planetary photographers were mostly disappointed, many observers were interested to see that the yellow colour and increased brightness meant that a weather event on a distant planet could actually be detected with the unaided eye - a very unusual thing in itself.
The three pictures above were taken on the evening of July 24, at 9:05, 9:51 and 11:34 pm. Although the fine details that are usually seen on Mars are hidden by the dust storm,
some of the larger features can be discerned, revealing how much Mars rotates in two and a half hours. Mars' sidereal rotation period (the time taken for one complete
rotation or 'Martian day') is 24 hours 37 minutes 22 seconds - a little longer than an Earth day. The dust storm began in the Hellas Desert on May 31, and after two months
it still enshrouded the planet. In August it began to clear.
Central meridian: 295º.
The two pictures immediately above were taken on the evening of September 7, at 6:25 and 8:06 pm. The dust storm is finally abating, and some of the surface features are becoming visible once again. This pair of images also demonstrates the rotation of Mars in 1 hour 41 minutes (equal to 24.6 degrees of longitude), but this time the view is of the opposite side of the planet to the set of three above. As we are now leaving Mars behind, the images are appreciably smaller (the angular diameter of the red planet has fallen to 20 arcseconds). Well past opposition, Mars on September 7 exhibited a phase effect of 92.65 %.
Central meridian: 180º.
Jupiter: This gas giant planet passed through conjunction with the Sun (on the far side of its orbit) on November 26. It is now in the pre-dawn eastern sky, but won't become observable until early in January next year.
Jupiter as photographed from Nambour on the evening of April 25, 2017. The images were
taken, from left to right, at 9:10, 9:23, 9:49, 10:06 and 10:37 pm. The rapid rotation of this giant planet in a little under 10 hours is clearly seen. In the
southern hemisphere, the Great Red Spot (bigger than the Earth) is prominent, sitting within a 'bay' in the South Tropical Belt. South of it is one of the
numerous White Spots. All of these are features in the cloud tops of Jupiter's atmosphere
Jupiter as it appeared at 7:29 pm on July 2, 2017. The Great Red Spot is in a similar position near Jupiter's eastern limb (edge) as in the fifth picture in the series above.
It will be seen that in the past two months the position of the Spot has drifted
when compared with the festoons in the Equatorial Belt, so must rotate around the planet at a slower rate. In fact, the Belt enclosing the Great Red Spot rotates around the
planet in 9 hours 55 minutes, and the Equatorial Belt takes five minutes less. This high rate of rotation has made the planet quite oblate. The prominent 'bay'
around the Red Spot in the five earlier images appears to be disappearing, and a darker streak along the northern edge of the South Tropical Belt is moving
south. Two new white spots have developed in the South Temperate Belt, west of the Red Spot. The five upper images were taken near opposition, when the Sun was
directly behind the Earth and illuminating all of Jupiter's disc evenly. The July 2 image was taken just four days before Eastern Quadrature, when the angle
from the Sun to Jupiter and back to the Earth was at its maximum size. This angle means that we see a tiny amount of Jupiter's dark side, the shadow being
visible around the limb of the planet on the left-hand side, whereas the right-hand limb is clear and sharp. Three of Jupiter's Galilean satellites are visible, Ganymede to
the left and Europa to the right. The satellite Io can be detected in a transit of Jupiter, sitting in front of the North Tropical Belt, just to the left of its centre.
Jupiter at opposition, May 9, 2018
Jupiter reached opposition on May 9, 2018 at 10:21 hrs, and the above photographs were taken that evening, some ten to twelve hours later. The first image above was
taken at 9:03 pm, when the Great Red Spot was approaching Jupiter's central meridian and the satellite Europa was preparing to transit Jupiter's disc.
Europa's transit began at 9:22 pm, one minute after its shadow had touched Jupiter's cloud tops. The second photograph was taken three minutes later at 9:25
pm, with the Great Red Spot very close to Jupiter's central meridian.
The third photograph was taken at 10:20 pm, when Europa was approaching Jupiter's central meridian. Its dark shadow is behind it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth photograph at 10:34 pm shows Europa and its shadow well past the central meridian. Europa is the smallest of the Galilean satellites, and has a diameter of 3120 kilometres. It is ice-covered, which accounts for its brightness and whitish colour. Jupiter's elevation above the horizon for the four photographs in order was 50º, 55º, 66º and 71º. As the evening progressed, the air temperature dropped a little and the planet gained altitude. The 'seeing' improved slightly, from Antoniadi IV to Antoniadi III. At the time of the photographs, Europa's angular diameter was 1.57 arcseconds. Part of the final photograph is enlarged below.
Jupiter at 11:34 pm on May 18, nine days later. Changes in the rotating cloud patterns are apparent, as some cloud bands rotate faster than others and
interact. Compare with the first photograph in the line of four taken on May 9. The Great Red Spot is ploughing a furrow through the clouds of the South
Tropical Belt, and is pushing up a turbulent bow wave.
Saturn:
Uranus:
Neptune: The icy blue planet reached eastern quadrature (transitting the meridian at Sunset) on December 6, so
will be best observable this month before 10 pm. In mid-December it can be found two-and-a-half handspans above the west-north-western horizon as soon as darkness falls.
The waxing gibbous Moon will be just above Neptune on December 15.
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 at 7:14 pm on September 09, 2018, 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.
Neptune, photographed from Nambour on October 31, 2008
Pluto: The erstwhile ninth and most distant planet reached opposition on July 12 and is poorly placed for viewing
this month. It will be only about a handspan above the western horizon as darkness falls on December 1. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth
that of Neptune. Located just east of the 'Teaspoon' which is north-east of the Sagittarius 'Teapot', it is a faint 14.1 magnitude object near the centre of Sagittarius. A
telescope with an aperture of 25 cm or more is necessary to observe Pluto. The waxing crescent Moon will be just to the right of Pluto during twilight on December 10.
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.
Phoenicids
December 6/7
New Moon, 1% sunlit ZHR =
6
Radiant: Near the star Achernar
Geminids
December 14/15
Waxing crescent
Moon, 40% sunlit ZHR =
90
Radiant:
Use this
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 wh<< ich 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:
Green comets in the news
Comet C/2018 V1 Machholz-Fujikawa-Iwamoto
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.
In modern astronomy, most comets are found by large mountaintop telescopes photographing the
skies under computer control. The photographs are scanned automatically to look for any new object that is not on the databases, such as an asteroid or
comet. These on-going robotic surveys discover most new comets before they are bright enough for amateur astronomers to catch them. Surprisingly, three
amateur astronomers (one in Arizona, two in Japan) have just discovered a bright new comet in the constellation Virgo that somehow escaped the notice of the automated
surveys. This morning it was near the magnitude 2.9 star Porrima (Gamma Virginis), and heading east through the background stars.
It was near the magnitude 3.38 star Zeta Virginis on November 18.
Named Comet Machholz-Fujikawa-Iwamoto after the three discoverers, it is plunging
toward the Sun and could brighten to naked-eye visibility later this month. It was
at its closest approach to Earth on November 27 and closest approach to the Sun
on December 4. Visit the November 12 and subsequent editions of
Spaceweather for
the full story.
Comet 46P/Wirtanen
UPDATE, 5 December:
Comet PANSTARRS (C/2017 S3)
A comet that may become visible to the naked eye exploded in brightness, suddenly increasing its luminosity 16-fold on July 1. Whatever happened on
Comet PANSTARRS
(C/2017 S3) has given it an expanding green atmosphere almost twice the size of the planet Jupiter. Visit the July 4 edition of
Spaceweather
and subsequent news releases for pictures and more information about this comet.
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
LINEARNearly all of these programs are based in the northern hemisphere, leaving gaps in the coverage of the southern sky. These gaps are the areas of sky where amateur astronomers look for comets from their backyard observatories.
To find out more about current comets, including finder charts showing exact positions and magnitudes, click
here. To see pictures of these comets, click here.
The 3.9 metre Anglo-Australian Telescope (AAT) at the Australian Astronomical Observatory near Coonabarabran, NSW.
Deep Space
Sky Charts and Maps available on-line:
There are some useful representations of the sky available here. The sky charts linked
below show the sky as it appears to the unaided eye. Stars rise four minutes earlier each night, so at the end of a week the stars have gained about half an
hour. After a month they have gained two hours. In other words, the stars that were positioned in the sky at 8 pm at the beginning of a month will have the
same positions at 6 pm by the end of that month. After 12 months the stars have gained 12 x 2 hours = 24 hours = 1 day, so after a year the stars have returned
to their original positions for the chosen time. This accounts for the slow changing of the starry sky as the seasons progress.
The following interactive sky charts are courtesy of Sky and Telescope magazine. They can simulate a view of the sky from any location on Earth at any time of day or night between the years 1600 and 2400. You can also print an all-sky map. A Java-enabled web browser is required. You will need to specify the location, date and time before the charts are generated. The accuracy of the charts will depend on your computer’s clock being set to the correct time and date.
To produce a real-time sky chart (i.e. a chart showing the sky at the instant the chart is generated), enter the name of your nearest city and the country. You will also need to enter the approximate latitude and longitude of your observing site. For the Sunshine Coast, these are:
latitude: 26.6o South longitude: 153o East
Then enter your time, by scrolling down through the list of cities to "Brisbane: UT + 10 hours". Enter this one if you are located near this city, as Nambour is. The code means that Brisbane is ten hours ahead of Universal Time (UT), which is related to Greenwich Mean Time (GMT), the time observed at longitude 0o, which passes through London, England. Click here to generate these charts.
_____________________________________
Similar real-time charts can also be generated from another source, by following this second link:
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
The view can be panned around the horizon, 45 degrees at a time. Scrolling down the screen will reveal tables showing setup and customising options, and an Ephemeris showing the positions of the Sun, Moon and planets, and whether they are visible at the time or not. These charts and data are from YourSky, produced by John Walker.
The charts above and the descriptions below assume that the observer has a good observing site with a low, flat horizon that is not too much obscured by buildings or trees. Detection of fainter sky objects is greatly assisted if the observer can avoid bright lights, or, ideally, travel to a dark sky site. On the Sunshine Coast, one merely has to travel a few kilometres west of the coastal strip to enjoy magnificent sky views. On the Blackall Range, simply avoid streetlights. Allow your eyes about 15 minutes to become dark-adapted, a little longer if you have been watching television. Small binoculars can provide some amazing views, and with a small telescope, the sky’s the limit.
In December, the Eta Carinae Nebula can only be viewed low in the south-south-east, in the hours after midnight. above the Southern Cross.
The Stars and Constellations for this month:
These descriptions of the night sky are for 10 pm on December 1 and 8 pm on December 31. Broadly speaking, the following
description starts in the west and heads east, then south, then high in the south.
The Great Spiral M33 in Triangulum. Above Triangulum is the zodiacal constellation of Aries, now past the meridian in the north-north-
west. There are three main stars in Aries, the brightest being a second magnitude orange star called Hamal. The other two are named Sheratan and 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 star in the constellation. Its common name is Diphda,
and it has a yellowish-orange colour. 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.
Mars will cross from Aquarius into Pisces on December 21. The spectacular constellations are in the eastern half of the sky tonight. Taurus, with its two star clusters the
Pleiades (also known as the Seven Sisters, or Subaru, or Santa's Sleigh) 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.
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 shows 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 (Beta Persei) 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 north-eastern horizon, the large pentagon of Auriga, the Charioteer has risen. The brightest star in this constellation
is Capella, which is directly above the north-north-eastern horizon. Just above Capella is a little triangle of stars,
known as 'The Kids'. The top star of Auriga's pentagon, Elnath, points towards Orion.
Actually, Elnath is the second-brightest star in Taurus, and forms the tip of
one of the Bull's horns. At 10 pm at the beginning of the month, the two bright stars of
Gemini, Castor
and Pollux, are just rising above the theoretical north-eastern horizon. These two first magnitude stars are known as
'The Twins'. If your horizon in this direction is very low, you may be able to glimpse them, both stars rising together
about five degrees apart. Pollux is the more easterly of the two. If your horizon is not low, you will have to wait a little to
see these stars appear. These two stars are the twins' heads, and their feet point towards Orion. Two-thirds of the way from
their heads to their feet, each twin is marked by a star at his hip, third magnitude Mebsuta for Castor, and fourth magnitude
Mekbuda for Pollux. Orion (see below) is about halfway up the sky in the east, and a bright star in Canis Minor (the Lesser Dog), Procyon, is now about
half-a-handspan above the eastern horizon. Around midnight at mid-month, Orion will cross the meridian (the north-south line across the sky, passing through the
zenith), at which time the constellation is said to ‘culminate’. The three stars marking Orion's Belt will be about twenty-five degrees (a little more than a handspan)
north of the zenith. Sirius, the brightest star in the night sky, is south-east of Orion, in the constellation Canis Major (the Greater Dog).
Setting on the western horizon is the zodiacal constellation Capricornus, the Sea Goat. Above it is Aquarius, the Water Bearer, in which the planets
Mars and Neptune are located. The Great Square of Pegasus is tilted over on its lower left corner, and is setting in the north-west, being followed by
Andromeda. Triangulum is above the north-north-western horizon. These constellations contain the well-known spiral galaxies M31 (in Andromeda,
illustrated at the bottom of this webpage), and M33 (in Triangulum, illustrated below).
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 and slightly to the left of Mirach tonight. 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 observed from the large observatories in the Northern Hemisphere. This month they are
best seen as soon as darkness falls, for then they will be closer to due north and therefore at their highest elevation. 
Very low in the south, Alpha Crucis (or Acrux), the brightest and most southerly star of Crux (Southern Cross) is beginning to
rise. Between Crux and Sirius is a large area of the Milky Way 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 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). Above the horizon, just to the left of due south and above Acrux, is the small constellation Musca, the Fly. Musca is
a circumpolar constellation, i.e. it is always in our sky, being too close to the South Celestial Pole to set. Out of the 88 constellations, it is the only insect. High in the south-east and two handspans from 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. At the present time, both Crosses are upside-down. At about thirty-five degrees above the south-western horizon, the flattened triangle of Grus, the Crane, is swinging down. To its right is the
bright star Fomalhaut, a bright, white first magnitude star in the faint constellation Piscis Austrinus, the Southern Fish. Fomalhaut means 'the fish's
mouth'. A planet revolving around Fomalhaut was recently photographed by the Hubble Space Telescope. Midway between -0.71 magnitude Canopus and
1.16 magnitude Fomalhaut is the 0.46 magnitude star Achernar, in the constellation Eridanus, the River. Eridanus winds its faint way across the sky from Achernar to Cursa, these two stars marking the ends of the river. Cursa
(Beta Eridani) is a 2.79 magnitude star about four degrees to the north-west of Rigel (see below). To the right of Achernar, the faint constellation of Phoenix may be seen. Its brightest star is
Ankaa, a mv 2.39 star which is about one third of the way from Achernar to Diphda. About a handspan south of Achernar is a small, faint glowing patch. This is the
Small Magellanic Cloud
(SMC). A handspan to its left is a larger glow, the Large Magellanic Cloud (LMC). Both of these glowing clouds are
galaxies in their own right, separate from our Milky Way galaxy. They are described in greater detail in
Two close galaxies
below. The LMC bridges two faint constellations, Dorado and Mensa. The SMC is in the constellation Tucana, the Toucan.
The zodiacal constellations visible tonight, starting from the south-western horizon and heading across to the north-east horizon, are Capricornus,
Aquarius, Pisces, Aries, Taurus and Gemini.
If you would like to become familiar with the constellations, we suggest that
you access one of the world's best collections of constellation pictures by clicking
The Winged Horse and the Chained Maiden:
About half-way up the sky from the north-north-western horizon (at the end of twilight on December 1) is a large star group called the
Great Square of Pegasus. The Square (actually slightly rectangular) is unusual for the sparcity of bright stars within it.
Pegasus was the winged horse of Greek legend, and is used as a symbol by the Mobil Oil Company and TriStar Pictures. By 8.00 pm the
Square will be tilted over to the west and it will begin to set in the north-west by 11.00 pm.
The stars making the four corners of the Square, starting from the top left and moving clockwise, are Markab, Algenib, Alpheratz and Scheat. Three of these are in the constellation of Pegasus, but Alpheratz is actually in the constellation Andromeda.
Between Alpheratz and the northern horizon, a very faint glowing oval may be seen if the sky is dark enough. You would need to be away from light pollution caused by bright lights. Binoculars will help you to find it, but it is visible faintly without optical aid. This oval is the Great Nebula in Andromeda (M31), a large spiral galaxy of over 100 million stars. The Andromeda spiral is almost a twin of our own galaxy, the Milky Way. Lying at a distance of 2.6 million light years, it is the furthest object that can be seen with the unaided eye (see The Andromeda Galaxy and the President of the United States at the bottom of this page).
The Hunter and his Dogs:
Two of the most spectacular constellations in the sky may be seen rising above the eastern horizon soon after sunset in December. 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 December
Orion rises south-east of Taurus. If you have a fairly low eastern horizon, you will
see most of the constellation rising above the eastern horizon as soon as it gets dark at the beginning of December. The bright Rigel is
highest in the sky. The red supergiant Betelgeuse is the last of Orion's bright stars to rise.
A little north of a line joining these stars is a tiny triangle of stars marking Orion’s head. In the centre of the constellation are three stars in a line forming Orion's Belt. These 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.
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 Orion’s Sword. Orion’s legs 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 leg stars to the south. It is quite a large star group, the Hunter being over twenty degrees (about 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*. A photograph of it appears below:
The Sword of Orion, with the Great Nebula, M42, at centre
By 8.00 pm in early December, a brilliant white star will be seen rising about one handspan to the south and lower than Orion.
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 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. The star at the root of the Dog's tail is called
Wezen, and the one at the tip of his
tail is called Aludra. The star Adhara marks his back foot. The Dog's head is shown by a faint outline of fourth and fifth magnitude stars. 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 on a night of good 'seeing', as
it is very close to brilliant Sirius and is usually lost in the glare.
Canis
Major as it appears in the east soon after sunset in December
This small constellation 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. Procyon rises in the east soon after 9.00 pm at the beginning of December.
Taurus, the Bull:
What is Orion hunting? About a handspan to the left of Orion's Belt is a bright orange star, Aldebaran. It is the brightest star in the constellation Taurus, the Bull. This orange star appears at the foot of a cluster of stars with the appearance of a capital A or inverted V. This cluster is called the Hyades, and seemed 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. Actually, Aldebaran is not physically connected with the Hyades cluster, being much closer, but in the same line of sight.
Half a handspan to the left of the Hyades cluster, and a little higher above the horizon, is another cluster, smaller and fainter. This is the Pleiades, a small group like a question mark. It is often called the Seven Sisters, although excellent eyes are needed to detect the seventh star without binoculars or telescope. 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 single gas cloud or nebula. There are actually more than 250 stars in the Pleiades, and this cluster marks the Bull's shoulder.
T
he Pleiades is the small cluster at centre left, while the Hyades is the much larger grouping at centre right.
Wisps of the nebula which surrounds the Pleiades can be seen around the brighter stars in the cluster.
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 Hevelius named it '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 part of 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.
Mira near minimum, 26 September 2008 Mira near maximum, 22 December 2008
Astronomers using a NASA space telescope have spotted an amazingly long comet-like tail behind Mira as the star streaks through space. The 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, including pictures
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 (Foot
of the Centaur, or 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 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 faster it burns its nuclear fuel and the shorter its life.
Close-up of the star field around Proxima Centauri
Knowing the orbital period of the two brightest stars A and B, we can apply Kepler’s Third Law to find the distance they are apart. This tells us that Alpha Centauri A and B are about 2700 million kilometres apart or about 2.5 light hours. This makes them a little less than the distance apart of the Sun and Uranus (the orbital period of Uranus is 84 years, that of Alpha Centauri A and B is 80 years.)
Albireo (Beta Cygni) is sometimes described poetically as a large topaz with a small blue sapphire. It is one of the sky’s most beautiful objects. The stars are of classes G and B, making a wonderful colour contrast. It lies at a distance of 410 light years, 95 times further away than Alpha Centauri.
Binary stars may be widely spaced, as the two examples just mentioned, or so close that a telescope is struggling to separated them (Acrux, Castor, Antares, Sirius). Even closer double stars cannot be split by the telescope, but the spectroscope can disclose their true nature by revealing clues in the absorption lines in their spectra. These examples are called spectroscopic binaries. In a binary system, closer stars will have shorter periods for the stars to complete an orbit. Eta Cassiopeiae takes 480 years for the stars to circle each other. The binary with the shortest period is AM Canum Venaticorum, which takes only 17½ minutes.Sometimes one star in a binary system will pass in front of the other one, partially blocking off its light. The total light output of the pair will be seen to vary, as regular as clockwork. These are called eclipsing binaries, and are a type of variable star, although the stars themselves usually do not vary.
Why are some constellations bright, while others are faint ?
If we look at ninety degrees to the plane, either straight up and out of the galaxy or straight down, we are looking through comparatively few stars and gas clouds and so can see out into deep space. These are the directions of the north and south galactic poles, and because we have a clear view in these directions to distant galaxies, these parts of the sky are called the intergalactic windows. The southern window is in the constellation Sculptor, not far from the star Fomalhaut. This window is well-placed for viewing this month, and 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 below the horizon in the evenings this month.
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.
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. Between the zenith and the south-western horizon are a number of small, faint constellations, Phoenix, Hydrus, Reticulum, Indus and Pavo. Clustered around the South Celestial Pole are Dorado, Octans, Apus, Chamaeleon, Mensa and Volans.
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 orange star at the top of the Cross (Gacrux) to the star at its base (Acrux) and continue straight on towards the south (to the left) 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 in the south-west to Achernar in the south-east. Bisect this imaginary line to locate the pole.
Neither of these methods will work in Queensland during December evenings as both the Southern Cross and Beta Centauri are below the horizon. However, they will have risen by 1 am.
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 seem to move during the exposure, being recorded on the film as short arcs of a circle. The arcs will be different colours, as 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 constellation Scorpius, just above the scorpion's sting. It lies in the direction of our galaxy's centre. The cluster itself is the group of white stars in the centre of the field. Its distance is about 380 parsecs or 1240 light years. M7 is not visible tonight.
Galactic Cluster M7 in Scorpius
Outside the plane of our galaxy, there is a halo of Globular Clusters. These are very old, dense clusters, containing perhaps several hundred thousand stars. These stars are closer to each other than is usual, and because of its great distance from us, a globular cluster gives the impression of a solid mass of faint stars. Many other galaxies also have a halo of globular clusters circling around them.
The largest and brightest globular cluster in the sky is
Although Omega Centauri is below the horizon in the evenings this month, there is another remarkable globular, second only to Omega, which is in a good position. Close to 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. Some regard this cluster as being more spectacular than Omega Centauri, as it is more compact, and the faint stars twinkling in its core are very beautiful. This month, 47 Tucanae is best placed for viewing at 8 pm.
The globular cluster 47 Tucanae
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 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.
* M7: This number means that this galactic cluster in Scorpius is No. 7 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:
At 10 pm on December 1, two faint smudges of light may be seen high in the south. 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 of the SMC, and is noticeably larger. They lie at distances of 190 000 light years for the LMC, and 200 000 light years for the SMC. They are about 60 000 light years apart. These dwarf galaxies circle our own much larger galaxy, the Milky Way. The LMC is slightly closer, but this does not account for its larger appearance. It really is larger than the SMC, and has developed as an under-sized barred spiral galaxy.
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 in the northern half of the sky, 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.
The Andromeda Galaxy and the President
of the United States:
In 1901, U.S.A. President William McKinley was assassinated and his Vice-President,
Teddy made the acquaintance of noted American naturalist, scientist, explorer and author
It is said that after dinner, Roosevelt, Beebe and their friends would step outside for cigars and lengthy discussions about world affairs. At the conclusion, they would look up at the starry sky. Roosevelt or Beebe would point out a small, faint smudge of light close to the Great Square of Pegasus and they would both recite, almost as a litany, something similar to the following:
"That is the Spiral Galaxy in Andromeda. It is as large as our Milky Way. It is one of a hundred million galaxies. It consists of one hundred million suns, many larger than our sun." The President would then turn to the others. "Now I think we are small enough," he would say. "Let's go to bed."
Whereas from the latitude of Washington D.C. the Andromeda Galaxy is visible for most of the year, from Australia it is so far north (41 degrees north declination) that it is only visible in the evenings during spring and early summer. For us, this magnificent galaxy is due north at 8 pm on December 1, about one handspan above the horizon.
The Great Galaxy in Andromeda, M31, photographed at Starfield Observatory with an off-the-shelf digital camera on 16 November 2007.
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