January
2019
Updated: 22 January 2019
Welcome to the night skies of Summer, featuring Andromeda, Cetus, Aries, Taurus, 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 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 zodiacal constellation of Sagittarius, the Archer. It leaves Sagittarius and passes into Capricornus, the Sea-Goat on January 20.
Partial Solar Eclipse, January 06:
There will be a partial solar eclipse occurring on this date. As its path crosses the north-western Pacific Ocean, it will not be visible from Australia.
Total Lunar Eclipse, January 21:
There will be a total lunar eclipse occurring on January 21. As the eclipse will occur in our daylight hours and will end an hour before moonrise, it will not be visible in Australia.Moon Phases:
February
Moon at 8 days after New, as on January 15.
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 pair of craters named after two Greek scholars living in the fourth century BCE, both being students of Plato and enrolled at his mathematical School of Philosophy at Akademia, a suburb of Athens. (Ever since, any advanced school has been called an 'academy'.) These two students were named Aristoteles and Eudoxus.
The largest crater in this picture is Aristoteles (90 km diameter), and south of it lies Eudoxus (70 km across). This image was taken at 7:37 pm on 18
September 2018. North is to the top, east to the right.
These two craters lie to the east of the Mare Imbrium (Sea of Rains) and north of the Mare Serenitatis (Sea of Serenity). Both are surrounded by melted rock that splashed out from the original impacts. Both have terraced walls where large sections of the walls have broken off the rim and slid down to the crater floor. Each of the two craters has a cluster of small mountains at its centre, which were formed when the shock wave from the impact travelled down to the Moon's bedrock, where it was reflected back up, lifting the newly-formed crater floor and fracturing it into blocks. Magma welled up through the fractures and pooled around the blocks where it cooled into a lava plain, giving each crater a flat floor with the tops of the blocks protruding through as a cluster of mountains. More magma welled up in Aristoteles, because it was a larger impact.
Eudoxus
The philosopher Eudoxus of Cnidus (ca. 410-347 BCE) was a student of Plato. who had taught him that the Earth was the centre of the universe, and the Sun, Moon, five planets and fixed stars circled around it on eight transparent, crystalline spheres, once per day. Plato eventually realised that this concept did not adequately explain the movements of the planets, whose paths against the starry background (the outermost 'sphere of fixed stars') were not regular but included loops, S-bends and variations in brightness. He asked his students to add extra spheres to improve the predictions of planetary movements. Eudoxus devised a nest of spheres for each planet, rotating at different speeds and at different angles. The Sun and Moon were controlled by three spheres each, the five planets by four each, and there was one sphere to which the stars were fixed, enclosing the rest. This made 27 concentric crystalline spheres in all, with the Earth at the centre. This became known as the Homocentric Spheres world system. Eudoxus also measured the length of a year as 365 days and 6 hours.
Eudoxus wrote two books on spherical astronomy, Phaenomena (Appearances) and Entropon, which contained lists of star positions. These were measured from his home at Cnidus and from Egypt during his travels there. We know that about twenty of the Assyrian, Chaldean and Sumerian constellations as recorded on stone by Babylonian astrologers (including the twelve of the Zodiac) had reached the Greek city-states by 450 BCE, as Eudoxus described them in his Phaenomena. Another ten had the same stars but different names, e.g. the Assyrian Swallow equated to the Greek Pisces. About eighteen Greek constellations were home-grown, including Hercules, Leo, Draco, Ophiuchus, Serpens, Delphinus, Andromeda, Perseus, Cepheus, Cassiopeia and Cetus. Most of these constellations were quite ancient, and had originated during a 400-year period around 2000 BCE. Eudoxus described the names and positions of a total of 43 constellations, and mentioned other objects in the sky as well. Although his Phaenomena has since been lost, in the following century the astronomer-poet Aratus of Soli (southern Turkey, 315-240 BCE) rewrote it in verse. This long, didactic, hexameter poem of 732 stanzas, also named Phaenomena, was intended, like the original, to be an introduction to the 43 constellations, which it listed and described. It was popular at the time and has survived, passing down to us not only the constellations, but also the proposed homocentric spheres of Eudoxus.
In the fourth century BCE, the philosopher Aristotle (384-322 BCE) studied natural history, philosophy and the natural sciences,
and his teachings and writings had an immense influence on future scientific thought. Aristotle (not his real name: it means "the best purpose") was a pupil of Plato
and knew Eudoxus. Later, as tutor to Alexander the Great, he became wealthy – legend has it that Alexander gave him an elephant as a present. Aristotle wrote the
Meteorologica, the first book on the weather, and described in detail an underwater device called a ‘diving bell’. Aristotle taught that people have five
senses (sight, hearing, taste, smell and touch), something still accepted today despite commonly experienced other senses such as heat, cold, hunger, thirst, balance, fear,
pleasure, pain etc. In astronomical matters, he supported the view that the Earth was spherical, but
agreed that it was at the centre of the universe, surrounded
by transparent spheres made of aether or 'quintessence', which carried the Sun, Moon, planets and stars around the Earth each day.
In his final conception (right), Aristotle placed an extra sphere outside the starry sphere that he called the Primum Mobile. This ‘First Moved’ sphere was black and opaque. It rotated east to west once in 23 hours 56 minutes at a constant angular velocity, and its motion turned the sphere of fixed stars at the same speed (a sidereal day). This motion was imparted downward from sphere to sphere, thus causing the whole universe to rotate. There was a little ‘lost motion’ or ‘slippage’, so that each main lower sphere turned slightly slower than the one above it. Two exceptions to this were Mercury and Venus, whose spheres rotated as one with the Sun’s sphere. This meant that each main sphere took a little longer to complete one rotation than the next one out, which accounted for the eastward drift of the seven planets through the constellations at varying speeds. The main sphere of the Sun took exactly 24 hours to complete a rotation, and the main sphere of the Moon about 24 hours 50 minutes.
This concept was later embraced by Christians, as they equated the Prime Mover with God.
We know that, despite his fixation with homocentric spheres, Aristotle reached some correct conclusions with regard to other astronomical matters. He observed
lunar occultations, and deduced that the Moon was the nearest sky object. He also found that the Moon was smaller than the Earth and agreed that it shone by
reflected sunlight. On the other hand, he mistakenly taught that heavier objects fall faster than light ones, stating categorically that an object ten times
heavier than another would fall ten times as fast. (No-one thought to test this by experimentation until the time of Galileo.)
His ideas were a blend of scientific information and philosophical argument, but were biased by his preconceptions of what constituted a good and perfect universe. We must admit that his hypotheses did allow planetary movements to be explained, calculated and predicted with an accuracy acceptable for his time. Although mistaken in many of his beliefs, he was searching after truth, and was considered the ultimate, common sense authority on science. It is a pity that Aristotle dismissed out of hand the case for a heliocentric (Sun-centred) universe whenever suggested by the followers of Pythagoras, Heracleides, and Plato in his old age, saying dogmatically that it did not fit the observations. Aristotle's erroneous ideas received much support from Ptolemy in his Almagest, and were embraced by Arab astronomers after 350 AD. These beliefs returned to Europe in the Middle Ages, and became the 'accepted wisdom' for astrologers.
The idea of crystalline spheres holding the Sun, Moon and planets in place while they circled around the Earth, was widely believed by the general public as the true model of the cosmos. The system was found by the Christian Church to be in accord with its religious beliefs, for they equated the 'Prime Mover' with God, and the Bible said that the Earth was fixed and could not be moved (Psalm 96.10). Therefore they gave it their full support. By AD 1300, such ideas took on a new power as the cosmogonies of Aristotle and Ptolemy (newly rediscovered in Europe after the Dark Ages) had been carefully melded with medieval theology. This blending of Christianity and ancient pagan Greek beliefs was undertaken by philosopher-theologians such as the Dominican friar Thomas Aquinas (AD 1225-1274) in his book Summa Theologiae (Principles of Belief), written between 1265 and 1274. In this book, he explains how he believes that Heaven (he calls it the "Empyrean") lies outside the Primum Mobile. The Empyean is described by him as "the Cœlum Empireum Habitaculum Dei et Omnium Electorum (Heavenly empire inhabited by God and all the Elect)". He explains how the crystalline spheres are pushed by angels to keep them moving as follows: the Seraphim (Guardians of God's throne) push the Primum Mobile, the Cherubim push the sphere of 'fixed' stars, the Thrones push Saturn, Dominions push Jupiter, and Principalities push Mars. Potentates (or Powers) push the sphere of the Sun, Virtues push Venus, Archangels push Mercury, and Angels such as Michael push the Moon. Thomas
Aquinas believed that the movements of the stars and planets were physical evidence of the angels, and were visible manifestations of God's will and control. The position of the Earth at the centre of the universe indicated the importance of humanity to God.Galileo Galilei, a Professor of Mathematics at the University of Padua near Venice, was the first to make a telescope that was not merely a toy but gave a useful magnification of 20 (later, 30) times. He turned it to the sky in late 1609 and found that Copernicus had been correct when he wrote in 1542 that the Earth travelled around the Sun and not the other way round as Aristotle had taught and the Churches believed. When he published this opinion in a book, A Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican, some priests led by Father Tommaso Caccini reported him to the Holy Inquisition, which tried him for heresy in 1632. Pope Urban
VIII, a one-time supporter and friend of Galileo, was angered and had Galileo placed under house arrest for life for publicising this opinion. (For more about Plato's concept of the universe and Copernicus' revolutionary theory, click
The photograph of Aristoteles and Eudoxus covers the area inside the rectangle above.
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.
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
February 2: Moon 1.4º north of Saturn at 18:08 hrs
February 2: Moon occults the star Pi Sagittarii (mv= 2.88) between 20:08 and at 20:41 hrs
February 3: Limb of Moon 42 arcminutes north of Pluto at 04:31 hrs
February 4: Saturn 24 arcminutes south of Omicron Sagittarii (mv= 3.76) at 07:02 hrs
February 5: Limb of Moon 7 arcminutes north of Mercury at 18:44 hrs
February 5: Limb of Moon 34 arcminutes north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 23:31 hrs
February 7: Mercury 45 arcminutes north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 10:16 hrs
February 7: Moon 2.3º south of Neptune at 20:01 hrs
February 11: Moon 5.5º south of Mars at 06:04 hrs
February 11: Moon 4.6º south of Uranus at 08:25 hrs
February 13: Mars 1º north of Uranus at 15:25 hrs
February 14: Moon 1.8º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 11:52 hrs
February 15: Moon occults the star Zeta Tauri (mv= 2.97) between 13:53 and 14:30 hrs
February 16: Saturn 1º south of the star Pi Sagittarii (mv= 2.88) at 04:14 hrs
February 16: Limb of Moon 43 arcminutes south of the star Mu Geminorum (mv= 2.87) at 09:11 hrs
February 18: Venus 6 arcminutes north of the star Pi Sagittarii (mv= 2.88) at 15:36 hrs
February 18: Venus 1.1º north of Saturn at 20:36 hrs
February 19: Mercury 40 arcminutes north of Neptune at 16:19 hrs
February 23: Venus 1.4º north of Pluto at 13:58 hrs
February 23: Jupiter 1.4º south of the star xi Ophiuchi (mv= 4.38) at 21:10 hrs
February 25: Mercury at perihelion at 18:02 hrs (diameter = 6.9")
February 27: Mercury at Greatest Elongation East (18 05') at 07:27 hrs (diameter = 7.2")
February 27: Moon 2.9º north of Jupiter at 23:06 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. On January 1 it will be about 16 degrees (a little less than a handspan) from the Sun, so may be difficult to find due to the brightness of the solar glare. Look above the eastern horizon prior to sunrise. The thin crescent Moon will be to the left of Mercury and below on the morning of January 5. Both will be near the horizon, in the constellation of Sagittarius. Mercury will pass through superior conjunction (on the far side of the Sun) on January 30, and will then return to our western twilight sky. It will become easily visible in late 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'.
It will be at greatest elongation west (maximum angular distance from the Sun)
on January 6. Currently in the constellation
Libra, it will
move into an outstretched claw of Scorpius on January 9, and then into the
non-zodiacal constellation of Ophiuchus on January 14. It will enter Sagittarius
on January 31. The waning crescent Moon will be close by Venus on the mornings of
January 2 and February 1. 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 now poorly positioned for viewing, as the Earth has left it far behind. Mars is much reduced in size, being only
7 arcseconds in
diameter on January 1. By January 31 its diameter will have fallen to 6 arcseconds, and during the month its phase will increase slightly from 87.3% to 89.3%. On
January 1 at
7:45 pm (end of twilight), Mars will be about 45º or two-and-a-half handspans
above the west-north-western horizon, in the constellation of Pisces. By January 31 at end of
twilight, Mars will be about 30º or one-and-a-half handspans above the west-north-west horizon,
still in the constellation Pisces.
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 and is now in the pre-dawn eastern sky. On January 1 it will be in the constellation Ophiuchus, and rising at 3 am. By the first light of dawn it will be about half-a-handspan above the east-south-eastern horizon. As the month progresses, Jupiter will become higher in the sky as dawn approaches, and on January 31 it will be two handspans above the horizon at 4:20 am (the beginning of dawn). On that date the waning crescent Moon will be to Jupiter's left.
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 seen this month as soon after sunset as possible. In mid-January it can be found
one handspan above the west-north-western horizon at 8 pm.
The thin, waxing crescent Moon will be just below and to the left of Neptune on
January 10.
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
impossible to observe
this month as it is very close to the Sun, reaching conjunction on January 11. 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 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.
Quadrantids
January 4
Waning crescent Moon, 4% sunlit ZHR =
95
Radiant: Near the star Vega. Named after the obsolete constellation Quadrans Muralis (Mural Quadrant)
Alpha Centaurids February
8
Waxing crescent Moon,
Moon, 9% sunlit ZHR =
10
Radiant: Near the star
Alpha Centauri.
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'. On an 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:
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 will be 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 will be at its closest approach to Earth on November 27 and closest approach to the Sun on December 4. The best time to observe it from November 13 to November 18 will be from 4 am to the first light of dawn, close to the due east horizon, and a little over half a handspan to the left of Venus. As the days go by and it becomes closer to the Sun, it will become lost in the solar glare. Visit the November 12 and subsequent editions of Spaceweather for the full story.
Comet 46P/Wirtanen
Last
month, Comet 46P/Wirtanen swept past Earth, making one of the ten closest approaches of a comet to our planet since 1960. It was faintly visible to the naked eye for two weeks. Although Wirtanen's nucleus is only 1.2 kilometres across, its green atmosphere became larger than the Full Moon, and was an easy target for binoculars and small telescopes. It reached its closest to the Sun (perihelion) on December 12, and then headed in our direction. It passed the Earth at a distance of 11.5 million kilometres (30 times as far away as the Moon) on December 16. In the week preceding it was at its brightest at magnitude 4, but this was a cloudy week at Nambour. It passed between the Pleiades and Hyades star clusters on the night of December 19-20, but the light of the almost Full Moon made it difficult to see. It then headed towards the star Capella in Auriga, which it passed on December 24-25. Visit Spaceweather and here for more information and chart
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 21P/Giacobini-Zinner
On September 10, another green comet will make its closest approach to Earth in 72 years. This small but active comet is named Comet 21P/Giacobini-Zinner. The 'P' indicates that it is a periodic comet in an elliptical orbit around the Sun, and returning regularly for us to see. After it passes Earth, it will swing around the Sun and head out towards the furthest point in its orbit, just beyond Jupiter. After 36 years it will head back towards the Sun.
This month it will shine at magnitude 7 so it will be easy to see in small telescopes and binoculars, but not with the unaided eye. It will only be observable in the hour
or so before dawn begins to light the sky, low to the north-east horizon. On September 10 it will be gliding through the stars of the constellation Auriga about 58 million
kilometres from our planet. In the week ahead, it will cross into Gemini and on September 15 it will pass right across the
rich star cluster M35, providing a spectacular photo-opportunity for amateur astronomers. Visit the September 9 edition of
Spaceweather
for details and observing tips.
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 mid-January at 9 pm, the Eta Carinae Nebula can be viewed about a handspan above the south-eastern horizon, above and to the left of the Southern Cross.
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 month hosts the planet Uranus and
Mars.
Mars is the only
bright object in this part of the sky at present, and is currently cruising
through Pisces and shining at magnitude 0.5 with an orange colour. It is similar
in brightness and colour to the star Aldebaran in Taurus (see next
paragraph). Uranus will cross into Aries on February 6 and Mars, travelling
faster, will enter Aries seven days later. Mars will overtake Uranus on February
13 and at that time they will be less than a degree apart, visible together in a
small telescope's field of view. 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
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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
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
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
Observers aiming their telescopes towards the SMC generally also look at the nearby 47 Tucanae, but there is another globular cluster
nearby which is also worth a visit. This is NGC 362, which appears to lie above 47 Tucanae as we see it in
mid-evening this month. It is less than half as bright as the other globular, but this is because it is more than twice as far away.
Its distance is 12.6 kiloparsecs or 41 000 light years, so it is about one-fifth of the way from our galaxy to the SMC. Both
NGC 104 and NGC 362 are always above the horizon for all parts of Australia south of the Tropic of Capricorn.
* M42:This number means that the Great Nebula in Orion is No. 42 in a list of 103 astronomical objects compiled and published in 1784 by Charles Messier. Charles was interested in the discovery of new comets, and his aim was to provide a list for observers of fuzzy nebulae and clusters which could easily be reported as comets by mistake. Messier's search for comets is now just a footnote to history, but his list of 103 objects is well known to all astronomers today, and has even been extended to 110 objects.
** NGC 5139: This number means that Omega Centauri is No. 5139 in the New General Catalogue of Non-stellar Astronomical Objects. This catalogue was first published in 1888 by J. L. E. Dreyer under the auspices of the Royal Astronomical Society, as his New General Catalogue of Nebulae and Clusters of Stars. As larger telescopes built early in the 20th century discovered fainter objects in space, and also dark, obscuring nebulae and dust clouds, the NGC was supplemented with the addition of 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.
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.
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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