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January

The Sun

Lying at a distance of 30 light years from the centre of the Milky Way galaxy the Sun takes over 200 million years to complete one orbit. It is the central body of the Solar System, containing 99.8% of its mass, and is the nearest star to the earth lying at an average distance of 149,600,000 km. Without its energy life on earth would not be possible. With a diameter of 1,392,000 km the Sun is classed as a yellow dwarf star that is 4.6 billion years old and is about half way through its life cycle. It will eventually expand into a red giant star and finally collapse to a white dwarf. The effective surface temperature of the Sun is around 5,800 degrees C while the centre of its core experiences temperatures of 15,000,000 degrees C. This core is extremely dense and although it contains only 1.5% of the volume of the Sun it contains over half of the Sun's mass.

The Sun is composed mainly of hydrogen (70%) and helium (28%) with about 2% of heavier elements. It generates its energy by the nuclear fusion process and is losing mass at a rate of 4 million tonnes every second. It can take as long as 20 million years for the energy created in the core to reach the Sun's surface. This energy is given off as heat and light and a low density stream of charged particles known as the solar wind which blows throughout the solar system at speeds of 450km/second.

Because the Sun is virtually a ball of gas it rotates at different speeds at different latitudes. At the equator the rotational period is around 25.4 days and near the poles the period is closer to 36 days. It has a period of energy fluctuations from high to low and back to high again. This is known as the sunspot cycle and is roughly 11 years long. The Sun's magnetic field reverses after each sunspot cycle and so takes 22 years to get back to where it was before. At the moment the solar cycle is at one of its low points and there are only a few small sunspots to be seen.

WARNING! Under NO circumstances try to observe the Sun without the proper solar filters.

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February

High in the northeast lies the constellation of Orion, The Mighty Hunter, with his three belt stars forming the base of the asterism we call the saucepan and his sword its handle. Check out the middle "star" of the handle with binoculars or a telescope, if you have one, and you will find that it is not a star at all but a great cloud of Hydrogen gas inside which new stars are being formed. These stars are lighting up the cloud and making it glow like a fluorescent light. The bright star to the south of the belt stars is the star Rigel, a blue-white super-giant, which lies 773 light years away. To the north of the belt stars lies Betelgeuse, a red super-giant star which is 400 times larger than our sun and lies at a distance of 427 light years. Note the colour contrast between these two stars.

Mythology has it that Orion was stung to death by a scorpion and is now forever pursued across the sky by this creature. As Orion sets in the west Scorpio rises in the east.

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March

Gemini-The Twins

One of the twelve constellations of the Zodiac, Gemini has the Sun passing through it from late June to late July. It lies between Taurus-The Bull in the west and faint Cancer-The Crab to the east. As darkness falls at the beginning of March its two brightest stars, Pollux and Castor can be seen high in the northern sky and 4 degrees apart. The southernmost and the brighter of the two, Pollux is an orange giant star and the nearest giant star to the Earth lying at a distance of only 34 light years. It appears yellowish to the naked eye.

Castor, the second of the twins is 52 light years away and is actually a system of six stars all bound to each other by gravity forming one of the most remarkable examples of a multiple star system in the heavens. To the naked eye Castor appears as a blue-white star but even small telescopes will separate the two main stars of this system into individual stars. These two stars orbit a common centre of gravity once every 470 years.
Castor and Pollux represent the heads of the twins and their bodies are depicted by two parallel lines of fainter stars heading off in a westerly direction like two stick figures. The constellation is best observed in a dark moonless sky away from the town lights.

The planet Mars is visiting the constellation at the moment and can be found around the feet of the twins for the whole month. Gemini also contains quite a few nice star clusters that are worth a look at through binoculars and the Geminid meteor shower appears to radiate from this constellation from December 7th to December 17th. The planet Uranus and dwarf planet Pluto were both in Gemini when discovered.

  Star Chart for Gemini  

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April

Virgo-The Virgin

The constellation Virgo is the second largest constellation in the sky and the only female among the constellations of the Zodiac. It lies between the constellations of Libra-The Scales and Leo-The Lion. She is often seen as a beautiful and virtuous maiden, the Roman goddess of justice, with neighboring Libra representing her scales of justice. The Sun passes through Virgo from mid-September to early November and thus is within Virgo's boundaries as the Sun crosses the celestial equator on its southward journey at the beginning of Spring in the southern hemisphere.

The constellations brightest star, Spica is the sixteenth brightest star in the sky and is a blue white star with a companion that passes in front of it every four days. This eclipsing star lowers the brightness of the main star very slightly but not enough to be readily noticeable with the naked eye. Spica is twice the size of the Sun but is in fact two thousand times brighter than the Sun and lies at a distance of 262 light years from The Earth. At the beginning of April it rises at 6:00pm and is visible all night long, crossing the meridian just after midnight about 60 degrees above the northern horizon. Don't be fooled by another star Arcturus which is much brighter and 32 degrees closer to the northern horizon.

Virgo contains a rich cluster of galaxies, some of which can be seen in moderately sized telescopes. This cluster is the nearest major galaxy cluster to us and is 65 million light years away containing around 3000 members.

  Star chart for Leo  

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May

Comets

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June

The Magellanic Clouds

Looking like fragments that have broken off the Milky Way and lying very close to the South Celestial Pole, the Magellanic Clouds are only clearly visible from the southern hemisphere. They are two of the nearest known galaxies to the Milky Way Galaxy and are easily visible to the naked eye high in the southern night sky on a moonless night. They were first reported in Europe by the survivors of Ferdinand Magellan's voyage around the Earth in 1615 but were not given their present names on star maps until much later. Their formal astronomical names are "Nebecula Major" and "Nebecula Minor". Though they appear quite close together in the sky they are in fact around 75,000 light years apart. There is a stream of cold hydrogen gas being pulled from the large cloud toward the Milky Way Galaxy, the result of a close passage of the Milky Way more than 200 million years ago. The stream contains about 1 billion times as much mass as our Sun. Until recently they were thought to be satellite galaxies of the Milky Way but current studies of their radial velocity seem to indicate that they are passing by at a rate of 480km/second, much too fast to be captured by the gravity of the Milky Way.

The large cloud lies at a distance of 160,000 light years and is located in the constellation Dorado-The Goldfish. It has a diameter of roughly 32,000 light years and contains around 15 billion stars. To the naked eye it appears as a fuzzy patch of light 12 times the apparent diameter of the moon. Astronomers around the world were excited when on the 24th of February 1987 one of the stars in the cloud exploded into a supernova and became 10,000 times brighter than it normally is. At its brightest it was brighter than all of the stars in the cloud put together and was the first naked eye supernova since 1604.

The small cloud is 230,000 light years away and resides in the constellation Tucana-The Toucan. It contains around 4 billion stars and is visible to the naked eye as a nebulous tadpole shaped patch 3.5 degrees across. Both clouds are visible all year round from the location of Mudgee.

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July

Moon Halos

We tend to take the Moon for granted and most of us hardly give it a second glance. However when atmospheric conditions are right the moon can produce some interesting and rather stunning effects. Perhaps the most common of these effects are the Moon halos, large rings which circle the moon on a hazy cold night and have a radius of either 22 degrees or 46 degrees. The most frequently seen seem to be the 22 degree halos. They are more intense when the Moon is at its brightest, around full Moon but can be witnessed to a lesser degree at most phases of the Moon. For the rings to appear there needs to be a layer of thin cloud containing millions of tiny ice crystals covering much of the sky high in the upper atmosphere. Each of these ice crystals behaves like a small lens and because most of these crystals have the same elongated hexagonal shape the moonlight entering one face of the crystal and exiting its opposing face is bent at an angle of 22 degrees. The larger 46 degrees ring which is far less typical may be caused by the light exiting at different angles from the crystals.

A much smaller ring, usually about 3 degrees in radius can often be seen around the Moon and this is known as a Lunar Corona. They are also caused by high thin cloud but the moonlight is being refracted by water droplets in the clouds and not by ice crystals. On rare occasions Moon Halos and Lunar Coronae can be seen together. The Lunar Coronae have been known to swell or shrink slightly depending on the size of the water droplets that are present in the passing clouds. There can also be colours present on the coronae but these colours are always very subtle and more likely to be obvious when the Moon is full.

So the next time you are walking home at night or just sitting out under the night sky check it out. If there is a bright Moon up there at the time it is well worth a look.

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August

The Moon

The only natural satellite of the Earth and the fifth largest natural satellite in the Solar System, the Moon is visible by virtue of reflected sunlight. It has a diameter of 3,476 km and lies at an average distance of 384,400 km from the Earth taking 27.322 days to complete one orbit at a speed of just over 1 km/second. This is the same amount of time it takes to spin once on its own axis and so always shows the same face to the Earth. Because the Moon wobbles slightly as it orbits we get to see about 59 percent of its surface. Since the Earth is moving in its orbit around the Sun as the Moon orbits the Earth it takes a little longer to go through its cycle of phases as seen from the Earth. From one full Moon to the next a period of 29.5 days elapses. Each spot on the Moon is subjected to two weeks of daylight during which surface temperatures reach 100 degrees Celsius and two weeks of darkness when the temperature plummets to -170 degrees . The daily rotation of the Earth and the slower eastward revolution of the moon in its orbit usually produce two high tides and two low tides in the Earth's oceans every 24 hours 50 minutes.

Traces of argon, helium, oxygen, methane, nitrogen, carbon monoxide and carbon dioxide have been detected in the Moon's atmosphere but this atmosphere is so thin as to be almost negligible. These gasses are, in part, the result of micrometeorites striking the surface of the Moon but because of the Moon's low gravity, about 17 percent that of the Earth's; they are very quickly lost into space. It is because of this lack of atmosphere that the face of the Moon hasn't changed significantly for millions of years. The temperature at the center of the Moon is around 1500 degrees Celsius and its core, consisting mainly of metallic iron alloyed with a small amount of sulfur and nickel, is around 1200 km in diameter. Directly above the core lies the mantle, 1100 km thick and composed mainly of iron, magnesium and silicates and this is covered by the crust with a depth of 60 km which consists primarily of calcium, aluminium and silicates.

The dark lowland plains are known as Maria, Latin for seas, because the first observers imagined them to be oceans of water. The name persists today even though we know that the Moon is waterless. The lunar highlands are named after mountain ranges on the Earth and so we have the lunar Alps and the Apennines. Because of the lack of atmosphere on the Moon there can be no twilight and the line, known as the terminator, dividing the night and day sides of the moon has bright daylight on one side and pitch blackness on the other. At times when viewed through a telescope sunlit peaks can be seen protruding from the night side of the terminator.
The origin of the Moon is still being debated with the Giant Impact Theory seeming to have taken favoritism. This proposes that a glancing high speed collision of a huge possibly Mars sized object with the primitive Earth, smashed what crust the Earth had and blasting it into space. The ejected material condensed to form a ring of orbiting debris and the Moon coalesced from this. It is our nearest celestial neighbor and so far the only other world upon which mankind has so far set foot.

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September

The Zodiacal Light

The Zodiacal light is often confused with the brightness of the sky due to a lingering twilight as it occurs in roughly the same area of the sky. It is produced by sunlight reflecting off cosmic dust particles in orbit around the Sun. Most of the particles that can be seen have sizes in the range from 10 to 100 micrometers and are produced by decaying comets and colliding asteroids and are spiraling slowly in towards the solar surface. The light is seen as a cone of light about 20 to 30 degrees wide at the horizon and on a clear night can extend halfway to the zenith. It is most noticeable when the Sun is more than 18 degrees below the horizon and astronomical twilight has ended. The best months to observe the Zodiacal light from southern latitudes are from September into November in the evening sky and February and March in the morning sky. These are the months when the ecliptic, the imaginary line plotted in the sky by the Sun's path, is at its largest angle to the horizon. A clear moonless night away from light pollution is a must to observe this delicate phenomenon.

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October

Asteroids

There seems to be two main theories as to how the asteroids came into existence. The first suggests that they are the remnants of a planet destroyed in a massive collision long ago and the second, and it seems the most popular, is that they are left over rocky matter that failed to coalesce into a planet during the formation of the Solar System. Most known asteroids (about 95 percent) orbit in the asteroid belt, or main belt, between the orbit of Mars and Jupiter at distances of 1.7 to 4.0 astronomical units from the Sun. (One astronomical unit (AU) is equivalent to the distance from the Earth to the Sun or 149,597,870 kilometers). These main belt asteroids orbit the Sun in periods between 3 and 6 years. Like the planets, most asteroids follow direct orbits within 10 degrees of the plane of the ecliptic (The average plane of the Earth's orbit around the Sun) although in a more pronounced ellipses than the orbits of the planets. Other asteroids have very different orbits from these main belt objects and indeed some, the Atens group even cross the orbit of the Earth and are the subject of close scrutiny by some of the world's major observatories. Another group, the Trojans, orbits the Sun 60 degrees ahead and behind Jupiter in its orbit.

Asteroids vary in size from the largest, the 933 km diameter Ceres, to objects much less than 1km wide. Ceres however has now been taken from the list of asteroids and classified as a "dwarf planet" along with Pluto. It was discovered on the first day of January 1801 by Giuseppe Piazzi and at first was thought to be a comet. They shine by reflected sunlight and although there have been several hundred thousand discovered only one, Vesta, is sometimes bright enough to be seen with the naked eye. Even though it is not the largest of these objects it has a surface that reflects sunlight better than any of the others. Despite the great number of asteroids so far discovered their total mass is less that of the Earth's Moon.

As a footnote to this article I must congratulate local Mudgee author, amateur astronomer and astrophotographer, Steve Quirk who has had an asteroid named after him. The asteroid, 18376 Quirk, was discovered on September 30th 1991 by Rob McNaught at Siding Spring Observatory. It is a main belt object that orbits the Sun every 4.5 years at a distance of 2 astronomical units to 3.3 astronomical units from the Sun.

The Orionid Meteor Shower

Don't forget the Orionid meteor shower which is active in October with the best nights around October 21st.

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November

The Constellations

The sky is divided into 88 sections, known as constellations, which simplify the locating and naming celestial objects. The main constellations of the sky were devised at the dawn of history by Middle Eastern people who imagined that they could see a likeness to certain fabled creatures and mythological heroes among the stars.

The largest constellation in the sky is Hydra-The Water Snake which winds its way over 100 degrees of the sky. It is by no means easy to identify on account of its faintness and apart from its brightest star Alphard, which marks the heart of the water snake, the only other recognizable feature is its head, made up of an attractive group of six stars lying just south of the constellation Cancer-The Crab. The smallest of the constellations is Crux-The Southern Cross which is one of the most recognizable and celebrated constellations in the heavens. Very few constellations look like the creature or object that they are named for but they are mainly meant to honor, not represent.

Our Modern constellations derive from a list of 48, recognized by the Greek astronomer Ptolemy in 150 AD. This list was expanded on by navigators and celestial map makers, notably the German Johann Bayer (1572-1625), the Pole Johannes Hevelius (1611-1687) and the Frenchman Nicolas Louis de Lacaille (1713-1762). Lacaille introduced 14 new constellations, named after instruments used by scientists and artists, in parts of the southern sky not visible from Mediterranean regions; other astronomers invented constellations to fill in the gaps between the figures recognized by the Greeks. The whole process sounds rather arbitrary, and indeed it was. A number of the newly devised patterns fell into disuse, leaving a total of 88 constellations that were officially adopted by the International Astronomical Union, astronomy's governing body, in 1930. Some of the brighter stars have proper names and these names are mainly of Arabic origin. The stars in each constellation are also labeled with a letter of the Greek alphabet, the brightest star usually (though not always) being termed α (alpha). Notable exceptions include the constellations Gemini and Orion in which the brightest stars are in fact marked β (beta) and because of the breaking up of the ancient constellation Argo Navis-The Ship into three smaller constellations, two of the resulting constellations namely Vela-The Sails and Puppis-The Stern have no alpha or beta stars.

There is no need to learn all of the constellations visible from your location on the Earth but learning to recognize some of the more familiar ones will be a great help in finding your way around the night sky. The more you look the more you will see and the greater will be your enjoyment of the magnificent starry spectacle. There are books available from the observatory or from good book shops that will help you to navigate and become familiar with the star patterns and will open up a whole new relaxing pastime.

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December

Mercury

Since the reclassification of Pluto from the "Planet" to the "Dwarf Planet" category, Mercury has now become the smallest of the main Solar System planets. The closest planet to the Sun, Mercury is a rocky planet with a diameter of 4,878 km orbiting the Sun in 87.97 days at an average speed of 47.89 km/second. Its distance from the Sun varies from 46,375,318 km to 70,310,966 km and has the largest known difference in daytime-nighttime temperatures in the Solar System. The sunlight striking Mercury when it is at its closest point to the Sun is 2.3 times as intense as it is at the most distant point in its orbit. Because it has almost no permanent atmosphere to shield it from the sunlight during the day and hold the heat during the night its daytime temperature reaches 447 degrees C and plummets to minus 153 degrees C at night. Strangely Mercury's day is longer than its year. The solar rotation period, noontime to noontime or one Mercurian day is 175.84 Earth days which equals two Mercurian years. Photography of half of Mercury's surface by Mariner 10 in 1974 and 1975 showed it to resemble the Moon in being heavily cratered with intervening areas of lava flooded plains with the largest impact feature detected being 1,300 km in diameter. Whereas the features on the Moon are named after scientists and astronomers, the features on Mercury are named after world cultural figures such as architects, composers, writers and artists. As Mercury orbits so close to the Sun it can only be observed from the Earth in either morning or evening twilight and at times can be quite elusive as at its best only ventures 28 degrees in angular distance from the Sun. Transits of Mercury across the Sun's disc as seen from the Earth are reasonably common but can only occur in the months of May or November with the intervals at which they occur being 3, 7, 10 or 13 years. The next will occur on May 9th 2016.

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