By Steve Harris
These pages are intended to help you find your way around the sky this month.
The chart above shows the night sky as it appears on 15th December at 21:00 (9 o’clock) in the evening Greenwich Mean Time (GMT). As the Earth orbits the Sun and we look out into space each night the stars will appear to have moved across the sky by a small amount. Every month Earth moves one twelfth of its circuit around the Sun, this amounts to 30 degrees each month. There are about 30 days in each month so each night the stars appear to move about 1 degree. The sky will therefore appear the same as shown on the chart above at 10 o’clock GMT at the beginning of the month and at 8 o’clock GMT at the end of the month. The stars also appear to move 15º (360º divided by 24) each hour from east to west, due to the Earth rotating once every 24 hours.
The centre of the chart will be the position in the sky directly overhead, called the Zenith. First we need to find some familiar objects so we can get our bearings. The Pole Star Polaris can be easily found by first finding the familiar shape of the Great Bear ‘Ursa Major’ that is also sometimes called the Plough or even the Big Dipper by the Americans. Ursa Major is visible throughout the year from Britain and is always quite easy to find. This month it is in the north east. Look for the distinctive saucepan shape, four stars forming the bowl and three stars forming the handle. Follow an imaginary line, up from the two stars in the bowl furthest from the handle. These will point the way to Polaris which will be to the north of overhead at about 50º above the northern horizon. Polaris is the only moderately bright star in a fairly empty patch of sky. When you have found Polaris turn completely around and you will be facing south. To use this chart, position yourself looking south and hold the chart above your eyes.
Planets observable in the night sky: Uranus and Neptune. Jupiter is visible from around 22:00 until dawn.
EXPLORING THE NIGHT SKY THIS MONTH
GUIDE TO THE NIGHT SKY - December 2014
The night sky looking south on 15th December
The chart above shows the night sky looking south at about 21:00 on 15th December. West is to the right and east to the left. The curved line across the sky at the bottom is the ecliptic. This is the imaginary line along which the Sun, Moon and planets appear to move across the sky. The constellations through which the ecliptic passes are known as the constellations of the ‘Zodiac’. This month the constellations we can see on the ecliptic are: (from west to east) Aquarius, Pisces, Aries and Taurus and Gemini (just off to the east).
The southern sky is now dominated by the conjoined constellations of Pegasus and Andromeda. Pegasus is larger than may be expected which sometimes makes it a little difficult to initially identify. However once it has been identified it is easy to find again in a clear dark sky. It is defined by the ‘Great Square’ with the stars Alpheratz, Scheat, Markab and Algenib at its corners. The constellation of Andromeda is joined to the upper left star of the Square of Pegasus called Alpheratz. From Alpheratz there are two fairly distinct lines of stars diverging away to the north east (upper left). Each of the lines is marked by four stars although the lower line is more distinct than the upper line.
Wrapped around the lower parts of Andromeda and Pegasus is the rather indistinct and rambling constellation of Pisces (the fishes). Uranus is currently residing in Pisces. To the east of Pisces is the equally indistinct constellation of Aries (the Ram). Aries has just one moderately bright star called Hamal. Just above Aries is the small constellation of Triangulum, defined by three stars. It hosts a beautiful galaxy M33.
The beautiful winter constellation of Orion (the Hunter) is now establishing itself to dominate the winter sky. It can be seen appearing over the horizon in the (east) soon after sunset at around 18:00. Orion will be at its very best around Christmas time as it dominates the southern sky. The most distinctive feature of Orion is the three stars that represent his belt. Orion has two hunting dogs called Sirius in Canis Major and Procyon in Canis Minor. Sirius can be found by following the line of Orion’s belt down and to the east (left). Sirius is always low and close to the horizon where there is a lot of turbulence in the air. This causes Sirius to twinkle very noticeably. It is the brightest star we can see from the UK.
Taurus (the Bull) is now quite obvious the night sky in the south east in the early evening. Taurus can be seen at the south east (left) of the chart above sitting astride the ecliptic. The first interesting object to appear over the eastern horizon in Taurus is the beautiful ‘naked eye’ Open Custer Messier 45 (M45) also known as the Pleiades or (the Seven Sisters).
To the east (left) of Andromeda and to the north of (above) Taurus is the constellation of Perseus, appearing as a line of stars leading towards the ‘W’ of Cassiopeia.
Joined to Taurus at the star Elnath is the Constellation of Auriga with its beautiful bright white star Capella. Capella will be directly overhead later in the evening and looks stunning through binoculars or a small telescope.
THE CONSTELLATION OF PISCES
The constellation of Pisces looks like a long serpentine line of stars wrapped around the south and east sides of the Square of Pegasus. It has a noticeable ‘head’ at the south western (lower right) end of the line. None of the stars are bright and a dark sky is needed to enable the shape to be traced out.
The ‘Ecliptic’ cuts across the southern part of the constellation thus making Pisces one of the constellations of the Zodiac. As the Sun, Moon and planets appear to move across the sky following the ecliptic they at times pass through the lower part of Pisces. The second most distant planet Uranus is currently located in Pisces.
Pisces has just one Messier object, this is M74. Messier 74 (also known as NGC 628) is a face-on spiral galaxy. It is at a distance of about 32 million light-years from Earth.
Messier 74 (M74) in Pisces
The galaxy contains two clearly defined spiral arms and is therefore used as an archetypal example of a Grand Design Spiral Galaxy. The galaxy's low surface brightness makes it the most difficult Messier object for amateur astronomers to observe. However, the relatively large angular size of the galaxy and the galaxy's face-on orientation make it an ideal object for professional astronomers who want to study spiral arm structure and spiral density waves. It is estimated that M74 is home to about 100 billion stars.
THE CONSTELLATIONS OF ARIES AND TRIANGULUM
The constellations of Aries and Triangulum
The constellations of Aries and Triangulum can be found nestled between the more prominent constellations of Andromeda, Perseus, Taurus, Cetus and Pisces. Aries follows, to the east of, Pisces on the ecliptic. Aries is defined by a very indistinct line of stars. Only one, Hamal, is easy to find even on a fairly clear night. There are no notable objects to be found in the constellation of Aries.
Triangulum is a small but it easy to find, looking like a triangle made up from three stars. Just inside the recognized boundary with Pisces to the west (right) of the triangle is the Messier object M33, known as the Triangulum Galaxy or the ‘Pinwheel’ Galaxy.
Messier 33 (M33) the Pinwheel Galaxy
M33 is a beautiful ‘face on’ spiral galaxy. From our vantage point we are looking straight down on the great disc of the galaxy enabling us to see the spiral arms. The disadvantage of having this view is that the total light from the galaxy is spread over a large area making it look quite faint. It needs a very dark sky to give contrast and a medium sized telescope to see it well. Even a small amount of light pollution will make it difficult to see. It is thought to be just less than 3 million light years from us and is estimated to contain around 40 billion stars.
THE CONSTELLATION OF AURIGA
The constellation of Auriga
Auriga is easy to find to the north east of Taurus. It actually shares the star Elnath with Taurus. Auriga’s brightest star is Capella and is almost overhead during the mid-winter months. Capella is the brightest star in the constellation Auriga. It is the forth brightest star to be seen in the northern hemisphere (from the UK) after Sirius, Arcturus and Vega.
Although it appears to be a single star to the naked eye it is actually a star system of four stars in two binary pairs. The first pair consists of two large, bright type-G giant stars, both with a radius around 10 times that of the Sun and two and a half times its mass. They are in close orbit around each other. Designated Capella Aa and Capella Ab, these two stars are thought to be cooling and expanding on their way to becoming red giants.
Diagram of Capella Aa and Ab with Capella H and L
The second pair is around 10,000 astronomical units (Earth – Sun distances) from the first pair. It consists of two faint, small and relatively cool red dwarfs. They are designated Capella H and Capella L. There are other close-by stars labelled Capella C through to G and I through to K. These are actually unrelated stars in the same visual field. The Capella system is relatively close, at only 42.2 light-years (12.9 pc) from Earth
There are three beautiful Messier ‘Open Star Clusters’ in the constellation Auriga. They are called M36, M37 and M38. Strangely they appear to be in a straight line with a forth cluster M35 a short distance away in the same line just over the border in Gemini.
Messier 36 (M36) in Auriga
Messier 37 (M37) in Auriga
Messier 38 (M38) in Auriga
Messier 35 (M35) in Gemini
THE SOLAR SYSTEM THIS MONTH
MERCURY will be in superior conjunction with the Sun on 8th December so will not be visible this month. The smallest of our planets can be seen located just to the east (left) of the Sun in the chart BELOW.
VENUS was in Superior Conjunction with the Sun on 25th October and is now moving eastwards away from the Sun. It will gradually make an appearance in the evening sky over the next few months. As Venus is currently on the opposite side of the Sun to Earth we will see the whole of the side facing us illuminated by the Sun. Therefore it will appear as a small but full disc. The position of Venus in the sky can be seen to the east (left) of the Sun on the chart above. Venus will rise over the eastern horizon at about 09:00 which is about an hour after the Sun rises and is therefore in daylight.
Venus will actually be in the sky just above the south western horizon as the Sun sets at about 16:00 but will be almost impossible to see.
MARS rises in the south east at about 11:00 which is in daylight and will set over the western horizon at around 19:30. It will just about be visible low in the south west in the constellation of Capricornus. Mars is a long way from Earth at the moment so will look very small at just 5.5 arc-seconds in diameter. It will just be visible above the south western horizon after the Sun sets at 15:50. Mars will set in the south west at 19:30 but will very difficult to see in the twilight sky.
JUPITER rises in the east at around 21:45 at the beginning of the month and 19:40 by the end of the month. This means it will well positioned before midnight, certainly by the end of the month. It will be very well positioned an hour or two after it rises just inside the western edge of the constellation of Leo after crossing the border from Cancer a few weeks ago.
By the end of the month it will be possible to observe Jupiter for at least three hours before midnight. However the King of the Planets will also still be at its best until the Sun brightens the sky in the morning.
Jupiter has four large moons that can be seen using a good pair of 10x50 binoculars. The moons were first reported by Galileo Galilei in 1610 and are referred to as the Galilean Satellites (or moons) in his honour.
Io is the closest of the large moons to Jupiter. Its orbit is 421,000 km from Jupiter and takes just 1.77 Earth days to circle the planet. Io is 3,630 km in diameter which is a little larger than our Moon (3,476 km).
Europa is a little smaller than Io and our Moon at 3,138 km in diameter. It orbits Jupiter every 3.55 Earth days on an orbit of 670,900 km.
Ganymede is the largest moon in our Solar System with a diameter of 5,262 km. It orbits Jupiter at 1,070,000 km from the planet and takes 7.16 days.
Callisto is 4,800 km in diameter and takes 16.69 days to orbit Jupiter a distance of 1,880,000 km
The four Galilean moons can easily be seen using a modest telescope (90mm aperture). The two inner moons called Io and Europa appear to move quite quickly especially when they are positioned close to the planet. They can be seen to have moved in periods of about 10 to 15 minutes. The position and motion of the Moons can be monitored recorded using sketches. If an accurate clock is used the time that the moons disappear behind or reappear from behind Jupiter can be compared to the time predicted using a computer planetarium application.
Jupiter is now appearing ‘side on’ as we look at it from Earth. This means we see the orbits of the moons in a flat plane which is almost aligned to the equator of Jupiter. As a consequence the moons pass in front or behind Jupiter on every orbit and not above or below as they do most of the time. This makes observing Jupiter very interesting. We can watch the moons approach the planet to disappear and then watch them reappear an hour or two later. We can also see their shadows as the pass in front and project an eclipse on to the planet.
When we see the orbits of the moons edge on, as we can now, Mutual events of Jupiter’s moons (eclipses and occultations) can be followed. An eclipse occurs when one moon casts its shadow on to another moon. Occultations occur when one moon passes in front of or behind another moon. More details to help observing these events in this magazine next month.
SATURN rises at about 07:30 but is effectively in conjunction with the Sun and will not be visible this month. Saturn rises and hour or so before the Sun in the east and sets before the Sun in the west. The actual conjunction took place on 18th November but Saturn is still very close to the Sun. See the chart on the previous page.
URANUS rises in the east in the constellation of Pisces at about 13:00 and will be observable in the south east from about 16:30 and then for the rest of the evening. Uranus will set over the western horizon at about 01:30. Uranus will require a medium sized telescope to see it as a blue disc, a 150mm reflector or a 100mm refractor.
Uranus, Neptune and Mars in the south at 18:00
It will be best to wait until about 9 o’clock 21:00 when the planet will be in the south and high in the sky well above the horizon and positioned away from dirty turbulent air. Uranus will be just 3.7 arc-seconds in diameter so will require a high magnification to see as a disc.
NEPTUNE rises in Aquarius at about 12:00 and will be observable as soon as it is dark. It will be just 2.3 arc-seconds in diameter so will be more difficult to see than Uranus. This elusive planet will be due south at 5 o’clock 17:00 and in its best possible position. It will set over the western horizon at about 22:00. Neptune will require a larger telescope to see it as a small blue disc.
THE MOON PHASES IN DECEMBER 2014
Full Moon will be on 6th December
Last Quarter will be on 14th December.
New Moon will be on 22nd December.
First Quarter will be on 28th December.
The best times for looking at the Moon through a telescope will be as the ‘Terminator' (the line between the light and dark sides) crosses the area to be observed. At this time that area will be at sunrise or sunset and the shadows of the features such as the craters will be elongated. The best times to observe the Moon this month will be from the beginning of the month until the 4th or 5th and from 23rd December through to the end of the month.
The best times to see the Maria (seas) [Mare singular] will be when the Moon is full or near full. A Moon filter will reduce the glare to give a more comfortable view and will increase the contrast and show the darker areas more clearly.
The Full Moon imaged by Steve Harris on 5th November 20014
The Sun rises at 07:50 at the beginning of November and at 08:00 by the end of the month. Solar activity has been relatively low during this cycle with fewer sunspots. However there have been occasional increases in activity over the past few months. There was a beautiful display of sunspots in November. These could easily be seen, looking much like the image below, using a small telescope fitted with a shop bought or even a homemade solar filter.
Sun spots imaged by SOHO on 18th November
Sunspots are caused by the magnetic force lines breaking the visible surface of the Sun to expose a deeper and cooler layer below.
To see what is happening on our closest star in real time visit the website for the Solar and Heliospheric Observatory (SOHO) at http://soho.nascom.nasa.gov/. This is NASA's space observatory that has instruments continuously watching the activity on the surface of the Sun.
A special solar filter must be fitted to a telescope to view the Sun or alternatively the image can be projected on to a screen.
DO NOT LOOK DIRECTLY AT THE SUN AS IT WILL CAUSE BLINDNESS