WHAT'S UP THIS MONTH - DECEMBER 2021
(Link to What's Up November 2021)
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THESE PAGES ARE INTENDED TO HELP YOU FIND YOUR WAY AROUND THE SKY
The chart above shows the whole night sky as it appears on 15th December at 21:00 (9 o'clock) 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 8 o'clock GMT at the beginning of the month and at 10 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 evening sky: Venus, Saturn, Jupiter and Neptune (in the early evening), Uranus later.
THE SOUTHERN NIGHT SKY THIS MONTH
The night sky looking south at about 22:00 GMT on 15th December
The chart above shows the night sky looking south at about 21:00 GMT on 15th December. West is to the right and east to the left. The point in the sky directly overhead is known as the Zenith and is shown (in red) at the upper centre of the chart. The curved brown line across the sky at the bottom is the Ecliptic or Zodiac. This is the imaginary line along which the Sun, Moon and planets appear to move across the sky. The brightest stars often appear to form a group or recognisable pattern; we call these ‘Constellations'.
Constellations through which the ecliptic passes this month are: Aquarius (the Water Carrier), Pisces (the Fishes), Aries (the Ram), Taurus (the Bull), Gemini (the Twins) and Cancer (the Crab).
The Summer Triangle that dominates the Summer Sky and was described in detail in September issue of this magazine is now moving over the western horizon. The triangle is defined by three obvious bright stars: Deneb in the constellation of Cygnus, Vega in Lyra, and Altair in Aquila.
Prominent is the southern sky is the constellation of Pegasus (the Winged Horse). The main feature of Pegasus is the square formed by the four brightest stars. This asterism (shape) is known as the Great Square of Pegasus. The square is larger than might be expected but once found is easier to find again . There is a very nice Globular cluster in Pegasus it is known as Messier 15 (M15). It is a lovely sight to see in a telescope.
Moving into view in the southern sky is the constellation of Taurus (the Bull). The most obvious star in Taurus is the lovely Red Giant Star called Aldebaran. It appears slightly orange to the ‘naked eye' but it is very obviously orange when seen using binoculars or a telescope. Aldebaran is located at the centre of the ‘flattened' X shape formed by the brightest stars in Taurus. At the end of the top right (upper west) arm of the ‘X' is the beautiful ‘naked eye' Open Star Cluster Messier 45 (M45) known as the Pleiades (or the Seven Sisters). It really does look magnificent using binoculars.
Following Taurus is the constellation of Gemini (the Twins). The two brightest stars in Gemini are Castor and Pollux and they are named after mythological twins. To the north of Taurus is the odd pentagon shape of Auriga (the Charioteer). Dominating Auriga is the brilliant white star Capella which is almost directly overhead. For those with a telescope there is a line of lovely open clusters to search out in Taurus and Auriga. These are M35 in Taurus and M36, M37 and M38 in Auriga.
To the south of Taurus is the winter constellation of Orion (the Hunter). Orion is easily found by looking for his very obvious three stars of his belt. Orion will be the constellation of the month in the January Magazine.
To the east (right) of Taurus is the rather indistinct constellation of Cancer (the Crab). The stars of Cancer are quite faint and can be difficult to discern especially in a light polluted sky. It is really worth searching out Cancer using binoculars or a telescope to see the Open Cluster M44 (the Beehive Cluster). M44 is older and further away than M45 (the Seven Sisters) so is fainter than M45 but still looks lovely. It has a group of stars that resemble an old straw Beehive with bees around it.
The Ecliptic was low in the sky during the summer months so the Moon and planets appeared close to the southern horizon. Saturn and Jupiter are well placed in the early evening but steadily moving towards the western horizon. The outer ‘Ice Giant' planets Neptune and Uranus are still fairly well placed in the evening for those who are fortunate enough to have access to a telescope. Due to their low altitude, the planets have not been at their best for observation this year. The thick, murky and turbulent air has caused the planets to appear quite unsteady.
Where to find the planets this month
Mercury is not well placed this month.
Venus is visible in the early evening in the west but it is very low and close to the horizon.
Mars is too close to the Sun and is not observable.
Jupiter is very bright in the south west during the early evening but moving towards the west.
Saturn is located to the west of Jupiter and moving towards the western horizon.
Uranus can be found in the south east in the early evening but really needs a telescope.
Neptune is located in the south but will need a telescope to see it.
CONSTELLATION OF THE MONTH – TAURUS
The chart above shows the night sky looking south at about 21:00 GMT on 15th November
The chart above shows the constellation of Taurus the Bull. There are many different representations of Taurus but he is generally shown with his horns tipped by the stars at the end of the obvious ‘>' shape. The bright red star Aldebaran is normally used to show the bull's eye.
An illustration of the constellation of Taurus
With a little imagination Taurus appears to be charging Orion in the illustration. It sits on the Ecliptic and is one of the star signs of the Zodiac. The asterism (shape) used to identify Taurus resembles a stretched ‘X'.
The bright red star Aldebaran is located at the centre of Taurus. It is easy to find and therefore helps to identify the constellation of Taurus. It is in fact a Red Giant Star and that is why it appears distinctly orange. A Red Giant is a star similar to our Sun (perhaps a little larger) that is approaching the end of life as a normal star. It has used up most of its Hydrogen fuel and has swollen into a giant . Its outer layers are now stretched over a larger area so the available heat is also spread over a bigger area so its surface is cooler and appears orange in colour.
Surrounding the bright red star Aldebaran is an Open Cluster of Stars known as the Hyades. It is an older cluster than M45 so its stars have begun to disperse. It is also quite far away from us so the stars appear quite faint. In a dark Moonless sky the cluster can be seen with the naked eye but is best seen using binoculars. The cluster is large, at 3.5° in diameter (about 7 Moon diameters) and well dispersed.
The Open Star Clusters Hyades and Pleiades
The real jewel of Taurus is without doubt the beautiful Open Cluster, Messier 45 (M45) also called the Pleiades or the Seven Sisters. An Open cluster is created as stars form in a giant cloud of gas and dust called a ‘Nebula'.
M45 is visible to the naked eye initially looking like a patch of light. Closer observation will reveal a cluster of up to seven stars. Using a good pair of binoculars many more stars will be seen. There are in fact about 300 young stars in the cluster that is estimated to be about 100 million years old. M45 is one of the closest open clusters to us at 400 light years.
The Pleiades look brighter than the stars of the Hyades because they are very bright large young stars and are relatively close to us. The largest and brightest is Alcyone which is 10 times the mass of our Sun and 1000 times brighter. The larger and brighter stars of the Pleiades are also rotating very fast.
Messier 45 (M45) the Pleiades (Seven Sisters)
The stars of the Pleiades cluster would have formed from the gas and dust of a Nebula. Gravity draws the atoms of the Nebula together to form denser clumps of gas that become ever denser. Eventually the gas is squeezed into dense spheres where the pressure and high temperature at the core causes Hydrogen atoms to combine through Nuclear Fusion. As Hydrogen atoms are fused into Helium atoms, heat is produced and the sphere becomes a shining star. Any left-over gas and dust is blown away by the intense radiation from the young stars and a cluster of new stars is revealed. This type of star cluster is called an ‘Open Cluster'.
The biggest and brightest stars of M45 (the Seven Sisters) have been named after seven sisters from Greek Mythology and are joined in the pattern we can see with our ‘naked eyes'. They were the seven daughters of the Titan god called Atlas and the sea-nymph Pleione. Atlas and Pleione are included as the naked eye stars but the 6 th & 7 th sisters are actually Sterope (Asterope) and Celaeno.
Names of Seven Sisters and Parents (shown in yellow)
Impressive as they are, the Seven Sisters are just the brightest (naked eye) stars in a cluster of around 300 young stars. In the images above the Seven Sisters appear to be surrounded by gas remaining from the original nebula. However it is now thought the cluster is just passing through a cloud of Hydrogen gas in space .
As M45 is so close to us the cluster has a relatively high apparent movement across the sky although it is still too slow for us to perceive. It will take 30,000 years to move a distance equal to the diameter of our Moon.
Although the cluster is moving through space the individual stars all have slightly different trajectories and relative speeds. Gradually over millions of years the stars will move further apart and the cluster will disperse, like the Hyades. Binoculars will reveal around 30 to 50 stars in the cluster and a telescope will reveal about 300 stars in the cluster. However the cluster is too large to fit into the field of view of most telescopes so the outline of the cluster will be lost.
There is another very interesting object in Taurus. At the end of the lower left (eastern) arm of Taurus is Messier 1 (M1) the Crab Nebula, see the chart above. It can be seen using binoculars in a dark and clear sky but really needs a telescope. From Aldebaran we need to look east to the star ? (xi) Tauri. Just above ? Tauri is a small smudge of light, this is M1.
Messier 1 (M1) the Crab Nebula imaged by Hubble
M1 is the remnant of a giant star that exploded as a Supernova about 7000 years ago. Its light took 6000 years to reach Earth and was observed by Chinese astronomers in the year 1054 AD. It can still be seen in a dark clear sky as a ‘fuzzy' patch of light using a medium sized telescope.
A Supernova is the ‘death' of a star more than three times the mass of our Sun. Giant stars consume their Hydrogen fuel at an experientially faster rate than smaller stars. Consequently bigger stars do not ‘live' as long as smaller stars. As stars begin to exhaust their supply of Hydrogen they develop into a Red Giant like Aldebaran. Even larger stars develop into even larger Red Super Giants like Betelgeuse in Orion.
A star like our Sun and those up to about twice the mass of our Sun eventually slowly collapse as their fuel eventually runs out. The outer layers of the Red Giant drift away to form a gas bubble. The core ‘gently' collapses to form a White Dwarf Star.
Stars that are over 2½ to 3 times the mass of our Sun come to a more dramatic end. As the fuel of a larger Red Giant Star finally runs out the star suddenly collapses and all the mass of the star falls inwards under the massive force of its own gravity. The collapse reaches a point where the pressure and heat causes a gigantic thermonuclear explosion. The outer regions are blown into space to create a Supernova Remnant like M1 and a dense Neutron Star about 12,000km in diameter. These tiny, super dense stars are also called ‘Pulsars'.
THE GEMINID METEOR SHOWER – December 13th and 14th
The Geminid Meteor Shower - Radiant at 01:00 14th December
In the middle of this month, around 8th to 17th December, there will be a meteor shower known as the Geminid shower. There will be a peak in activity during the evening of the 13th and morning of 14th December. The very best time to watch for the meteors will be during the early morning hours on 14th December (at 07 :00 the shower should be at its maximum as seen from the UK ).
The gibbous Moon will be in the south in the early evening of 13th December but will move into the west by midnight and before any serious meteor watching has started. Conditions look promising, weather permitting and the sky will be fairly dark but not moonless.
The type of meteor that occurs in showers usually originates from a comet and is much more common than the ‘Fireballs' that originate from asteroids. The Geminid shower for this reason is unusual because it is thought to originate from an asteroid known as 3200 Phaethon. This means that some of the meteoroids (the particles moving through space) may be of a rocky nature so they will often be bright and survive for quite a long time. When they enter Earth's atmosphere about 100km up they might produce a bright and persistent trail.
The Geminid meteors also enter the atmosphere comparatively slowly at about 35 km/second compared with other showers that enter at over 75 km/second. As a result of this slower entry and some having a more robust make up, the Geminid meteors may appear brighter and their trails across the sky last longer.
The actual peek of activity is expected to occur at 07:00 around on 14th December so should be visible from the UK before dawn. Observers in the USA will be luckier as they will be able to see it in their darkness.
Because the constellation of Gemini is above the horizon from early evening, the meteors can be seen for most of the night and in almost any part of the sky. By midnight the constellation will be almost due south and high in the sky. If you are intending to have a look remember to wrap up warm before you go out because you will soon feel very cold and that will spoil your enjoyment of the shower. Make yourself comfortable in a garden chair and spend at least an hour looking .
Geminid meteor shower composite by Clint Spencer
COMET LEONARD (C/2021 A1) - DECEMBER 2021
The location of Comet Leonard (C/2021 A1) at 16:15 GMT on 15th December
There is a ‘naked eye' Comet visible in the night sky at the moment so that is the good news but unfortunately there is some bad news. The bad news is the comet called Leonard (C/2021 A1) is close to the Sun from our point of view and difficult to see. Its location is shown on the chart above just above the horizon after sunset in the south west and will be very difficult to see.
Comet Leonard (C/2021 A1) is an inbound (approaching the Sun) long period comet discovered by G. J. Leonard at the Mount Lemmon Observatory. It was first spotted on 3rd January 2021 (a year before perihelion) when the comet was 5 AU (750 million km) from the Sun. Jupiter's orbit is (5 AU from the Sun) is at the frost line where methanol (CH 3 OH) and water start sublimation into gas. [1 Astronomical Unit ‘AU' is a unit of distance between Earth and the Sun equal to 150 million kilometres.]
This was the first comet discovered in 2021 and has a retrograde orbit. On 12th December 2021 the comet will be 0.233 AU (34.9 million km) from Earth and on 18th December 2021 will be 0.028 AU (4.2 million km) from Venus. It will make its closest approach to the Sun on 3rd January 2022.
The trajectory of Comet Leonard (C/2021 A1)
On the morning of 6th December 2021 the comet will be about 5 degrees from the star Arcturus. On 14th December 2021 the comet will be just 14.7 degrees from the Sun and will quickly become southern hemisphere object not visible from the northern hemisphere and the UK. The forward scattering of light could cause the comet to brighten to as much as magnitude 2.
The comet may reach naked eye visibility in December 2021 from the southern hemisphere. At an apparent magnitude of 4, it (in theory) should be a good binocular comet. On 10th October the comet showed a short but dense dust tail. As of mid-November the comet has gained a total magnitude (coma + nucleus) of around 10.
A comet similar to what Leonard may look like
Comet Leonard C/2021 A1 has been inside of the orbit of Neptune since May 2009. Using an epoch of 1950 which is well before the comet entered the planetary region of the Solar System, a barycentre orbit solution suggests the comet had roughly an 80,000 year orbital period. Therefore the comet had spent the last 40 thousand years inbound from approximately 3,700 AU (550 billion km). After perihelion (closest point to the Sun) the comet will be ejected from the Solar System. The barycentric orbit will remain hyperbolic after September 2022.
THE SOLAR SYSTEM - DECEMBER 2021
The planets at 17:30 GMT on 15th December
The chart above shows the location of the planets along the Ecliptic. The sky has been darkened to make the planets visible. The visible planets: Venus, Saturn, Jupiter, Neptune and Uranus are visible along the Elliptic from the West (right) to East (left). The planets appear low in the sky and are not well positioned for observing.
MERCURY will be very close the Sun after sunset in the west and will not be visible this month.
VENUS will be visible in the early evening sky after sunset. It will be easy to find but will it require a clear view to the south western horizon. Venus has emerged from its excursion behind the Sun when it was in ‘Conjunction' with the Sun. It appeared at its Greatest Easterly Elongation on 29th October and it is now moving back towards the Sun and appearing bigger in diameter but as a narrowing crescent.
Venus as it appears at sunset on 15th December
Also showing location of Comet Leonard (C/2021 A1)
MARS has now moved out of view and will not appear in the evening sky again until September 2022. It is very close to the Sun this month will be too close to the Sun and too small in diameter to be observable.
JUPITER will be at its best and visible in the south west as the sky darkens. Jupiter was at opposition and its best on 20th August. It will be at its rather poor best in the south at about 17:00 but it will be moving towards the western horizon to set at about 21:15.
SATURN is now bidding us goodbye until next year. It will be difficult to observe in the turbulent air close to the south western horizon. Saturn will be at its best as soon as it is dark and in the south. It will be moving west and will set over the western horizon at about 19:30.
URANUS will be observable this month and will be best at 21:00 when it will be due south and at its highest point above the horizon but is small and faint at +5.7.
NEPTUNE will be just visible to the east of Jupiter and will be at its best at 18.00. It is small a difficult to see at only 2.4 arc-seconds in diameter and at magnitude +7.7.
The location of the planets this month
Diagram showing the location of the inner planets this month
The diagram above shows the location of the inner planets in relation to Earth during the month of December. Earth is shown with the daytime illuminated in yellow (sunshine) and the night time dark. The arrow shows the direction of rotation of Earth on its axis every day. The line between day and night is called the ‘Terminator' and the point on Earth on the left of the Terminator is sunset and the point on the right is sunrise.
So as Earth rotates anticlockwise (as we are looking down at it) an observer will move from daylight into night time. As our observer moves into the evening the planets will appear to pass over the western horizon in succession – Mercury then Venus. Mercury is actually too close to the Sun to be seen. Mars would then appear over the eastern horizon in the morning as our observer moves from night across the terminator into daylight. Mars is however too close to the Sun to be seen this month.
Diagram showing the location of the outer planets this month
The diagram above shows the location of the outer planets in relation to Earth during the month of December. Earth is shown with the daytime illuminated in yellow (sunshine) and the night time dark. The arrow shows the direction of rotation of Earth on its axis every day. The line between day and night is called the ‘Terminator' and the point on Earth on the left of the Terminator is sunset and the point on the right is sunrise.
So as Earth rotates anticlockwise (as we are looking down at it) an observer will move from daylight into night time. As our observer moves into the evening the planets will appear to pass over the western horizon in succession – Saturn (at 19:30), Jupiter (at 21:30), Neptune (at 23:30) and Uranus (at 04:20). The diameter of the orbits Jupiter and Saturn have been reduced and the orbits of Uranus and Neptune have been shrunken and placed on top of each other so the orbits of all the planets can fit on to this diagram.
The Sun rises at about 07:50 at the beginning of the month and 08:00 by the end. It sets at 15:50 at the beginning of the month and 16:00 at the end of the month. It will be at its lowest point in the sky on 21st December and at the Summer Solstice. It will be the longest night at 16 hours and shortest day at just 8 hours long. There have been a few nice Sunspots during October and November.
THE MOON PHASES DURING DECEMBER
New Moon will be on 4th December
First Quarter will be on 11th December
Full Moon will be on 20th December
Last Quarter will be on 27th December
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