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Constellations of the month

 

 
CURRENT MOON

Saturn

Saturn, will be seen west of south as darkness falls at the start of the month. Then, its disk is ~16 arc seconds across and its rings - which are still, at ~24 degrees, nicely tilted from the line of sight - spanning some 36 arc seconds across. During the month its brightness remains +0.6 with an angular size of 15.4 arc seconds. Sadly, now in Sagittarius and lying on the south-eastern side of the milky way, it is at the lowest point of the ecliptic and will only have an elevation of ~12 degrees after sunset. As with Jupiter, an atmospheric dispersion corrector will help improve our view.

Jupiter

Jupiter, shining on the 1st at magnitude -1.8 and with an angular size of 32 arc seconds, can be seen very low in the southwest as darkness falls at the start of December but, soon after, will be lost in the Sun's glare. Jupiter lies in the southeastern part of Ophiuchus and is heading towards the southernmost part of the ecliptic so, as it appears in the twilight, will only have an elevation of ~6 degrees. With its low elevation, atmospheric dispersion will take its toll and an atmospheric dispersion corrector would greatly help to improve our views of the giant planet and it four Gallilean moons.

Mars

Mars can be seen towards the southeast in the pre-dawn sky at the start of its new apparition. It rises some two and a half hours before the Sun at the start of the month and will have an elevation of ~15 degrees before it is lost in the Sun's glare. It then has a magnitude of +1.7 and a 3.9 arc second, salmon-pink, disk. By month's end it will be seen further round towards the south before dawn and its magnitude will have increased slightly to +1.6.

Venus

Venus may just be glimpsed in the south-west at the start of the month, but will be difficult to see due to the fact that the ecliptic is at a shallow angle to the horizon and so Venus will have a very low elevation. As the month progresses, it will rise higher in the sky and on the 31st will have reached an elevation of 14 degrees as darkness falls. During December, its magnitude remains at about -4 and its disk increases from 11.6 to 13 arc seconds across. A low horizon and possibly binoculars will be needed to spot Venus, but please do not use them until after the Sun has set.

Mercury

Mercury. Following its transit of the Sun and reaching greatest elongation west on the 28th of November, can be seen in the pre-dawn sky low in the southeast at the start of December. On the 1st it will have a magnitude +0.29 and will rise around an hour before the Sun. It will then have an elevation of some 9 degrees before being lost in the Sun's glare. With an angular size of ~5 arc seconds, it will then fall back towards the Sun and be lost from view by the middle of the month.

The Night Sky In and Around December 2019

Slide Presentation

The following presentation was given at our last meeting on the 18 October, 2019 by Robin Wilkey

Image of the Month

Saturn's Moons
Image: Cassini Imaging Team, SSI, JPL, NASA.

Compiled by Prof. Ian Morison
The planets this month
December 10th - after sunset: Venus lies below Saturn

Venus lies below Saturn
Image: Stellarium/IM
After sunset, low in the southwest, Venus will lie just 2 degrees below Saturn.
The constellations Pegasus and Andromeda
Pegasus

The Square of Pegasus is in the south during the evening and forms the body of the winged horse. The square is marked by 4 stars of 2nd and 3rd magnitude, with the top left hand one actually forming part of the constellation Andromeda. The sides of the square are almost 15 degrees across, about the width of a clentched fist, but it contains few stars visibe to the naked eye. If you can see 5 then you know that the sky is both dark and transparent! Three stars drop down to the right of the bottom right hand corner of the square marked by Alpha Pegasi, Markab. A brighter star Epsilon Pegasi is then a little up to the right, at 2nd magnitude the brightest star in this part of the sky. A little further up and to the right is the Globular Cluster M15. It is just too faint to be seen with the naked eye, but binoculars show it clearly as a fuzzy patch of light just to the right of a 6th magnitude star.

Andromeda

The stars of Andromeda arc up and to the left of the top left star of the square, Sirra or Alpha Andromedae. The most dramatic object in this constellation is M31, the Andromeda Nebula. It is a great spiral galaxy, similar to, but somewhat larger than, our galaxy and lies about 2.5 million light years from us. It can be seen with the naked eye as a faint elliptical glow as long as the sky is reasonably clear and dark. Move up and to the left two stars from Sirra, these are Pi amd Mu Andromedae. Then move your view through a rightangle to the right of Mu by about one field of view of a pair of binoculars and you should be able to see it easily. M31 contains about twice as many stars as our own galaxy, the Milky Way, and together they are the two largest members of our own Local Group of about 3 dozen galaxies.


M31 - The Andromeda Nebula
M33 in Triangulum

If, using something like 8 by 40 binoculars, you have seen M31 as described above, it might well be worth searching for M33 in Triangulum. Triangulum is

the small faint constellation just below Andromeda. Start on M31, drop down to Mu Andromedae and keep on going in the same direction by the same distance as you have moved from M31 to Mu Andromedae. Under excellent seeing conditions (ie., very dark and clear skies) you should be able to see what looks like a little piece of tissue paper stuck on the sky or a faint cloud. It appears to have uniform brightness and shows no structure. The shape is irregular in outline - by no means oval in shape and covers an area about twice the size of the Moon. It is said that it is just visible to the unaided eye, so it the most distant object in the Universe that the eye can see. The distance is now thought to be 3.0 Million light years - just greater than that of M31.


M33 in triangulum - David Malin
December 12th - before dawn: Mars and the double star Alpha Librae

Mars close to the double star Alpha Librae
Image: Stellarium/IM
If clear before dawn on the 12th, one will see Mars (magnitide +1.67) just above the double star Alpha Librae (Magnitudes +2.74 and +5.15) or Zubwnelgenubi. Despite its name it is the second brightest star in Libra. This would make a nice image using a small telescope.
Saturn's moons
Saturn now holds the record for having the most detected moons within the Solar System with a known total of 82 as compared to Jupiter's 79. All of the 20 newly found satellites are very small, 5 km or so in diameter. The Cassini image shows 5 of the moons: left to right are Janus and Pandora (179 and 81 km across), 504 km diameter Enceladus and Mimas at 396 km diamater. Cut off at the right hand side of the image is Rhea, Saturn's second largest moon, some 1,528 km across.
December 14th and 15th after midnight: the Geminid Meteor Shower

The Geminid Meteor shower
Image: Stellarium/IM

The early mornings of December 14th and 15th will give us the chance, if clear, of observing the peak of the Geminid meteor shower. The Moon is at First Quarter and will set around 11 pm so, when Gemini is highest in the sky, its light will not hinder our view. The Geminids can often produce near-fireballs and so the shower is well worth observing if its clear. An observing location well away from towns or cities will pay dividends. The relatively slow moving meteors arise from debris released from the asteroid 3200 Phaethon. This is unusual, as most meteor showers come from comets. The radiant - where the meteors appear to come from - is close to the bright star Castor in the constellation Gemini as shown on the chart. If it is clear it will be cold - so wrap up well, wear a woolly hat and have some hot drinks with you.

The constellation Taurus
Taurus is one of the most beautiful constellations and you can almost imagine the Bull charging down to the left towards Orion. His face is delineated by the "V" shaped cluster of stars called the Hyades, his eye is the red giant star Aldebaran and the tips of his horns are shown by the stars beta and zeta Tauri. Although alpha Tauri, Aldebaran, appears to lie amongst the stars of the Hyades cluster it is, in fact, less than half their distance lying 68 light years away from us. It is around 40 times the diameter of our Sun and 100 times as bright.
To the upper right of Taurus lies the open cluster, M45, the Pleiades. Often called the Seven Sisters, it is one of the brightest and closest open clusters. The Pleiades cluster lies at a distance of 400 light years and contains over 3000 stars. The cluster, which is about 13 light years across, is moving towards the star Betelgeuse in Orion. Surrounding the brightest stars are seen blue reflection nebulae caused by reflected light from many small carbon grains. These relfection nebulae look blue as the dust grains scatter blue light more efficiently than red. The grains form part of a molecular cloud through which the cluster is currently passing. (Or, to be more precise, did 400 years ago!)

VLT image of the Crab Nebula
Close to the tip of the left hand horn lies the Crab Nebula, also called M1 as it is the first entry of Charles Messier's catalogue of nebulous objects. Lying 6500 light years from the Sun, it is the remains of a giant star that was seen to explode as a supernova in the year 1056. It may just be glimpsed with binoculars on a very clear dark night and a telescope will show it as a misty blur of light.

Lord Rosse's drawing of M1
Its name "The Crab Nebula" was given to it by the Third Earl of Rosse who observed it with the 72 inch reflector at Birr Castle in County Offaly in central Ireland. As shown in the drawing above, it appeared to him rather lile a spider crab. The 72 inch was the world's largest telelescope for many years. At the heart of the Crab Nebula is a neutron star, the result of the collapse of the original star's core. Although only around 20 km in diameter it weighs more than our Sun and is spinning 30 times a second. Its rotating magnetic field generate beams of light and radio waves which sweep across the sky. As a result, a radio telescope will pick up very regular pulses of radiation and the object is thus also known a Pulsar. Its pulses are monitored each day at Jodrell Bank with a 13m radio telescope
December 22nd/23rd - late evenings: the Ursid Meteor Shower

The Ursid meteor shower
Image: Stellarium/IM
The late evenings of the 22nd and 23rd of December are when the Ursid meteor shower will be at its best - though the peak rate of ~10-15 meteors per hour is not that great. Sadly, this year Full Moon is on the 21st, so its light will greatly hinder our view. The radiant lies close to the star Kochab in Ursa Minor (hence their name), so look northwards at a high elevation. Occasionally, there can be a far higher rate so its worth having a look should it be clear.
The constellation Orion
Orion, perhaps the most beautiful of constellations, will be seen in the south at around 11 - 12 pm during January. Orion is the hunter holding up a club and shield against the charge of Taurus, the Bull up and to his right. Alpha Orionis, or Betelgeuse, is a read supergiant star varying in size between three and four hundred times that of our Sun. The result is that its brightness varies somewhat. Beta Orionis, or Rigel, is a blue supergiant which, at around 1000 light years distance is about twice as far away as Betelgeuse. It has a 7th magnitude companion. The three stars of Orion's belt lie at a distance of around 1500 light years. Just below the lower left hand star lies a strip of nebulosity against which can be seen a pillar of dust in the shape of the chess-board knight. It is thus called the Horsehead Nebula. It shows up very well photographically but is exceedingly difficult to see visually - even with relativly large telescope.

The Horsehead Nebula: Anglo Australian Observatory
Beneath the central star of the belt lies Orion's sword containing one of the most beautiful sights in the heavens - The Orion Nebula. It is a region of star formation and the reddish colour seen in photographs comes from Hydrogen excited by ultraviolet emitted from the very hot young stars that make up the Trapesium which is at its heart. The nebula, cradling the trapesium stars, is a beautiful sight in binoculars or, better still, a telescope. To the eye it appears greenish, not red, as the eye is much more sensitive to the green light emitted by ionized oxygen than the reddish glow from the hydrogen atoms.
The Orion Nebula

The Orion Nebula: David Malin