The night sky in December 2017

posted 30 Nov 2017, 01:48 by Pete Collins
by Anne Holt

Sunrise      1st:  08.01           31st:  08.25
Sunset       1st:  15.53           31st:  15.59

Astronomical darkness
1st:  18.01  to  05.54         31st:  18.09  to  06.14

The Winter Solstice is Thursday 21st at 16.27,  the shortest day at 7 hours 28 minutes 39 seconds. It is also the day of the HPAG Christmas party.

Latest Sunrise:   29th & 30th,  08.25
Earliest Sunset:  13th,   15.49

Full Moon:  3rd Dec at 15.46, known as The Cold Moon for obvious reasons.
New Moon: 18th Dec at 06.30

Highlights

This month we have one major meteor shower and a few minor ones, around 12 hours of astronomical darkness each night, we have the shortest day and, more importantly for astronomers, the longest night.

On the night of 30th/31st the Moon passes the Hyades - the V shaped Head of the Bull asterism in Taurus.  Aldebaran, the Eye of the Bull, is occulted in the early hours of 31st, disappearing a few minutes after 1am and reappearing about 15 minutes before 2am. 
We also have a perigee-syzygy Moon.  OK, call it a Supermoon if you really must. It will be the biggest and brightest full Moon of the year - though the next one, in early Jan, will be even better.


Constellations

Orion, with the stars of his belt pointing down to the brightest star in the sky, Sirius, is now well above the horizon by midnight, and is a beautiful sight especially from a dark sky site.  By month end these will be visible from 10pm - weather permitting.  Taurus and the Pleiades precede him across the sky.

Gemini, including the 'twins' Castor and Pollux, and Auriga with the bright Capella are also very prominent.
Aries and Pisces, while not particularly bright - or often not even visible in our light polluted skies - are both quite high this month.

Perseus, Andromeda and the Great Square of Pegasus  are also well placed for most of the night. The Plough starts the night quite low in the Northern sky, with Cassiopeia high overhead.  Because of the long winter nights, these last two will have changed places before dawn as they rotate around the celestial north pole.


Planets

Mercury:  in Sagittarius, mag 0.2.  Not easy to see this month, it starts December as an evening object, very low in the SW after sunset.  On 1st it sets at 16.47, just over an hour after the Sun.  On 6th, when it has faded to mag 1.5, it is only 1.5 degrees from Saturn - the pair may be visible, from a site with an unobstructed SW horizon, about 25 minutes after sunset.  On 10th it moves into Ophiuchus and 2 days later reaches inferior conjunction.  It then becomes a morning object.  On 15th it is 2 degrees from Venus, but both are very low in the SE and very difficult to see. On 17th it is at mag 2.4 and lies 1 degree South of the very thin crescent Moon. By 31st it rises almost 2 hours before the Sun and will have brightened to mag -0.2. 

Venus:  in Libra, mag -3.9.  Another planet which appears very close to the Sun this month.  On 1st it rises about an hour before it, then a few minutes later each day.  It may be seen for a short while before Sunrise in the first week of December.  Moves into Scorpio on 4th, Ophiuchus on 8th and Sagittarius on 23rd.  On 31st it rises only 5 minutes before the Sun.

Mars:  in Virgo, mag 1.7.  Because of its relatively fast eastward motion against the background stars it rises around the same time throughout December,  03.42 on 1st, then about quarter of a minute earlier each day.  it also brightens very slightly during the month as its position improves. On the morning of 25th it rises at 03.36 and should be visible for a while before the sky brightens.  However it is hardly a Christmas star in the East, at a mag of only 1.5.  On the morning of 31st it rises almost 5 hours before the Sun and is 3 degrees from the much brighter Jupiter.

Jupiter:  in Libra, mag -1.7.  Another improving morning object.  On 1st it rises at 05.17, then 2 or 3 minutes later each day during the month.  On 25th it rises at 04.10 and, at mag -1.8, is a much better candidate for the Christmas star. On 31st it rises at 03.52 and culminates at 08.26, just as the Sun is rising. 

Saturn:  in Sagittarius, mag 0.5.  Appears very close to the Sun throughout December.  On 1st it sets at 17.04 and might be seen very close to the SW horizon soon after Sunset.  Unlikely to be visible after the first few days of the month as it approaches conjunction on 21st.  By month end it is a morning object, rising just over an hour before the Sun on 31st.
  
Uranus: in Pisces, mag 5.7.  Once again this is the best positioned planet for observing - it's a pity that it isn't a bright naked eye object.  On 1st it reaches it's highest point in the sky at 20.58 and sets at 03.55, 2 hours before astronomical darkness ends.  By 31st it is culminating 2 hours earlier and sets around 2am.  During the month it fades slightly, to mag 5.8,  still theoretically a naked eye object - if both the conditions and the eye in question are perfect.  Binoculars or, better still a telescope, should show the small blue green disc.

Neptune:  in Aquarius, mag 7.9.  Past its best now, visible only in the early part of the night.  On 1st it culminates at 18.18. soon after astronomical darkness begins, and sets 20 minutes before midnight. By month end it culminates just after Sunset and sets at 21.41. It may be visible in good binoculars under ideal conditions but is best seen in a telescope in order to see its rich blue colour.


Meteor Showers

One major shower, the Geminids: active Dec 4th to 19th, peak in the early hours of 14th.  ZHR could be as high as 120 but is likely to be far fewer. These bright, medium speed meteors are the only major shower which originate from an asteroid - 3200 Phaethon - rather than a comet.  This shower often has near maximum rates for several hours either side of the peak.  The 13.5% lit Moon rises at 03.53 on 14th, so won't cause too much interference.

Minor showers

Ursids, active Dec 17th to 26th, peak 22nd,  ZHR usually 5 -10 but can be as many as 25. Occasional outbursts of 50 per hour have been observed - last seen in 1945 and 1986.  These are medium paced meteors, parent comet 8P/Tuttle.  The Moon sets at 20.09 on 22nd so no interference.

Leo Minorids, active Dec 5th to Feb 4th, peak 20th, ZHR 5.  A weak but long lasting shower.

Coma Berenicids, active 12th to 23rd, peak 16th, ZHR 3.  Another weak shower best seen around 1am when the radiant is high in the sky.   Again, no Moon interference. These were once thought to be part of the Leo Minorids but are now regarded as a separate shower.

Minor planets and Asteroids

Two days after the peak of the Geminids, its parent object, asteroid 3200 Phaethon passes Earth at a distance of only 10.3 million Km/6.4 million miles. At maximum mag 10.9 it isn't visible to the naked eye but might be of interest to astrophotographers.
For exact position on any day see https://in-the-sky.org/data/object.php?id=25877  (or id=a3200 which appears to be exactly the same)

Ceres in Leo and Makemake in Coma Berenieces are both well placed for imaging.
For more info on these, and all planets, minor planets and asteroids see:-

Comets

No comets brighter than mag 10 this month, however for astrophotographers, we have several between mag 11 & 17.
or  https://in-the-sky.org/data/comets.php
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