(Please note all times are UT and are based on an observing location of Belfast and covers the month of December)
At the start of the month, the Sun rises at 08:25 and sets at 16:00. By month's end, it rises at 08:45 and sets at 16:05.
11th pm Venus and Saturn (very low to horizon) – Venus 1.8 degrees to the South of Saturn.
Mercury will appear in the morning sky at the start of the month when it rises at 06:25 in Libra and is mag -0.5.
Venus is in the evening sky albeit very low. At the start of the month, it sets at 17:30 and is mag -3.8 in Sagittarius. By month’s end, it sets at 19:00 and is still mag -3.9 but now in Capricornus. It has a close encounter with Saturn (see above).
Mars is visible in the morning sky in Libra. It rises at 05:25 and brightens from mag +1.7 to mag +1.6 during the month.
Jupiter is at conjunction on the 27th. It is visible in the evening sky in Sagittarius at the start of the month when it sets at 17:10 and is mag -1.7.
Saturn visible in the evening sky in Sagittarius. At the start of the month, it sets at 18:40 and by month’s end it sets at 16:55. It brightens from mag +0.6 to mag +0.5 during the month.
Uranus is visible in the evening sky in Aries. It rises during daylight hours during the month and sets at 02:55 by month’s end. It maintains its brightness at mag +5.7 during the month.
Neptune is at eastern quadrature on the 8th and is visible in the evening sky in Aquarius. During the month, it rises during daylight hours and by month’s end it sets at 22:15. It maintains its brightness at mag +7.9 during the month.
The first quarter moon is on the 4th (06:58) with the full moon on the 12th (05:12). The last quarter moon is on the 19th (04:57) with the new moon on the 26th (05:13).
4th pm the 55% waxing gibbous lies S of Neptune at 18:00.
8th pm the 88% waxing gibbous lies S of Uranus at 18:00.
10th pm the 98% waxing gibbous lies S of M45 – The Pleiades and NW of Aldebaran (Alpha (α) Tauri, mag +0.9) at 18:00.
11th pm the near full moon lies E of Aldebaran (Alpha (α) Tauri, mag +0.9) at 18:00.
16th pm the 75% waning gibbous lies N of Regulus (Alpha (α) Leonis, mag +1.4) at 23:00.
17th pm the 64% waning gibbous lies SE of Regulus (Alpha (α) Leonis, mag +1.4) at 23:00.
21st am the 28% waning crescent lies E of Spica (Alpha (α) Virginis, mag +1.0) at 04:00.
23rd am the 10% waning crescent lies E of Mars at 06:00.
28th pm the 7% waxing crescent lies S of Venus at 17:00.
29th pm the 12% waxing crescent lies E of Venus at 17:00.
The best time to observe meteor showers is when the moon is below the horizon; otherwise its bright glare limits the number you will see especially the fainter ones. Below is a guide to this month's showers.
The Geminids peak during daylight hours on the 14th with a ZHR of 140 (the actual visible rate is always less than this). The radiant is visible from 19:00 on both the nights of 13th/14th and 14th/15th, but unfortunately the shower this year is ruined by a bright waning gibbous also rising in Gemini on both nights.
There may be additional minor showers this month, details of which can be found in the below Information Sources and Links Section.
There are no bright asteroids at opposition this month.
Finder charts and further information about other fainter asteroids can be found in the below Information Sources and Links Section.
C/2017 T2 (PanSTARRS) is slowly brightening and is predicted to be mag +10 in December. It will be circumpolar all month. It starts in Auriga, before into Perseus. It then spends a short time in Camelopardalis before ending the month in Perseus. On the night of the 13th/14th, it lies NW of NGC 1545 (mag +6.3 open cluster in Perseus). On the night of the 15th/16th, it lies NW of NGC 1528 (mag +6.4 open cluster in Perseus). On the night of the 19th/20th, it lies NW of NGC 1496 (mag +9.6 open cluster in Perseus).
Finder charts and further information about the above and other fainter comets can be found in the below Information Sources and Links Section. Any of the above estimates are based on current information at the time of writing the guide and can be wrong - “Comets are like cats; they have tails, and they do precisely what they want”, David H Levy.
On the deep sky front this month, galaxies M81 and M82 can be observed in Ursa Major. In Andromeda, M31 - The Andromeda galaxy can be observed along with its satellite galaxies M32 and M110. In Perseus, there is the open cluster M34 and the excellent Double Cluster - NGC 869 and 884. In Triangulum, there is the galaxy M33. In Auriga there are three open clusters M36, M37 and M38 and also M35 in Gemini. Taurus has the excellent Pleiades - M45, the Hyades and also M1 - The Crab Nebula. Orion returns to our skies with M42 - The Great Orion Nebula and also Cancer with M44 - The Beehive Cluster.
Always keep an eye out for Aurorae. The winter solstice is on the 22nd which sees the shortest day of the year and after this date the nights shorten and the days lengthen. This also sees the beginning of winter. Other interesting naked eye phenomena to look out for include the Zodiacal Light and the Gegenschein. Both are caused by sunlight reflecting off dust particles which are present in the solar system.
The Zodiacal Light can be seen in the West after evening twilight has disappeared or in the East before the morning twilight. The best time of year to see the phenomenon is late-Feb to early-April in the evening sky and September/October in the morning sky - it's then that the ecliptic, along which the cone of the zodiacal light lies, is steepest in our skies. The Gegenschein can be seen in the area of the sky opposite the sun. To view either, you must get yourself to a very dark site to cut out the light pollution. When trying to observe either of these phenomena, it is best to do so when the moon is below the horizon. A new appendix has been added explaining some of the more technical terms used in the guide.
The ZHR or Zenithal Hourly Rate is the number of meteors an observer would see in one hour under a clear, dark sky with a limiting apparent magnitude of 6.5 and if the radiant of the shower were in the zenith. The rate that can effectively be seen is nearly always lower and decreases as the radiant is closer to the horizon. The Zenith is the overhead point in the sky.