(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 08:50 and sets at 16:05.
Mercury is at inferior conjunction on the 13th and is visible in the morning sky towards the end of the month. At month’s end, it rises at 07:00 and is mag -0.2 in Ophiuchus.
Venus is visible at the start of the month as a morning object, rising at 07:35 and is mag -3.8. It is lost to the twilight after the 1st week of the month.
Mars is a morning object this month, moving from Virgo to Libra. It rises at around 04:05 and brightens from mag +1.7 to mag +1.5 during the month.
Jupiter is a morning object this month in Libra. It rises at 05:45 at the start of the month and by month’s end, it rises at 04:20. It brightens from mag -1.6 to mag -1.7 during the month.
Saturn is at conjunction on the 21st and not visible this month.
Uranus is visible in the evening sky this month in Pisces. During the month, it is visible as soon as darkness falls and sets at 02:05 by month’s end. It fades from mag +5.7 to mag +5.8 and lies near to Torcularis Septentrionalis (Omicron (ο) Piscium, mag +4.3) during the month.
Neptune is at eastern quadrature on the 3rd and is visible in the evening sky this month in Aquarius. During the month, it is visible as soon as darkness falls and sets at 21:50 by month’s end. It maintains its brightness at mag +7.9 and lies near to Lambda (λ) Aquarii, mag +3.7 during the month.
The full moon is on the 3rd (15:47). The last quarter moon is on the 10th (07:51) with the new moon on the 18th (06:30). The first quarter moon is on the 26th (09:20). The full moon on the 3rd falls within the definition of the perigee-syzygy of the Earth-Moon-Sun system phenomenon.
8th pm 65% waning gibbous and Regulus (Alpha (α) Leonis, mag +1.4) – between around 21:25 and 22:20. This event will have started prior to moonrise at 22:07.
31st am 94% waxing gibbous and Aldebaran (Alpha (α) Tauri, mag +0.9) – between around 01:00 and 02:00.
3rd pm the full moon lies E of Aldebaran (Alpha (α) Tauri, mag +0.9) at 18:00.
13th am the 22% waning crescent lies NE of Spica (Alpha (α) Virginis, mag +1.0) and N of Mars at 05:00.
14th am the 14% waning crescent lies E of Mars and N of Jupiter at 06:00.
15th am the 8% waning crescent lies SE of Jupiter at 06:00.
24th pm the 34% waxing crescent lies E of Neptune at 18:00.
27th pm the 64% waxing gibbous lies S of Uranus at 18:00.
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 in the morning of the 14th with a ZHR of 120. It is one of the main showers of the year and is well placed to put on a good show. The radiant rises at around 20:00 on the 13th. A 15% waning crescent moon rises at 04:10 on the 14th in Libra, but is unlikely to interfere too much. The meteors are of a medium speed – 35 km/s.
The Ursids peak in the morning of the 22nd with a ZHR of 10. The radiant is circumpolar and this combined with the 10% waxing crescent moon setting in Capricornus by 19:15 on the 21st, allows for a full night’s window to observe this shower. Meteor speeds are similar to the Geminids – 32 km/s.
There may be additional minor showers this month, details of which can be found in the below Information Sources and Links Section.
Asteroid (349) Dembowksa is at opposition on the 1st in Taurus. It is mag +9.6 and visible from 18:00 on the evening of the 1st.
Asteroid (20) Massalia is at opposition on the 17th in Taurus. It is mag +8.4 and visible from 18:00 on the evening of the 17th.
Finder charts and further information about other fainter asteroids can be found in the below Information Sources and Links Section.
There are no bright comets this month.
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 21st 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.