The top sights to observe or image this month
DON’T MISS: December comets
BEST TIME TO SEE: 1-16 December
RECOMMENDED EQUIPMENT: Binoculars or small/medium telescope
There are three reasonably bright comets and one wild card in this month’s sky. Comets 67P Churyumov-Gerasimenko, 4P/Faye and C/2019 L3 Atlas are all located in the general region centred on Gemini, highest around 00:30 UT on 1 December, 23:30 UT mid-month and 22:30 UT by the month’s end.
67P is the periodic comet visited by the Rosetta spacecraft from 2014 to 2016. It reached perihelion on 2 November when it was brightest at mag. +10.7. During December, 67P fades from mag. +10.9 to +11.7. The comet is relatively easy to find as it never wanders far from mag. +4.0 Iota (ι) Cancri, staying within 5˚ of the star throughout December.
Periodic comet 4P/Faye reached perihelion on 8 September and like 67P is now fading. On 1 December it is predicted to be at mag. +11.9, not significantly dimmer than its mag. +11.5 perihelion peak. By the month’s end, it will have dimmed to mag. +12.5. 4P/Faye follows a gently curving track to the west this month, in an area of sky about 9˚ to the south of Alhena (Gamma (ζ) Geminorum).
Comet C/2019 L3 Atlas is the brightest of the three, starting the month at mag. +9.9 and ending at mag. +9.7. Its December track starts conveniently 5˚ to the north of Castor (Alpha (α) Geminorum) and from there follows an almost linear path southwest, moving just 7˚ throughout the month. Actually, all three comets have relatively short paths this month making it both easier to keep track of these objects and photograph them.
At the start of 2021, newly discovered comet C/2021 A1 Leonard was announced. Excitingly this had the possibility of becoming bright during December and exceeding the threshold of naked-eye visibility. Current estimates have it near mag. +4.3 Beta (β) Comae Berenices on the 1st, shining at mag. +9.3. It’s then expected to brighten to about seventh magnitude mid-month, an easy binocular object. An effect known as forward-scattering enhancement may boost its brightness so it becomes visible to the naked eye.
Forward-scattering enhancement may help A1 Leonard become visible to the naked eye between 10–16 December, peaking at mag. +4.0 around the 13th. The comet’s sky location isn’t that favourable from the UK. The best prospects will be during the mornings of 1–13 December. Its morning track is shown on page 53, while on this page we’ve shown how the comet moves relative to Venus in the evening sky. Unless the comet brightens more than expected, the evening appearance will probably go unnoticed from the UK due to low altitude.
Geminids 2021
BEST TIME TO SEE: Early hours of 13, 14 and 15 December
RECOMMENDED EQUIPMENT: Naked eye
The Geminid meteor shower is one of the favourite observing events for the year. As with any meteor shower, the shower’s success is dictated by the phase of the Moon and the weather.
There’s not much we can do about the latter, except to move to a location where the clouds are less likely, but the Moon’s phase is very predictable, as is its rise and set time.
The Geminid shower reaches its peak around 07:00 UT on 14 December making the nights of the 13/14 December and 14/15 December optimum for viewing.
The Moon is in an advanced waxing phase around this time. 78% illuminated in a waxing gibbous phase on the evening of 13/14 December, it sets around 03:00 UT on the 14th. Although this isn’t ideal, long dark December nights mean there are still around three hours of darkness left to enjoy what the shower has to deliver.
The Geminid shower has an excellent zenithal hourly rate (ZHR) of 140–150 meteors per hour. A medium entry speed for the meteoroids also makes Geminid trails easier to photograph. A bright event can look pretty spectacular.
The best observing advice to spot a Geminid this year will be to get some sleep on the night of 13/14 December, setting your alarm clock for a 02:00 UT wake up. Prepare yourself with warm clothing and perhaps a hot drink in a flask. A sun lounger or deck chair makes a great viewing platform as it allows you to watch in relative comfort. Aim to view the sky at an altitude of 60º or so. Any direction is fine, although bear in mind that trails will be shortest closer to the radiant and longest 90° from the radiant. Convention dictates that a good compromise is to look 40–50° from the radiant position, which is located near the star Castor (Alpha (α) Geminorum) in Gemini.
Seven planets at sunset
BEST TIME TO SEE: From 28 Dec to 3 Jan, 30 minutes after sunset
RECOMMENDED EQUIPMENT: Naked eye or binoculars
Seven planets are on view in the evening twilight sky towards the end of December. Given a good flat southwest horizon, mag. –4.3 Venus appears around 7˚ up, 30 minutes after sunset. On 28 December, mag. –0.7 Mercury can be seen 4.3˚ below Venus.
Over the following evenings, Mercury maintains its brightness and improves in position compared to Venus. Binoculars are the best bet for finding it, but make sure the Sun has set properly.
As Mercury and Venus approach the southwest horizon, look further east (up and left from the UK) to locate mag. –2.0 Jupiter. Midway between Venus and Jupiter lies dimmer mag. +0.9 Saturn.
True darkness occurs just after 18:00 UT and this is the time to look for Uranus and Neptune. Extend a line from mag. +2.0 Hamal (Alpha (α) Arietis) through mag. +5.2 Eta (η) Arietis for approximately three times again to locate a pair of sixth magnitude objects. The southern one is mag. +5.7 Uranus. Neptune is a binocular planet too. Shining at mag. +7.9, it’s currently located 3.2˚ to the east-northeast of mag. +4.2 Phi (φ) Aquarii.
If you’re wondering where the seventh planet is in this scenario, it’s all around you of course – Earth! The only main planet currently absent from the evening sky is the morning planet Mars.