The top sights to observe or image this month
DON’T MISS
Mars disappears at opposition
BEST TIME TO SEE: 8 December, 04:30 UT–06:30 UT
RECOMMENDED EQUIPMENT: Small/medium telescope
The Moon passes in front of Mars on the morning of 8 December in an event known as a lunar occultation. Although the Moon appears large, bright and dominant when in the night sky, its size is deceptive and in reality, its apparent diameter is pretty small at 0.5° across.
As it appears to travel around the sky over the course of a month, the Moon frequently passes in front of dim stars. Occultations of medium-bright stars occur too, but these are less frequent, and occultations of bright stars are uncommon enough to be quite special. But lunar occultations of planets are rare and to catch one is very special indeed.
You’ll frequently hear the Moon described as our nearest neighbour in space, orbiting at an average distance of 384,400km. This relatively close proximity introduces an effect known as parallax and means that when viewed from locations that are widely separated, the Moon’s position against more distant background objects appears to shift. From the UK, we’ve been lucky in terms of lunar occultations of planets in 2022. In September the Moon moved in front of Uranus, an event that repeats this month on 5 December (see opposite).
On the morning of 8 December, we get our third occultation of a planet when Mars will be hidden by the full Moon. Mars reaches opposition on 8 December too, when it will be opposite the Sun in the sky. This is also why the Moon is full on this date; it too is technically at opposition.
Mars will be shining at mag. –1.9 and presents a disc 17.1 arcseconds across. Consequently, it takes the Moon an extended time to fully cover and subsequently reveal Mars.
From the centre of the UK, disappearance begins at 04:57 UT, Mars taking around 36 seconds to fully disappear. Reappearance begins at 05:57 UT, the Red Planet also taking 34 seconds to reappear. Now remember the mention of parallax. Your location within the UK will affect the timings slightly, varying them by up to a few minutes. This is because locations away from the UK’s centre make the Moon appear in a slightly different position relative to Mars.
The best observing strategy for this event is to start watching from 20 minutes or more before the stated event times, to make sure you don’t miss anything. The event is suitable for viewing with the naked eye, binoculars or a telescope. A mid-power magnification will give an amazing view, showing the Moon well and Mars as a tiny disc next to it.
Lunar occultation of Uranus
BEST TIME TO SEE: 5 December, 16:20 UT–17:50 UT
RECOMMENDED EQUIPMENT: Binoculars
As a prelude to the lunar occultation of Mars on the morning of 8 December, the planet Uranus will also be hidden by the Moon on the evening of 5 December. This is the second lunar occultation of Uranus this year and, in terms of timing, the least favourable of 2022’s three lunar occultations of planets. The Moon will be in an advanced waxing phase at the time of the occultation, 94% lit. This will mean it is the dark preceding limb that hides Uranus, with the planet then reappearing from behind the bright following limb.
Disappearance from the centre of the UK is at 16:50 UT, 50 minutes after sunset, when the sky will be darkening but not fully dark. Shining at mag. +5.6, Uranus will be harder to see because of this. Reappearance is better-timed, but not by much, Uranus reappearing at 17:17 UT from behind the Moon’s northeast limb. The sky will still be twilit, but with a small or medium telescope it should be possible to spot the planet pop out from behind the Moon.
Uranus is a distant world and appears quite small, even through a large telescope. With an apparent diameter of 3.8 arcseconds, it doesn’t take the Moon very long to cover it. However, unlike the 8 December event involving Mars, in this occultation Uranus disappears close to the Moon’s northern limb. As a consequence, the disappearance and reappearance occur at a shallower angle to the Moon’s edge and it takes a fraction longer to cover Uranus than it would if the event were more square-on.
Favourable Ursids
BEST TIME TO SEE: Night of 22/23 December
RECOMMENDED EQUIPMENT: Naked eye
The full Moon on 8 December means that there will still be a bright lunar presence in the sky five days later when the Geminid meteor shower reaches its peak on the night of 13/14 December. Unfortunately that 70%-lit waning gibbous Moon will have a detrimental effect on the number of Geminid meteors that can be seen. However, there is another shower that peaks in December which, while it may not have the pazazz of the Geminids in terms of peak hourly rate, is still a worthy event to observe.
The Ursid shower has its radiant position close to the star Kochab (Beta (β) Ursae Minoris). Although not optimal, this isn’t too bad. The shower has a typical peak rate of 10 meteors per hour and, like the Quadrantids which peak early in January, the maximum activity period is short. For the best views you need to be observing within 12 hours of the peak, which this year is expected around 22:00 UT on the night of 22 December. That bright Moon that will hinder Geminid meteor observation will be completely out of the way on this date, being new at 10:17 UT on 23 December. Consequently, if the sky is clear on the evening of 22 December, this is a good year to try to record some Ursid meteors.