This December, we’ll be treated to a rare show: The Moon will occult Mars on the same night the latter reaches opposition.
The opposition of Mars
When Mars, the Sun and the Earth line up every 26 months, the Red Planet is said to be in opposition. That’s also the best time to observe it, since the planet’s closeness to Earth means it appears brighter in the sky and bigger in our telescopes.
The 2022 opposition of Mars occurs on December 8 at 12:34 a.m. EST. However, due to celestial mechanics and the planets’ orbital shapes, Mars will be slightly closer to Earth one week earlier: The Red Planet will be only 81 million kilometres from us, but the difference is minimal compared to the evening of the opposition.
The conditions for observing the planet will be best during the weeks before and after opposition. A telescope will reveal details that would otherwise be difficult to see when Mars is further from the Earth.
This year, Mars lies in the constellation Taurus during its opposition. You can look for it in the east-northeast in the early evening: Its red hue and incredible brightness make it easy to identify as soon as twilight sets in. The planet then rises in the sky until it culminates at about 69 degrees high around midnight—ideal conditions for checking out our celestial neighbour. It then sinks toward the west-northwestern horizon where it disappears at daybreak.
An occultation and an opposition at the same time
By sheer coincidence, this year’s Martian opposition takes place on the same night as the full Moon. Not only will Mars and the Moon be in the same region of the sky, opposite the Sun, but observers in some parts of the world will see the lunar disc approach the Red Planet and eclipse it completely for about 45 minutes on the evening of December 7. This phenomenon, known as occultation, is relatively frequent with stars, but occultations of planets are rare. This one will be visible to all observers in Quebec. In Montreal, the Moon will begin to occult Mars at 10:41:22 p.m. EST. In less than two minutes, the Red Planet will have completely disappeared behind the mountain peaks along the lunar disc’s limb. The planet will then reappear in the sky between 11:29:02 p.m. and 11:30:22 p.m. (Note that the exact times depend on your precise location and may vary by several dozens of seconds depending on your geographical coordinates.)
Pull out your optical instruments to fully enjoy the moment: Even regular binoculars or a small telescope will provide a better view of this very rare phenomenon. The conditions will be excellent for us in Quebec, since the show will take place very high in the sky. We can only hope the weather will be on our side...
Jupiter and Saturn in the evening sky
Several other planets are clearly visible in the evening skies of December, starting with the gas giants Jupiter and Saturn.
On December 1 at twilight, the waxing gibbous Moon can be found 4 degrees to the lower right of Jupiter in the south-southeast. Later that same evening, as the two celestial bodies sink toward the west-southwestern horizon, the gap between them narrows to just 2½ degrees.
By month’s end, the Moon sidles up to Saturn. In the early evening of December 26, you’ll be able to spot the celestial duo above the southwestern horizon; the lunar crescent will lie to the lower left of the Ringed Planet.
The Moon then comes back to visit Jupiter. On the evening of December 28, turn your gaze to the southwest and you’ll find the crescent Moon hanging just below Jupiter. The following evening, it will move to the upper left of the giant planet.
Mercury and Venus form a duo
On Christmas Eve at twilight, the thin crescent Moon will form a triangle with the planets Mercury and Venus. You’ll be able to observe the trio approximately 30 minutes after sunset, very low on the southwestern horizon. Venus is by far the brighter of the two planets, but it will be closer to the horizon; the lunar crescent lies 6 degrees to its left and Mercury hangs directly above, completing a beautiful isosceles triangle.
On December 28 and 29 at twilight, the two planets move to a mere 1½ degrees from each other, again very low in the southwest, approximately 30 minutes after the Sun sets. Mercury sits above Venus on the 28th, then slides to the planet’s right the next day. This is an excellent opportunity to spot Mercury in the sky!
The Moon interferes with the Geminids
The Geminid meteor shower will peak during the night of December 13 to 14. This typically active shower often produces several dozen meteors per hour. Unfortunately, the waning gibbous Moon will spoil the show this year when it rises after 9 p.m. That means the best time to observe the Geminids is in the early evening, when conditions are most favourable.
Lastly, astronomical winter in the Northern Hemisphere starts on the solstice, which occurs December 21 at 4:48 p.m.