Occultations of Uranus and Mars

Two of Earth’s neighbours perform disappearing acts during December

When: Uranus – 5 December 2022 and

Mars – 8 December 2022

Equipment to use: For Uranus, long focal-length telescopes with large apertures (200mm+) are needed to ensure it appears larger than a speck. Use high frame-rate planetary cameras for imaging.

For Mars, a reflecting telescope (125mm+) will show details before the Moon washes out too much detail. Consider moving to a lower magnification telescope and eyepiece for the occultation.

December welcomes two lunar occultations within a few days of each other, when two different planets pass behind the Moon, ‘disappearing’ on one side before ‘reappearing’ on the other. Because of parallax, occultations are location dependent; one place may see it, while others won’t because their view of the event can put the two objects further apart.

First, watch as Uranus disappears behind the Moon due east, in the constellation of Aries, the WaterBearer. Catch it about an hour after sunset. While not fully dark, UK astronomers should be able to see it.

To find Uranus as it occults, pop an RA of 02h52m40s and a dec. of 16˚08’N into your Go-To. If you are looking at the Moon as a clock face, Uranus will disappear at the 10 o’clock position, at 16:46 UT. It then reappears at 17:23 UT in the 1 o’clock position.

Mars‘s lunar occultation follows early on 8 December, when the Moon is at full illumination. Mars will also be at opposition, at 0.54 AU (81 million km) from Earth. The relative distance between Mars and Earth won’t be as small as this again until May 2031, making December 2022 one of the standout occasions to view Mars through a telescope in many years! UK observers can see both the disappearance and reappearance of Mars, meaning a good opportunity for imagers to capture a composite sequence.

Set your alarm for about 04:30 UT and look to the west, where the Moon will be at an altitude of 29˚ between Taurus, the Bull and Auriga, the Charioteer. Mars disappears at 04:55 UT and reappears at 05:56 UT while the Moon is still 20˚ above the horizon.

Get prepared

Expect Uranus to appear as a tiny blue-green disc; it’s challenging not only due to its distance, it’s also not as illuminated by the Sun as the inner planets. A 94 per cent illuminated Moon may disrupt views as the planet gets closer. Imagers can vary short exposures to capture both bodies, boosting ISO or gain to increase signal from Uranus. Don’t expect to capture surface detail without infrared filters.

Because Mars is also at opposition, observers and planetary imagers might want to make a night of it and view Mars from 20:00 UT on 7 December. Find a clear western horizon to capture the full occultation early the following morning; Mars disappears at the 10 o’clock position and reappears at about 4 o’clock.