Comet Leonard in the east on December 9, 2021, pre-dawn | SkyNews
Comet Leonard in the east on December 9, 2021, pre-dawn (Chris Vaughan, Starry Night Education)

This Week’s Sky: December 6 to 12, 2021

It’s an eventful week — the Moon meets Venus, Jupiter, Saturn and Neptune, while Comet Leonard graces pre-dawn skies.

Monday, December 6 – Crescent Moon passes Venus (after sunset)

Crescent Moon passes Venus on December 6, 2021 | SkyNews
Crescent Moon passes Venus on December 6, 2021 (Chris Vaughan, Starry Night Education)

Look in the lower part of the southwestern sky after sunset on Monday, December 6 to see the slim crescent of the young Moon shining a few finger widths below (or three degrees to the celestial south of) the very bright planet Venus, close enough to share the view in binoculars (green circle). Viewed in a telescope that night, Venus will display a 24-per-cent-illuminated crescent phase — similar to, but somewhat thicker than the Moon’s. Ensure that the Sun has fully set before aiming optical aids low in the western sky.

Tuesday, December 7 – Moon meets Saturn (early evening)

Moon meets Saturn on December 7, 2021, early evening | SkyNews
Moon meets Saturn on December 7, 2021, early evening (Chris Vaughan, Starry Night Education)

The Moon’s monthly visit with the bright gas giant planets Saturn and Jupiter will kick off after dusk in the southern sky on Tuesday, December 7. Before the sky has fully darkened, try using binoculars (green circle) to find the yellowish dot of Saturn positioned a palm’s width to the upper left (or 6 degrees to the celestial northeast) of the waxing crescent Moon. Or wait until Saturn is visible with your unaided eyes. Much brighter and whiter Jupiter will be shining off to their upper left. Even brighter Venus will gleam to their lower right.

Wednesday, December 8 – Moon hops past Jupiter (evening)

Moon hops past Jupiter on December 8, 2021 | SkyNews
Moon hops past Jupiter on December 8, 2021 (Chris Vaughan, Starry Night Education)

In the southwestern sky for several hours after sunset on Wednesday, December 8, the crescent Moon will shine below and between the bright, white dot of Jupiter and somewhat fainter and creamy-colored Saturn. Very bright Venus will blaze to their lower right (celestial west) until it sets at about 7 p.m. local time. On Thursday night, the Moon will hop east to sit a generous palm’s width to the left of Jupiter. The Moon will then bid adieu to those bright planets until January 4-5.

Thursday, December 9 – Comet Leonard in the east (pre-dawn)

Comet Leonard in the east on December 9, 2021, pre-dawn | SkyNews
Comet Leonard in the east on December 9, 2021, pre-dawn (Chris Vaughan, Starry Night Education)

If current predictions hold, the mornings around Thursday, December 9 will offer fine opportunities to see Comet C/2021 A1 (Leonard) while it is higher and shining in a dark, moonless sky. Although the comet won’t reach peak brightness for several more days, its location one-third of the way up the eastern sky, near the circle of stars that form the head of Serpens Caput (the Snake’s Head), should provide good views of it in binoculars (green circle) — and possibly with your unaided eyes. On December 9, the comet will rise at about 3:30 a.m. local time — but make your viewing attempt before 5:30 a.m., when the dawn sky will begin to brighten. Comets are notoriously unpredictable — but skywatchers are looking forward to a great show from this one.

Read more about how to catch Comet Leonard this month!

Friday, December 10 – Half Moon near Neptune and Pallas (evening)

Half-moon near Neptune and Pallas on December 10, 2021 | SkyNews
Half Moon near Neptune and Pallas on December 10, 2021 (Chris Vaughan, Starry Night Education)

In the southwestern sky on the evening of Friday, December 10, the waxing, half-illuminated Moon will be positioned near the faint planet Neptune and the main belt asteroid designated (2) Pallas. The magnitude 7.9 planet will be located a palm’s width above (or 5.5 degrees to the celestial north of) the Moon, and the fainter, magnitude 9.9 planet will be a similar distance to the Moon’s lower right (celestial west). The nearby bright Moon will make them harder to see, so find the tight trio of stars named Psi Aquarii above the Moon and use them as a guide to Neptune and Pallas on a subsequent night when the Moon has moved away. On Friday evening, observers in India and much of China can see the Moon occult the asteroid.

Friday, December 10 – Two shadows and the Great Red Spot cross Jupiter (evening)

Two shadows and the Great Red Spot cross Jupiter on December 10, 2021 | SkyNews
Two shadows and the Great Red Spot cross Jupiter on December 10, 2021 (Chris Vaughan, Starry Night Education)

On Friday evening, December 10 in the Americas, observers with telescopes can watch the small round shadows of two of Jupiter’s moons cross the planet — one of them accompanied by the Great Red Spot! At 5 p.m. EST (or 22:00 UTC), the shadow of Callisto will be completing a crossing of the planet that began at 18:15 UTC. A few minutes later, the Great Red Spot and the shadow of Europa will rotate into view on the opposite side of Jupiter’s disk. The shadow and the spot will complete their own crossing several hours later, at about 8 p.m. EST (or 1:00 UTC on December 11).

Saturday, December 11 – First quarter Moon (at 1:35 UTC)

First quarter Moon on December 11, 2021 | SkyNews
First quarter Moon on December 11, 2021 (Chris Vaughan, Starry Night Education)

The Moon will complete the first quarter of its monthly journey around Earth at 1:35 UTC on Saturday, December 11. That translates to 8:35 p.m. EST on Friday, December 10. At first quarter its 90 degree angle from the Sun will cause us to see the Moon exactly half-illuminated – on its eastern side. At first quarter, the Moon always rises around mid-day and sets around midnight, so it is also visible in the afternoon daytime sky. The evenings around first quarter are the best ones for viewing the lunar terrain while it is dramatically lit by low-angled sunlight, especially along the terminator, the pole-to-pole boundary separating its lit and dark hemispheres.

Sunday, December 12 – The Lunar Straight Wall (evening)

The Lunar Straight Wall on December 12, 2021 | SkyNews
The Lunar Straight Wall on December 12, 2021 (Chris Vaughan, Starry Night Education)

On Sunday evening, December 12, the pole-to-pole terminator that divides the lit and dark hemispheres of the waxing gibbous Moon, will fall just to the left (or lunar west) of Rupes Recta, also known as the Lunar Straight Wall. The rupes, Latin for “cliff,” is a north-south aligned fault scarp that extends 110 kilometres across the southeastern part of Mare Nubium, which sits in the lower third of the Moon’s Earth-facing hemisphere. The wall, which is very easy to see in good binoculars and backyard telescopes, is most prominent a day or two after first quarter, and also the days before last quarter. For reference, the very bright crater Tycho is located due south of the Straight Wall.

Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the Universe together.

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