Great Conjunction of Jupiter and Saturn (Chris Vaughan, Starry Night Education)

This Week’s Sky: December 21 to 27, 2020

Saturn and Jupiter meet in their closest conjunction since 1623, a minor meteor shower peaks and the Moon’s Lunar X will be visible for western Canadians.

Monday, December 21 – Northern Winter Solstice (at 10:02 GMT)

Northern Winter Solstice (Chris Vaughan, Starry Night Education)

Winter in the Northern Hemisphere will officially commence on Monday, December 21 at 10:02 GMT (or 5:02 a.m. EST and 2:02 a.m. PST). At that time the Sun will reach the solstice — its southernmost declination for the year, resulting in the lowest noonday Sun, the shortest amount of daylight of the year for the Northern Hemisphere, and the longest amount for the Southern Hemisphere. After the December solstice, the daylight hours will begin to increase for the Northern Hemisphere.

Monday, December 21 – Great Conjunction of Jupiter and Saturn (after sunset)

Great Conjunction of Jupiter and Saturn (Chris Vaughan, Starry Night Education)

In the southwestern sky after sunset on Monday, December 21, Jupiter’s faster orbital motion will bring it within 0.1 degrees of slower Saturn, causing the two planets to appear, to the unaided eye, as a single bright object. The two planets haven’t been as close together since Galileo was using his spyglass in 1623 — and they won’t meet so closely again until 2080. The two planets will easily appear together in the field of view of a backyard telescope at high magnification (inset with red circle). You’ll need to start observing them as soon as you can find them in the darkening sky, because they will set in the west at about 7 p.m. local time. Don’t wait until December 21 to view this spectacular conjunction. They’re already a pretty pair, and they’ll still be telescope-close for more than a week on either side of that date.

Monday, December 21- First quarter Moon (at 6:41 p.m. EST)

First quarter Moon (Chris Vaughan, Starry Night Education)

When the Moon completes the first quarter of its orbit around Earth at 6:41 p.m. EST (or 23:41 GMT) on Monday, December 21, the relative positions of the Earth, Sun and Moon will cause us to see it half-illuminated on its eastern side. At first quarter, the Moon always rises around noon and sets around midnight, so it is also visible in the afternoon daytime sky. The evenings surrounding first quarter are the best for seeing the lunar terrain when it is dramatically lit by low-angled sunlight.

Monday, December 21 – Lunar X (starting around 7 p.m. PST)

Lunar X (Chris Vaughan, Starry Night Education)

Several times a year, for a few hours near its first quarter phase, a feature on the Moon called the Lunar X becomes visible in strong binoculars and backyard telescopes. When the rims of the craters Purbach, la Caille and Blanchinus are illuminated from a particular angle of sunlight, they form a small, but very obvious X-shape. The Lunar X is located near the terminator, about one third of the way up from the southern pole of the Moon (at 2° East, 24° South). The prominent round crater Werner sits to its lower right. The X is predicted to become apparent after about 7 p.m. PST on Monday, December 21, peak at about 11 p.m. PST, and then continue until about 1 a.m. This event should be visible anywhere on Earth where the Moon is shining in a dark sky during that time window. The viewing window corresponds to 3:00 to 7:00 GMT on December 22. Simply adjust for your difference from the Pacific Time zone. For observers in the easterly parts of North America, the Moon will set while the X is visible.

Tuesday, December 22 – Ursid Meteor Shower peak (pre-dawn)

Ursids Meteor Shower peak (Chris Vaughan, Starry Night Education)

The annual Ursids meteor shower, produced by debris dropped by periodic comet 8P/Tuttle, runs from December 17 to 23. The shower will peak during the early hours of Tuesday, December 22, when seeing 5 to 10 meteors per hour is possible under dark skies. The best time to watch will be the hours before dawn. A waxing, half-illuminated Moon will have set at around midnight, leaving the sky nice and dark for seeing meteors. True Ursids will appear to radiate from a position in the sky above the Little Dipper (Ursa Minor) near Polaris, but the meteors can appear anywhere in the sky.

Wednesday, December 23 – Gibbous Moon meets Mars (evening)

Gibbous Moon meets Mars (Chris Vaughan, Starry Night Education)

In the southern sky on Wednesday, December 23, the waxing gibbous Moon will sit a palm’s width below (or 5 degrees to the celestial south of) reddish Mars. By the time the duo sets in the west after midnight, the diurnal motion of the sky will lift the Moon to Mars’ left. You can also try to spot Mars in binoculars during the late afternoon by using the Moon below it as your guide.

Wednesday, December 23 – Moon occults star Nu Piscium (from 10:10 to 11:17 p.m. EST)

Moon occults star Nu Piscium (Chris Vaughan, Starry Night Education)

On the evening of Wednesday, December 23, observers in most of the continental United States and Canada can see the waxing gibbous Moon pass in front of (or occult) the medium-bright star designated Nu Piscium (or ν Psc). The exact start and end times for the event vary by location. Use Starry Night, or another astronomy app, to find out the timing where you live. The event can easily be viewed in a backyard telescope, but your telescope will likely flip and/or invert the regular, binoculars view. Be sure to start watching a few minutes before ingress and egress.

Friday, December 25 – Sinus Iridum’s Golden Handle (all night)

Sinus Iridum’s Golden Handle (Chris Vaughan, Starry Night Education)

On Friday night, December 25, the pole-to-pole terminator that divides the lit and dark hemispheres of the waxing gibbous Moon will fall to the left (or lunar west) of Sinus Iridum, the Bay of Rainbows. The circular, 155 mile (249 km) diameter feature is a large impact crater that was flooded by the same basalts that filled the much larger Mare Imbrium to its right (lunar east), forming a rounded, handle-shape on the western edge of that mare. You can see it with sharp eyes, and easily in binoculars and backyard telescopes. The “Golden Handle” is produced when slanted sunlight brightly illuminates the eastern side of the prominent, curved Montes Jura mountain range that surrounds the bay on the top and left (north and west), and by a pair of protruding promontories named Heraclides and Laplace to the bottom and top, respectively. Sinus Iridum is almost craterless, but hosts a set of northeast-oriented dorsae or “wrinkle ridges” that are revealed under magnification at this phase.

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 to tour the universe together.