2020 is drawing to a close and with it the lovely planetary show that has graced our skies these past few months, as the planets gear up for their final act before retreating or disappearing altogether. Skywatchers will be wowed by the lovey-dovey duo of Jupiter and Saturn, while Venus and Mars linger on the celestial stage... amid a backdrop of shooting stars.
Closer than ever
The show-stopping event of December is without a doubt the Great Conjunction of Jupiter and Saturn. With orbital periods of 12 and 30 years, respectively, the two giant planets appear close together in the sky—from our viewpoint on Earth—only every 20 years or so. However, the minimum gap varies from one conjunction to another depending on the planets’ relative position to the nodes of their slightly inclined orbits. On December 21, 2020, Jupiter and Saturn will be separated by just 6 arc minutes, or 1/10th of a degree! As a reminder, the full Moon is ½ a degree across. This is the closest Great Conjunction since 1623, and we won’t see Jupiter and Saturn this close together again before 2080.
Here are a few tips to ensure you get the best views of this unique show. Look for the planetary duo very low on the horizon as the Sun sets. You’ll only have a short viewing window before the planets sink below the horizon, barely two hours after the Sun. If you have a clear view of the southwestern horizon, try your luck at spotting these remarkably and persistently bright beacons of light as soon as darkness gathers. To the naked eye, the two planets will shine almost as one, but with Jupiter blazing ten times brighter than Saturn, the duo will appear “lopsided.” Any instrument will deliver sharper views of the finer details. Even a medium-power telescope (150X) will capture both giant planets and some of their moons in the same field of view. You’ll see the four Galilean satellites gracefully orbiting Jupiter, with Titan—due north of Saturn—appearing to loom over the entire scene.
December 21 marks the climax of this Great Conjunction, but the two planets rub shoulders throughout the month. They appear less than one degree apart from December 12 to 29. Take the time to admire their progression before and after the key date of December 21, bearing in mind that the viewing window (before the planets set for the night) shrinks with every passing evening. On December 16, a very thin crescent Moon glides about 3 degrees below the planetary duo; a similar scene repeats the next day, as the growing lunar crescent sits slightly above the pair.
Venus, a fading morning beacon
Since reaching its greatest elongation on August 12, Venus has been slowly working its way toward the Sun. The planet then rapidly heads downward in December, in a clear “changing of the guard” move. It gradually exits the morning sky, and by spring 2021, will begin to illuminate our evening skies. Venus can still be easily spotted just west of Libra at the beginning of the month, from two hours before sunrise until the crack of dawn. However, as the planet heads toward the Sun, it crosses the entire constellation in just two weeks. After a brief pass over Scorpio’s pinchers, it ventures into Ophiuchus as of December 21. So, anyone hoping to catch a glimpse of its glow in the hour before the Sun rises must have a clear, unobstructed view of the southeastern horizon. A romantic scene awaits you on December 12 and 13, when a very thin crescent Moon snuggles up to Venus. Your best bet is the 12th, when the lunar crescent will be slightly thicker and higher than on the 13th, and located a mere 4½ degrees to the upper right of the Shepherd’s Star.
Mars, still bright red and spectacular
The Red Planet has lost some of its glory since its opposition on October 13 but can still be easily seen in December. It reigns supreme in the constellation Pisces, where no other objects compete for attention, except of course for a lovely waxing gibbous Moon that drops in for a visit on the 23rd. Mars is still perched high in the south in the early evening, and its westward dive takes place in the first half of the night. As the month progresses, you’ll see its brightness fade by half after passing the symbolic magnitude –1 threshold on December 5.
An exceptional year for the Geminids
The Geminids are considered the most consistent and reliable meteor shower year after year. And 2020 is shaping up to be an excellent year as this shower, known for its relatively slow but bright meteors, is expected to peak on the moonless night of December 13. The radiant, which lies near the star Castor, is visible all night long and culminates around 2 a.m., less than 15 degrees from the zenith. This means we can hope to see about 70 shooting stars per hour under perfectly clear skies away from light pollution! What a great way to enjoy those beautiful, cold winter nights.