By Bruce McClure and Deborah Byrd
December 2020 will show you three bright planets as soon as darkness falls: Mars, Jupiter and Saturn. Venus, the brightest planet, is in the east before sunrise.
On December 21, Jupiter and Saturn will have their once-in-20-years conjunction in our sky, appearing closer than they have for centuries, only 1/5 of a moon-diameter apart.
Jupiter and Saturn come out first thing at nightfall in December 2020, and will continue to do so until the year’s end. These two worlds appear bright and beautiful at nightfall, though they sit low in the southwest sky. Day by day, the twosome appears a bit lower in the sky at nightfall, and sets sooner after dark.
To see these worlds, find an unobstructed horizon in the direction of sunset, and seek them out rather low in the sky as soon as darkness falls.
For the first time since the year 2000, Jupiter and Saturn will have a great conjunction this year, on December 21.
Great conjunctions of these two giant worlds happen every 20 years, but this year’s event will be the closest Jupiter-Saturn conjunction since the year 1623.
Astronomers use the word conjunction to describe meetings of planets and other objects on our sky’s dome. They use the term great conjunction to describe a meeting of Jupiter and Saturn. The last great Jupiter-Saturn conjunction was May 28, 2000. The next one will be December 21, 2020. Watch for these worlds to edge closer and closer together throughout the first three weeks of December 2020.
If you have a telescope, it’s best to use it when Jupiter and Saturn are highest up for the night at dusk/nightfall. In other words, you have to catch them soon after sunset. Typically, the view of Jupiter’s four major moons and Saturn’s glorious rings through the telescope is sharper when these worlds are higher up than lower down.
The thickness of the Earth’s atmosphere near the horizon tends to blur the view of Jupiter’s moons and Saturn’s rings.
Look first for brilliant Jupiter; Saturn is the bright object immediately to Jupiter’s east for the first three weeks of December. Although Saturn is easily as bright as a 1st-magnitude star – as bright as the brightest stars in our sky – the ringed planet can’t compete with the the king planet Jupiter, which outshines Saturn by some 11 times.
After all, Jupiter almost always ranks as the fourth brightest celestial object, after the sun, the moon and the planet Venus, respectively, although Mars temporarily reigned as the fourth-brightest celestial body – and Jupiter as the fifth-brightest – in October.
Watch for the moon in the neighborhood of Jupiter and Saturn for a few days, on December 16 and 17.
Mars lords over the southeastern sky as darkness falls in December from northerly latitudes. From the Southern Hemisphere, Mars is seen high in the northern sky at nightfall.
Its fiery-red splendor lights up the nighttime well past midnight. In October 2020, Mars was brighter than it will be again until September 2035. That’s because, on October 13, Mars reached opposition in our sky, when it was opposite the sun as seen from Earth.
At opposition, Earth was sweeping between Mars and the sun. Now – in its smaller, faster orbit – Earth is now rushing along, leaving Mars behind. Thus, in the months ahead, Mars will slowly but surely dim in our nighttime sky.
Even so, Mars remains bright and beautiful all throughout December. Let the moon help guide your eye to Mars for several nights centered on or near December 23.
Venus – the brightest planet – reached its greatest elongation from the sun in the morning sky on August 12 or 13 (depending upon your time zone). But dazzling Venus will remain bright and beautiful as a morning “star” for the rest of this year, and for the first few months of 2021.
Throughout December, Venus in its faster orbit around the sun will be going farther and farther away from Earth. As viewed through the telescope, Venus’ waxing gibbous phase will widen, yet its overall disk size will shrink. Venus’ disk is 89-percent illuminated in early December, and 94-percent illuminated by the month’s end; Venus’ angular diameter, on the other hand, will shrink to about 91-percent of its initial size by late December.
Watch for the waning crescent moon to shine with Venus in the morning sky for several days, centered around December 12.
Mercury won’t be easy to catch this month from anywhere around the world. Mercury starts out the month in the morning sky and ends the month in the evening sky. Look for this world in your western sky after sunset in January 2021.
By bright planet, we mean any solar system planet that is easily visible without an optical aid and that has been watched by our ancestors since time immemorial.
In their outward order from the sun, the five bright planets are Mercury, Venus, Mars, Jupiter and Saturn. These planets actually do appear bright in our sky.
They are typically as bright as – or brighter than – the brightest stars. Plus, these relatively nearby worlds tend to shine with a steadier light than the distant, twinkling stars.
You can spot them, and come to know them as faithful friends, if you try.