Dec. 21 is the shortest day of 2020, and that’s good news if you’re planning to scan the night sky for a celestial event that hasn’t been seen to this degree in nearly 800 years.
Jupiter and Saturn, the solar system’s largest planets, can now be seen creeping closer together in the night sky every evening this week. On the winter equinox, those planets will line up almost perfectly to look like a bright double planet in the night sky.
The two planets will be so close that they will appear to be touching, separated by one-fifth the diameter of a full moon. When celestial bodies align, astronomers call it a conjunction, but since this one involves our solar system’s two biggest gas giants, it’s technically a “great conjunction.” Because the event is landing on a holiday week, many have begun calling the formation the “Christmas Star.”
The planets are likely already visible on the southwest horizon after sunset, and dedicated viewers can see them form into alignment over the next week. Though the two planets will look close together by next Monday, they actually will be more than four times the distance between Earth and the sun.
“Look for them low in the southwest in the hour after sunset,” NASA officials said in a news release. And on December 21st, the two giant planets will appear just a tenth of a degree apart – that’s about the thickness of a dime held at arm’s length. This means the two planets and their moons will be visible in the same field of view through binoculars or a small telescope. In fact, Saturn will appear as close to Jupiter as some of Jupiter’s moons.”
It may be helpful to look down from the moon to the horizon and then begin scanning to the right to spot the planets. If you are still having trouble, several available smartphone apps can be used to identify the stars and planets in the night sky.
Dec. 21 will be the closest Jupiter and Saturn have appeared since March 4, 1226. There was a closer conjunction of the planets some 400 years ago, but that event was not visible to many here on Earth, according to Space.com.
Great conjunctions happen on average every 19.6 years. However, the next great conjunction of similar proximity won’t happen until 2080, according to NASA.