Despite this most recent orbit being dominated by earthly struggles, 2020 has been an incredible year for astronomy. Whether it's SpaceX making headlines by launching Starlink satellites to provide wi-fi for the world or sending astronauts to the International Space Station (twice!) or the unexpected delight of watching Comet NEOWISE journey across the sky, there have been some inspiring reasons to keep gazing up all year long.
Before the ball drops and we reset our calendars, there's one more spectacular astronomical experience to mark on your calendar: the Great Conjunction of Jupiter and Saturn Dec. 21, 2020. This celestial event is a once-in-a-lifetime opportunity to see the gas giants of our solar system appear so close together in the sky that they appear to touch. (They won't in fact and will actually be 400 million miles apart – it's all a matter of perspective!)
Based on their orbits, Jupiter (which orbits the sun every 11.9 years) and Saturn (every 29.5 years), the two planets appear close together roughly every 19.6 years. When they do, it's called a Great Conjunction, and the last one occurred in the dawn hours of May 28, 2000.
This year's Great Conjunction is particularly special, as it's the closest these two planets will appear in the sky since the 13th century. "This conjunction is exceptionally rare because of how close the planets will appear to one another," said Patrick Hartigan, an astronomer at Rice University in Texas in a press release. "You'd have to go all the way back to just before dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky." In more common language, it's been nearly 800 years since Jupiter and Saturn have appeared this close together, due to the two planets' orbits – as well as our earthly one. (Although there was a close Jupiter-Saturn conjunction in 1623, it was too near to the sun to be seen without a telescope and so was likely not observed by many — the telescope being a new object at the time.)
The planets will appear very close: less than 1/5th the diameter of a full moon, or roughly 0.1 degrees apart in the sky. Astronomers use degrees as the largest unit of distance between objects in the sky; most times when two planets appear close together, they're within 2-4 degrees of one another. With Saturn and Jupiter being so close together, they may look like a double planet. "For most telescope viewers, each planet and several of their largest moons will be visible in the same field of view that evening," Hartigan said. If you don't have a telescope, you can still see them with a pair of binoculars, a clear horizon and some patience.
The Great Conjunction will be visible across Earth, though the timing will depend on your location and latitude. The best viewing prospects are near the equator, though those in the Northern Hemisphere will have a shorter viewing window before the planets set beyond the horizon. For American viewers, the best time to observe this might be twilight. You can start looking from Dec. 17 through Christmas, though the closest approach is Dec. 21.
And even though we had a very long wait to see this event, it's going to happen again on March 15, 2080, a relatively short 60 years from now.