The planets’ orbits are nearly co-planar, a vestige of their common origin. Since no two move at the same speed, they periodically pass one another in the sky, known as planetary conjunctions.
Some planetary pairings happen more frequently than others. The slowest naked-eye planets, Jupiter and Saturn, only line up in our sky every 18-20 years (a so-called “great conjunction”). Great conjunctions occasionally arrive in groups of two or three should they occur when the planets are being overtaken by the faster-moving earth, which makes them appear to backtrack and pass each other over several months’ time.
The fastest planets, Mercury and Venus, pass one another up to five times a year, frequently when the innermost planets are too close to the sun to observe.
In short, there’s no simple pattern to how often any two planets will appear together in the sky.
Jupiter, which passed behind the sun around Thanksgiving, is now emerging from morning twilight, just as Mercury is making its best morning appearance of the year. They’ll pass within less than a degree (two full-moon widths) of one another on Friday, Dec. 21.
The last five Jupiter-Mercury conjunctions were too close to the sun to view. Before that was May 27, 2013, when they were over two degrees apart. So next week’s rendezvous between the largest and smallest planet will be worth a look.
The tricky part will be seeing the planetary pair in morning twilight. They’ll rise in the east-southeast just after 6:30 a.m. With clear skies, an unobstructed horizon, and a bit of luck, you might catch them before they’re swallowed up by the pre-dawn glow. Binoculars can improve your chances—just be sure to put them away before sunrise, just after 8:00 a.m., so you don’t risk eye damage.
Next column: The largest stars.