Jupiter Closes in on Saturn as Great Conjunction Approaches

Jupiter (right) and Saturn huddle about 2° apart on Tuesday night, Dec. 1 at 5:30 p.m. local time. They're headed for an exceptionally close conjunction on Dec. 21. Bob King

Closer... closer... closer... It seems like it's taking forever for these two evening planets to reach conjunction, but rest assured it will happen. The big night is Monday, Dec. 21, when the two outer planets will squeeze together so tightly you'll barely be able to split them apart with the naked eye. 

Jupiter and Saturn move slowly across the sky because they're distant from Earth. Jupiter is the closer of the two, so it moves more quickly, taking about a year to pass through each constellation of the zodiac and completing one orbit in 12 years. Farthest of the classical planets, Saturn takes 29 years to make a spin around the sun. Every 20 years, faster Jupiter laps Saturn in a Great Conjunction. 

Speedier Jupiter shrinks the apparent distance between the two planets until they're closest on the winter solstice. Stellarium with additions by the author

Both planets move to the east as they orbit the sun, but because Jupiter travels faster it gains on Saturn and decreases the apparent distance between the two. Back on the first day of fall they were 7.5° apart or about four fingers held together at arm's length. Tonight they're just shy of 2°. Come Dec. 21, the first day of winter, they'll stand 20 times closer — just 0.1­° apart! 

Because each planet's orbit is tilted slightly with respect to the plane of Earth's orbit the separation between them varies at every Great Conjunction. This time around we're in for a real treat. Jupiter and Saturn will squeeze together more tightly than at any other time since July 16, 1623. That makes this a not-to-miss event. 

Right now, the duo is visible rather low in the southwestern sky during evening twilight starting about 45 minutes after sunset. Although they set around 8 o'clock local time it's best not to wait too late to catch a look. My favorite viewing window is from an hour to 90 minutes after sunset. Click here to find your sunset time.

Amateur astronomer Piqui Diaz of Ezeiza, Argentina used a basic cell phone to take pictures of Jupiter and Saturn through a pair of 7x binoculars on Oct. 20 and Nov. 30 and then combined them into an animation showing the planets' changing separation. Whether you use binoculars or a straight camera, now is a good time to begin photographing your own sequence of the merging planets. To create the animation upload your photos to gifmaker.me. Piqui Diaz 

If you want to get an idea of just how close Jupiter will come to Saturn when they conjoin on the winter solstice, direct your gaze 10° (one balled fist at arm's length) above Jupiter. There you'll see two faintish stars: Algedi (Alpha Capricorni) and Dabih (Beta Capricorni) in the constellation Capricornus. The upper star, Algedi, is an "optical" double star — not a real, physical binary sun but two separate stars seen along the same line of sight. 

One Dec. 21 Jupiter and Saturn will be as close together as the Algedi (Alpha Capricorni) pair visible near the top of the frame. Bob King

The pair is separated by 0.1°, the same distance Jupiter and Saturn will be during their Great Conjunction. How's that for convenience? If I look carefully I can split Algedi into two stars. Can you? I'm curious if Jupiter's extra brilliance might "hide" fainter Saturn. Will you need binoculars to split them? We'll just have to wait and see.