Heaven knows that Williams College astronomer has seen his share of eclipses

Jay Pasachoff, professor of astronomy at Williams College, says that while the “great conjunction” of Jupiter and Saturn on Monday will be a sight to behold, such alignments have been known for centuries.

As you look into the night sky around sunset these days, you might see Jupiter and Saturn, two unblinking dots amid the stars, drawing closer together. On Monday, this solar system’s biggest planets will appear closer to each other than they have since Galileo Galilei was alive.

During the “great conjunction,” the two planets will appear to be just one-tenth of a degree apart in the night sky — despite being separated by hundreds of millions of miles. While they pass by each other once every couple of decades on their separate orbits around the sun, they have not appeared this close since 1623.

Monday’s great conjunction will not be matched until 2080, when Saturn and Jupiter once again will appear less than a finger’s width apart in the sky.

Jay Pasachoff, professor of astronomy at Williams College, explains the phenomenon.

What is happening?

“On Dec. 20 and 21, Jupiter and Saturn will be so close together that they’re less than a fifth of the diameter of the moon away from each other. If you’re looking with a telescope, they’re so close you’d see them in the same view.”

How do I see it?

“If you look up at the sky, any clear night, right after sunset, Jupiter and Saturn are in the southern sky, a little to the left of south. Jupiter’s about 10 times brighter than Saturn, but they both stand out in the sky. Every day until Monday, they’re going to look closer and closer together.

“For the general public, it doesn’t really matter which night you look up for the next couple of weeks. I saw them Saturday night, and they look very pretty in the southern sky.”

Are they actually close together?

“No. One is hundreds of millions of miles farther away than the other. Jupiter is about five times farther away from the sun than the Earth is. And Saturn is 10 times farther away from the sun than the Earth is. So, Saturn is more than 400 million miles further away from us than Jupiter is. They just happen to be in close to the same direction, but they’re really quite far apart.”

Why do these conjunctions happen?

“Jupiter’s going around the sun more rapidly than Saturn is. So, it overtakes Saturn every 20 years or so.”

How long have we known about this phenomenon?

“People have noted the positions of the planets in the sky for thousands of years. [William] Shakespeare wrote about a conjunction in ‘Henry IV, Part 2.’ [German astronomer] Johannes Kepler, in his 1603 almanac, described one of these conjunctions that was going on then.”

{div class=”asset-tagline text-muted”}Francesca Paris can be reached at fparis@berkshireeagle.com and 510-207-2535.{/div}

Francesca Paris covers North Adams for The Berkshire Eagle. A California native and Williams College alumna, she has worked at NPR in Washington, D.C. and WBUR in Boston, as a news reporter, producer and editor. Find her on Twitter at @fparises.