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Since September, Jupiter and Saturn have appeared to be moving closer and closer toward each other in the night sky. Tonight, December 21, they will be the closest they have been to each other since 1623.

Although the planets will be hundreds of millions of miles apart, Jupiter and Saturn will appear just a tenth of a degree apart -- roughly the thickness of a dime held at arm’s length, NASA explains.

Why It’s Happening

When two objects appear close to each other in the sky, it’s called a conjunction. A conjunction of Jupiter and Saturn is called a great conjunction because it happens so infrequently.

Here’s why: Earth, of course, completes its orbit of the sun in one year, but Jupiter’s orbit of the sun takes 11.86 years. Saturn’s orbit takes even longer: 29.46 years.

Consequently, the great conjunction of Jupiter and Saturn only happens once every 20 years.

This year, however, the two planets will appear exceptionally close, which is even rarer. Indeed, you’d have to go back to 1623 -- just 13 years after Italian astronomer Galileo Galilei first pointed his telescope to the night sky -- for a closer alignment of the two planets.

When You Can See The Great Conjunction

Jupiter and Saturn will become visible at twilight tonight, weather permitting. Look for them low in the sky above the southwestern horizon in the Northern Hemisphere, or the western horizon in the Southern Hemisphere. They will set within a couple of hours, so it’s important to have a clear view of the horizon.

If you want to see exactly where and when to look for the planets at your specific location, check out Stellarium’s online web app. All you need to do is enter the date, time, and location, and the app will show when it’s best to look for the planets.

What You Will See

The best way to see the great conjunction will be with a telescope. Joe Rao, an instructor at the Hayden Planetarium in New York, writes in an article on Space.com that under high magnification of a telescope, “you’ll be able to see both planets -- Saturn with its famous ring system and Jupiter with its cloud bands and Galilean satellites -- simultaneously in the same field of view!”

The planets will also be visible to the naked eye because they shine so brightly low on the horizon. However, in an interview with USAToday, Rice University astronomer Patrick Hartigan cautioned people to keep in mind that the two planets “are not going to suddenly look like a brilliant ‘Christmas star.’”

Don’t miss this chance to view the great conjunction. The next great conjunction will occur on November 2, 2040, and April 7, 2060. The minimum separation of Jupiter and Saturn on both of those occasions will be 1.1 degrees. That means they will be 11 times farther apart than they will be tonight.

How To Photograph The Great Conjunction

Astronomy author, photographer, and writer from blog The Amazing Sky, Alan Dyer, explains that photographing the conjunction will be challenging because the planets will be so close to each other.

However, for best results, he offers these tips:

  • Be sure to capture the pair of planets over a scenic landscape or urban skyline to place them in context.
  • Keep in mind that the sky and horizon might be bright enough to allow a camera’s auto-exposure and auto-focus systems to work.
  • Use a wide-angle (24mm) to short telephoto (85mm) lens to frame the scene and exposures of no more than a few seconds at ISO 200 to 400 with the lens at f/2.8 to f/4.
  • For most locations, the planets will appear no higher than about 15 degrees to 20 degrees above the southwestern horizon.

December is a big month for stargazers. The big conjunction is only one of three starry spectacles this month. For more travel news, you can read the latest here.

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