Photos reveal Jupiter and Saturn aligned in the sky as a 'Christmas star' that hadn't been seen for 800 years

• Photographers from around the world captured Jupiter and Saturn aligning almost perfectly in the night sky this week.
• Monday was the closest the two planets have appeared in 800 years — the last time they were both this close and visible was in 1226.
• The two planets move into alignment, or conjunction, every 20 years. But this year they were close enough to look like a "double planet."

On the eve of the winter solstice, Jupiter and Saturn seemed to touch in the night sky.

The last time the two planets looked this close from Earth's vantage point was nearly 800 years ago. The astronomical event is called a conjunction, the term for an alignment of celestial bodies. Since this conjunction involved the two biggest gas giants in our solar system, it's known as the great conjunction.

The conjunction wasn't a one-night event: The planets' approach in the sky started December 17, and they'll continue to appear close together until Christmas Day. Monday was just when Jupiter and Saturn were at their very closest. So if you haven't seen it yet, you've got a few more nights to try.

This "Christmas star," as the conjunction has been nicknamed, appeared brighter than nearly every star in the sky, according to NASA. That made the two planets easy to photograph.

Katherine Trouche, a science communicator with NASA's Jet Propulsion Laboratory, snapped an impressive photo (below) using her telescope and an iPhone XR.

"To be able to see five moons and two planets over 400 million miles away using nothing but pieces of glass and metal is nothing short of astonishing," she told Business Insider.

A double planet

The distance between Jupiter and Saturn is actually more than four times that between Earth and the sun.

But on Monday, the planets were separated in the sky by a distance equal to about one-fifth of a full moon's diameter. That's so close that the two points of light formed a kind of double planet.

People from around the world captured the rare alignment over the past week.

Photographer Alireza Vafa snapped this view of the moon, Jupiter, and Saturn above the Alborz Mountains in Iran.

Con Kolivas, an engineer and amateur astronomer from Australia, took this photo on Sunday.

Gary Hershorn, a photographer based in New York City, encountered cloudy conditions on Monday. But he captured a shot of the two planets approaching one another the week before, above the Statue of Liberty.

"As a photographer, I have been looking forward to seeing the 'Christmas Star,'" he told Business Insider. "The two planets have been setting next to the Statue of Liberty for the past week getting a bit closer together each day. It was a beautiful sight to see last week as they came down behind the statue."

The best time to catch a glimpse of the conjunction is at twilight, during the hour after sunset. Look low in the sky, toward the southwest. (Websites like Stellarium can help you orient your telescope.)

For those who want to see the event online on Tuesday, the Amateur Astronomers Association of New York is hosting a live stream event from its telescopes on YouTube and Facebook Live starting at 4:30 p.m. ET.

A rare celestial event

Jupiter and Saturn align to some degree every 20 years.

"But it is fair to say that this conjunction is truly exceptional in that the planets get very close to one another," Patrick Hartigan, a professor of physics and astronomy at Rice University, explained on his website.

In the last 2,000 years, there were just two times that Jupiter and Saturn came closer in the sky than they got this year. One was in 1623, but the sun's glare made it impossible to see. The other was in 1226.

Viewers will have to wait until March 15, 2080 to see Jupiter and Saturn as close as they were on Monday.

"The major challenge there is you'll have to stay alive for another 60 years to see it!" Hartigan said.