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Planets align to form rare trio this weekend

The milky way and a starry night sky is seen over a forest in springtime.
The milky way and a starry night sky is seen over a forest in springtime.Adobe Stock

Make sure to look up at the evening sky this weekend. Otherwise, you might miss a special planetary alignment that won’t be seen again anytime soon.

Mercury, Jupiter, and Saturn will form a planetary trio that will be visible through Monday evening. Like other trios, this triangular cluster of planets has come together for just a few days to form a rapidly changing and beautiful sight, said Amy Oliver, a spokeswoman for the Harvard-Smithsonian Center for Astrophysics.

“This shape is just a blip in time,” Oliver said. “We won’t see it exactly like this ever again, since it probably isn’t going to happen again in this exact same way — at least not in your lifetime.”

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While these three planets can often be seen from Earth, it’s unusual for them to come so close together. Oliver said the last time they came anywhere near each other was in 2000, largely because they move at different speeds around the sun.

“Mercury has somewhere to go...” she said. “Because [Mercury and Jupiter] are orbiting slower, it’s going to take longer for them to get farther apart. After the next few days, they won’t all be in the same plane of view together.”

Jupiter will shine brightest and be visible at the top of the triangle, with Mercury and Saturn making up its base angles, said Federica Spoto, an astronomer at the Harvard-Smithsonian Center for Astrophysics.

The shape will be visible just above the southwestern horizon about 30 to 45 minutes after sunset, but don’t be late. The trio will disappear around 15 or 20 minutes after it shows up.

Saturday and Sunday evening will have clear skies perfect for viewing the stars and planets, according to the National Weather Service. However, a few clouds could obscure the planetary trio on Monday night.

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“As always, it is best to find dark places where the bright lights of the city won’t affect visibility,” Spoto said. “Be sure that you don’t have landscape covering the horizon.”

Anna Frebel, an astronomer and associate professor at the Massachusetts Institute of Technology, said you can see the planets better with binoculars, or by zooming in on them with your cell phone camera.

“Everything moves, albeit slowly, and ever so often we find ourselves presented with the opportunity to witness the gentle passages of our planetary neighbors,” Frebel said. “For folks new to the night sky, this is an excellent chance to... watch the many moons of Jupiter and the marvelous rings of Saturn.”

Saturday will also be the last evening where Jupiter and Saturn will be visible with the naked eye, even though binoculars may already be needed for Saturn, Spoto said. In the next few days, both planets will disappear from the night sky for several months.

This trio won’t be as close together as the conjunction between Jupiter and Saturn was in December. During that event, the two planets were as far a part as the thickness of a dime held out at arms length, Oliver said.

This weekend, Mercury, Jupiter, and Saturn will be about five degrees apart, or the distance between your ring finger, middle finger, and pointer finger when held up against the sky, she said.

“It’s so much different to see it with your own two eyes than through pictures or live streams, of which there will be plenty tomorrow night,” Oliver said. “It’s just so much more amazing to see this way.”

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The last planetary trio was in 2015, and one among Mercury, Venus, and Jupiter will occur on Feb. 13, Spoto said. After that, another trio won’t be seen until April 2026, when Mercury, Mars, and Saturn will come close together.

“In ancient times, people looked at the sky because they thought it could influence their lives,” Spoto said. “We know now that’s not how it works, since the positions of the planets don’t influence our lives, but this doesn’t make these events less beautiful.”

Caroline Enos can be reached at caroline.enos@globe.com. Follow her on Twitter @CarolineEnos.