Mercury, Jupiter, and Saturn are clustering in the night sky in a rare, three-planet conjunction event. Here's how to see it on Monday.

• Mercury, Jupiter, and Saturn are aligned in the night sky, appearing closer together than they've been since 2000.
• When three planets move into such a conjunction, it's known as a planetary trio.
• Here's how to see the conjunction on Monday night.

Mercury, Jupiter, and Saturn are currently clustering together in the night sky.

Monday is the final night that the three planets will be lined up and visible at twilight. They appeared the closest they'd been in more than two decades on Sunday, forming an equilateral triangle.

"This shape is just a blip in time," Amy Oliver, a spokeswoman for the Harvard-Smithsonian Center for Astrophysics, told the Boston Globe. "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."

An astronomical event in which celestial bodies align like this is called a conjunction. A triple alignment like this is known as a planetary trio.

If you hold your palm up to the sky and all three planets cluster within a circle that fits in the space between your ring finger and your pointer finger, that's a trio.

Here's how to see the planetary trio before it's gone.

Head out at twilight, and bring binoculars

On Monday, head out at twilight, between half an hour and 45 minutes after sunset. Look to the southwestern sky. The clearer the sky is, and the father from city lights you are, the easier it will be to see the conjunction.

Jupiter will look brightest to the naked eye (it's about 10 times bright than Saturn), followed by Mercury, then Saturn.

Since Saturn is so dim, it may not be distinguishable from the sun's afterglow with the naked eye. So the best way to spot the planetary triangle is to focus your eyes on Jupiter, which will be near the top, then point a pair of binoculars at it. Mercury and Saturn should appear in the same binocular field as Jupiter, according to EarthSky.

After Monday, Jupiter and Saturn will dip below the horizon and no longer be visible, while Mercury will continue rising in the sky night after night - moving steadily away from the other two planets.

Although the three worlds seem to almost touch during the planetary trio, Jupiter and Saturn are actually separated by almost five times the distance between Earth and the sun. Mercury and Saturn are separated by nearly twice that distance.

The last time these 3 planets aligned so closely was in 2000

Astronomers turned their telescopes skyward last month to catch another conjunction event, when Jupiter and Saturn aligned more closely than they had for centuries.

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

Planetary trios, by contrast, are far more common. The last one was in October 2015. Another trio, involving Mercury, Venus, and Jupiter, will happen on February 13, according to EarthSky.

The last time Mercury, Jupiter, and Saturn formed a triangle was in May 2000.