Mercury, Jupiter and Saturn are currently converging in the night sky.
Monday is the last night on which the three planets will be erected and visible in the twilight. On Sunday, they looked the closest in more than two decades, 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’ll never see it that way again, because it’s probably not going to happen exactly the same way again – at least not in your lifetime.”
An astronomical event in which celestial bodies are so aligned is called a conjunction. A triple alignment like this is known as a planetary trio.
If you hold your palm up in the air and converge all three planets within a circle that fits in the space between your ring finger and your index finger, it’s a trio.
Here’s how to see the planetary trio before it disappears.
Drive out at dusk and bring binoculars
The surface of Mercury, as photographed by the Mariner 10 spacecraft in 1974.
Space Frontiers / Hulton Archive / Getty
Go out on Monday at dusk, between half an hour and 45 minutes after sunset. Look at the southwest sky. The brighter the sky, and the greater the father of the city lights, the easier the connection will be seen.
Jupiter will appear to the naked eye the brightest (it is about ten times brighter than Saturn), followed by Mercury, then Saturn.
Since Saturn is so faint, it can not be distinguished with the naked eye from the night glow of the sun. So the best way to see the planetary triangle is to set your eyes on Jupiter, which will be near the top, and then point a pair of binoculars at it. According to EarthSky, Mercury and Saturn must appear in the same binocular field as Jupiter.
After Monday, Jupiter and Saturn will sink below the horizon and no longer be visible, while Mercury will continue to rise in the sky night after night, gradually moving away from the other two planets.
Although the three worlds seem to be almost touching during the planetary trio, Jupiter and Saturn are actually separated by almost five times the distance between the earth and the sun. Mercury and Saturn are separated almost twice.
The last time these 3 planets aligned so closely was in 2000
A photograph of Saturn and two of its moons, taken by Voyager 1 in 1980.
SSPL / Getty
Astronomers turned their telescopes skyward last month to see another collaborative event, when Jupiter and Saturn brought in closer alignment than they had for centuries.
In the last 2000 years, Jupiter and Saturn have only twice come closer to the sky: one was in 1623, but the brightness of the sun made it impossible to see. The other was in 1226.
Planetary trios, on the other hand, are much more common. The last one was in October 2015. Another trio with Mercury, Venus and Jupiter will take place on February 13, according to EarthSky.
The last time Mercury, Jupiter and Saturn formed a triangle was in May 2000.