Skywatchers will be treated to a cosmic duet tonight (Dec. 17), as the brightest comet of 2021 and the brightest planet pair up for a night sky double feature.
Comet Leonard, also known as Comet C/2021 A1, will be visible from the Northern Hemisphere as it passes near the planet Venus — sometimes referred to as the "evening star." The comet can be seen shortly after the sun goes down in the southwest sky, very low above the horizon.
The comet will be located near Venus, making its closest approach to the bright planet tonight at 9:08 p.m. EST (Dec. 18 at 0208 GMT). The comet is expected to travel within 2.6 million miles (4.2 million kilometers) of Venus. By comparison, the bright comet made its closest approach to Earth on Dec. 12, passing within a distance of about 21 million miles (34 million kilometers). Given Venus' brightness and prominence in the night sky, the planet may help skywatchers locate Comet Leonard, according to EarthSky.
Under clear, dark skies, you can observe Venus with your unaided eye, but to get a good view of Comet Leonard, you'll want to use binoculars or a telescope. The pair will also be close together Saturday evening (Dec. 18) if weather doesn't cooperate today.
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Soon after its Venus flyby, Comet C/2021 A1 — discovered by and named for astronomer Greg Leonard in January 2021 — will continue on its tour of the inner solar system. The comet is expected to make its closest approach to the sun, or perihelion, on Jan. 3 at a distance of 57.2 million miles (92 million km), at which point it will be lost to our view on Earth.
"This is the last time we are going to see the comet," Leonard said in a statement from the University of Arizona. "It's speeding along at escape velocity, 44 miles [71 km] per second. After its slingshot around the sun, it will be ejected from our solar system, and it may stumble into another star system millions of years from now."
Despite its incredible speed, the comet will actually appear to be moving very slowly across the night sky, due to its distance from Earth. The comet may also brighten as it heads toward the sun, which warms the icy body, releasing glowing, ionized gas.
"Comets are typically brightest around perihelion, and the comet has been brightening and is still getting brighter," according to EarthSky. "And, as recent activity shows, there's always the possibility of brightness outbursts as Comet Leonard draws nearer and nearer the sun."
As you look up at the night sky for Venus and Comet Leonard this weekend, keep an eye out for December's full Cold Moon, which rises on Saturday, Dec. 18 at 11:36 p.m. EST (Dec. 19 at 0426 GMT). Even to the casual stargazer, the moon will appear full the night before and after its peak.
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