A much-anticipated comet – C/2021 A1 (Leonard) – is likely to be 2021’s best comet, and its brightest comet by year’s end. Astronomer Greg Leonard discovered the comet as 2021 began. Discovery images showed a tail for the comet, suggesting we might see a nice tail as Comet Leonard draws closer to the Earth and sun. And telescopic observers and astrophotographers do now see a tail, as photos on this page show. The comet is currently heading sunward, toward its perihelion (closest point to the sun) on January 3, 2022. Comets are typically brightest around perihelion. Comet Leonard has been in the morning sky, but, in December, it will become visible in the evening sky. All in all, it’s time to look for a comet!
Over the coming month, as Comet Leonard heads sunward, it’ll sweep closest to Earth on December 12. It won’t be particularly close at its closest, passing more than 21 million miles (34 million km) away. But six days later – on December 18 – the comet will have an exceptionally close pass of Venus of just 2.6 million miles (4.2 million km). Then it’ll round the sun on January 3, 2022, at a distance of about 56 million miles (0.6 AU, or 90 million km).
Will the comet get bright enough to see with the eye alone in December? It’s possible that Comet Leonard might reach 4th magnitude before its early January perihelion. Comets are diffuse bodies, not pinpoints, so a 4th-magnitude comet won’t appear as bright to your eye as a 4th-magnitude star. The star would be easily visible in a dark sky. The comet? At 4th magnitude, it would be a good binocular comet. It would be fun to see!
Nature provides us with sky events seen once in a lifetime. Comet Leonard might be one of these, if it gets bright enough.
Despite its incredible speed through the vast space of our solar system, don’t expect to see this comet swoosh across the sky. Like planets, comets do move in front of the star background, but only very slowly due to the large distances involved. Observers using telescopes will have to take a close look at the comet’s position relative to background stars. Then compare the view five or 10 minutes later to detect its motion, because its great distance will cause it to appear as a very slow moving object.