Comet C/2020 F3 NEOWISE will continue to be visible after sunset this week. It’s beginning to diminish in brightness, but you can still see it with your unaided eyes against a dark sky. All you will need are clear skies to the northwest and a view that is free of obstructing trees and buildings. If you live in an apartment with western or north-facing windows or a balcony, you’re in luck.
This comet looks its best when seen through binoculars. What you should expect to see is a small, bright, fuzzy spot, possibly with a greenish hue — and the comet’s faint tail extending generally upwards and to the right, away from the Sun. Actually, look for two tails pointed in slightly different directions — a brighter one composed of debris the comet is dropping behind it, and a fainter, blue-tinted one composed of ionized gas. That second tail will always point directly away from the Sun, since it is being pushed by the solar wind. While your telescope will magnify the comet’s head nicely, its tail, which will extend beyond your limited field of view, won’t be as impressive.
NEOWISE’s trajectory will bring it closest to the Earth (or perigee) at about 9 p.m. EDT on Wednesday, July 22, when it will be 103.5 million kilometres or 0.692 Astronomical Units (1 AU is the mean Earth-Sun separation) away from us. Unfortunately, as the comet nears Earth and looms larger, it will also be experiencing less heating from the Sun, causing it to fade in visual brightness due to less gas production. It’s a trade-off that makes looking at the comet at the first opportunity your safest bet. What’s more, the pesky Moon will return to the evening sky soon.
From our point of view on Earth, the comet is heading in a northwesterly direction — along the belly and legs of the Great Bear, Ursa Major. The northerly component of that motion has caused the comet to become circumpolar for observers living north of about 41 degrees North latitude. That means it never drops below the horizon, but it also sits very close to the horizon during the night, making it best visible only during the post-sunset and pre-dawn periods. Even better, that northerly motion will also lift the comet nearly a thumb’s width (or 2 degrees) higher every night, making viewing much easier.
The comet is carried with the stars as the Earth turns. If you see something rapidly crossing the sky, it’s a plane or a satellite — such as the International Space Station.
After 10 p.m., the comet will become visible without binoculars — if your skies are clear and not too light-polluted. The easy way to find the comet in evening is to look for the bright stars Dubhe and Merak high in the northwestern sky. They mark the bottom edge of the Big Dipper’s bowl. The comet will be directly below them Monday evening. If your view to the north is unobstructed, you should be able to watch the comet until at least midnight.
Each night this week, the comet will shift left with respect to the Big Dipper’s stars. On Wednesday, the night when the comet is closest approach to Earth, the path of Comet NEOWISE will carry it just a couple of finger widths above a pair of medium-bright stars named Tania Australis and Tania Borealis. They form the bear’s rear paws. On the coming weekend, the comet will sit about 1.5 fist diameters to the lower left of the Big Dipper’s bowl star Phecda.
Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.