Sunday Is Your Last Chance — Don’t Miss It

Its time for one last look at comet Tsuchinshan-ATLAS. This long-period comet from the Oort Cloud — a sphere around the solar system that’s home to millions of comets — comet Tsuchinshan-ATLAS (also called C/2023 A3 and Comet A3) is now on its way back there on a 80,000 years-long journey.

It’s fading as it speeds away from the inner solar system, but rising quickly into the night sky. It’s now past it best in terms of brightness, but this weekend is still a great time grab a final glimpse, though you’ll need binoculars and patience to see it, particularly from urban areas with light-polluted skies.

Note: times and viewing instructions are for observers at mid-latitudes in the Northern Hemisphere. Check the exact time of sunset where you are and the comet’s setting times on Stellarium Web for times accurate for your exact location.

Tonight, Sunday, Oct. 20, is likely the last night to see the comet with the naked eye from the northern hemisphere, though probably only if you observe away from light pollution. Try looking from somewhere that looks dark on a light pollution map or from a Dark Sky Place. If you find it, here’s how to photograph it.

“Around Oct. 20, a window of true darkness begins to open up between twilight and moonrise if you’re far from the skyglow of city lights,” states Sky & Telescope. “But by then, the comet, now high in the sky, will have started to fade and shrink. It will diminish into the distance in the following days, becoming invisible to the unaided eye later in the month even under ideal, dark-sky conditions.”

Here’s exactly when and where you need to look to see the comet on Sunday, Oct. 20.

How To Locate Comet Tsuchinshan-ATLAS: Sunday, Oct. 20

Position: west, 45 degrees from the sun in Ophiuchus

Time: 45 minutes after sunset where you are

Magnitude: +2

Comet’s distance from the sun: 66.5 million miles (107 million kilometers)

Comet’s distance from Earth: 57 million miles (91.7 million kilometers)

This evening, the comet will be visible high above the western horizon about 45 minutes after sunset, setting around two hours 45 minutes later. That means it will be in the sky for about 10 minutes longer than on Saturday.

That means the comet will be high in the sky and remain so as twilight turns to astronomical darkness. Find bright Venus and the bright star Arcturus; the comet will be high above them as the third point of a vast triangle. Scan with a pair of binoculars until you find it.

Why Does Comet Tsuchinshan-ATLAS Have A Tail?

“Comets are like cats: they have tails, and they do precisely what they want,” goes a famous saying by author David H. Levy in Comets: Creators and Destroyers.

There’s more than a grain of truth in that saying, but there are also plenty of grains in the tail of a comet. “The main tail visible for this comet is its dust tail, made of dust released from the nucleus at the head of the comet mostly over the last few weeks to months,” said astronomer Dr. Qicheng Zhang of Lowell Observatory in Flagstaff, Arizona, who has been monitoring the comet, in an email. “That dust gets pushed by the radiation pressure of sunlight away from the sun into a tail.”

Check my feed every day this week and next for a daily “comet tracker” with sky-charts and tips for viewing the comet.

Wishing you clear skies and wide eyes.