51¾«Æ·ÊÓƵ

We may be seeing mostly cloud cover in 51¾«Æ·ÊÓƵsburgh, but there really is a comet in the skies. And it’s worth looking up for this once-in-a-lifetime phenomenon — Tsuchinshan-ATLAS won’t be back for 80,000 years.

To learn more, 51¾«Æ·ÊÓƵwire spoke to Edward Potosky (A&S ’74), a tour guide at the Allegheny Observatory. He holds a bachelor’s degree in physics and has been stargazing since he was a kid.

What are comets made of?

Years ago, people used to describe them as dirty snowballs. Now it’s more like an icy dirtball. It’s not clear their exact volumes and composition, but there is a lot of ice and dust and dirt.

What is the tail made of?

Because it’s ice and dirt, when a comet gets closer to the sun the ice boils off, leaving the trail of gas we see in the sky.

Where did Tsuchinshan-ATLAS come from?

Comets can have different orbits. Some come close to the earth and then go out past Jupiter and Saturn. Every few years, they come around. Others can go out further, like Halley’s Comet, which returns on average every 76 years.Ìý

Scientists think this one came from the Oort Cloud, a sphere beyond the orbit of Pluto made of icy, rocky debris. The debris stays there until something happens to perturb it. Then it’s pulled toward the sun, which whips it around, sending it flying away.

When is the best time to see Tsuchinshan-ATLAS?

Through October, it will get higher in the sky each night. But that will be counterbalanced by the fact that it’s getting farther from the sun. Look for it in the western skies, about an hour after sunset. You can use an app like Google Sky. The comet might be hard to find at first, so binoculars may help. But if the app is updated, it should point you right to Tsuchinshan-ATLAS.

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Photography by Edward Potosky