A barely-visible comet is now appearing low in the western sky shortly after sunset. Comet watchers hope it will grow brighter and easier to see in the nights ahead.

The Comet Pan-STARRS has caught the attention of professional and amateur astronomers around the world since it was first recorded by an automated telescope (the Pan-STARRS  telescope) in June of 2011. Astronomers have been watching it through their telescopes for months as it moves into our solar system. Now it is time for sky watchers in the northern hemisphere to see it, hopefully without scopes.

On Tuesday evening, about an hour after sunset, a thin crescent moon will appear low in the western sky. If you can see the day-old moon, the comet will appear above and to the left. Photographers around the world will painstakingly try to capture that critical moment, but only a few will be successful. A clear, unobsructed horizon and ideal viewing conditions are critical.

The path of Pan-STARRS has been challenging to watch and predict. Early forecasters were more optimistic about its brilliance than they are now. Some thought the comet would glow and be almost as easy to see as a planet. The latest forecast puts the comet at the brilliance of a star. It will be harder still to predict the brightness of the long tail.

Comets are mysterious and interesting. Imagine a large, dusty snowball made up of stone and ice speeding toward the sun at breakneck pace. As it gets closer to the sun, the sun’s heat and radiation melts the ball of ice. The melting comet emits frozen materials; that debris is what we see as a long tail.

Comet Pan-STARRS is closest to the sun on Sunday. After that the comet heads back out to space.

The comet will get higher in the sky in the nights ahead, making it easier to see. But as it moves farther away from the sun, it will fade.