Montana Earth Science Picture of the Week

Robin Loznak took this week's photo from the summit of Big Mountain near Whitefish. She pointed her camera at Polaris ("the North Star"), and then left the shutter open for 70 minutes, recording the "motion" of stars in the northern part of the sky. Such "motion" is referred to as "apparent" because it is not caused by the movement of the stars, but rather the spin of the Earth on its axis. It's like being on a merry-go-round and thinking that your surroundings are moving. The Sun and Moon "move" across our for the same reason. Since Polaris is directly above the North Pole, it stays put while the other stars seem to circle around it.

A rock among stars . . .
The straight bright line is a meteor (a.k.a. "shooting star"). Meteors are not stars at all. Instead they are small rocks that burn up in our atmosphere as they are pulled in from space by our planet's gravity. On any night, at any location, a few meteors can be seen each hour. Occasionally, intense meteor displays fill the sky with tens, hundreds, or even thousands of meteor trails. These displays, called meteor showers, can be predicted because they repeat every year when the earth passes through the path of a comet. The bits of debris left behind by the comets, most no larger than a grain of sand, create a spectacular light show as they enter the earth's atmosphere. The meteor in this week's photo was part of the Perseid Shower that occurs every year around August 12-13 as a result of debris left behind by Comet Swift-Tuttle.

Term: circumpolar stars, meteoroid