## How long does it take the Big Dipper to move in the sky? (Intermediate)

What length of time is required for the Dipper to change from one position to the other?

It depends on which is the "one position" and which is the "other position". :)

In the Northern Hemisphere, all of the stars appear to rotate about a point in the sky that is due north, and at an elevation equal to the latitude at which you're standing. This also happens to be the point in the sky occupied by the North Star. (The same thing happens in the Southern Hemisphere, but there things rotate about a point in the south.)

This apparent rotation of the sky is actually a result of the fact that the Earth itself is rotating. So the sky does one complete rotation every 24 hours. Of course, you can't see the stars during the day, but the sky is still "rotating" then nonetheless.

So you see, there aren't just two positions for a constellation like the Big Dipper. But over the course of an entire night (~12 hours), you should be able to see it move from one end of its "path around the North Star" to the opposite end.

#### Christopher Springob

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies.  He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.

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