## Can you tell time by the Big Dipper? (Beginner)

My fourth grade students just read a story about the stars and constellations. It mentioned that you can tell time by using the North Star and the Big Dipper. I was perplexed and had not heard of it. Do you have any information about this?

Sure. Just like the sun, the stars rise and set; constellations near the North Star like the Big Dipper appear to circle the North Star once per day. This motion, like the sun's daily motion, is due to the Earth's rotation. Because of this motion, if you know your astronomy, then you can tell the time. When the "pointer" stars of the big dipper are directly above the North Star, it is 11:00 siderial time. When they are directly below the big dipper, it is 23:00 sidereal time, etc. Unfortunately, the time told by the stars (sidereal time) is slightly different than the time told by the sun (solar time), which is what we are used to using. To convert sidereal time to solar time, you should add 4 minutes from the sidereal time for each day after March 21. And that's how you tell time by the stars!

#### Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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