What is sidereal time? (Beginner)

I have read about different times called sidereal time, etc. I am a bit confused about all of it. Could you please enlighten me on it?

Solar time is the sort of time we're used to, where a day is 24 hours, the average time it takes for the Sun to complete one trip around the sky and return to its original position. (Technically, civil time and time zones are based on mean solar time.) Sidereal time is measured according to the positions of the stars in the sky. A sidereal day is the time it takes for a particular star to travel around and reach same position in the sky. A sidereal day is slightly shorter than a mean day, lasting 23 hours, 56 minutes, and 4.1 seconds. A sidereal day is divided into 24 sidereal hours, which are each divided into 60 sidereal minutes, and so on.

The reason that sidereal days are shorter is that while the Earth rotates on its axis, it is also moving around the Sun. Both motions are counter-clockwise as viewed from above Earth's north pole. You may find it helpful to draw a diagram. The Sun can be represented with a point. Draw the Earth. Let it be noon for an observer on the Earth, so sketch a little stick person with his feet on the Earth and his head pointed at the Sun, because at noon, the Sun is directly overhead. Draw a line from the Earth to the Sun, and let it extend far beyond the Sun. Draw a star on this line. From the observer's point of view, the star is also overhead, although of course it would be hidden behind the Sun. Now, imagine that the observer is carried for one mean day on the Earth as it makes a rotation while also moving through space. Draw the Earth at its new position in the orbit (it's okay to exaggerate this motion for purposes of illustration) and notice that when you add the person pointing at the Sun, he's no longer pointed toward the star! More than one sidereal day has passed!

You might ask whether the star's distance would affect the length of the sidereal day. Try moving the star farther from the Sun, and you'll notice that as the star gets very far away, the differences become quite small. Even the closest stars to us are so far away that the sidereal day is the same, no matter what star you use to measure it.

This page was last updated on September 5, 2016.

About the Author

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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