## What is the speed of Earth's rotation?

Because Earth is a solid body, not a liquid or gaseous one, all parts of Earth have to move around the axis a full 360 degrees every day. But, the speed of the planet at its surface depends on how many miles the surface moves in space to complete a 360-degree rotation in 24 hours. Near the equator, it has to travel its full circumference at that latitude which is 2piR, where R is Earth's radius. However, at any other latitude, Earth's circumference decreases by the cosine of the latitude so that you get 2PiR cosine(latitude). You can see this with a piece of string and a globe. At the equator, Earth's circumference is 25,600 miles, and the day is 24 hours long so the speed of rotation is 1070 miles per hour. This decreases by the cosine of your latitude so that at a latitude of 45 degrees, cosine(45) = 0.707 and the speed is 0.707 x 1070 = 755 miles per hour. You can use this formula to find the speed of rotation at any latitude.

 This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011 Return to Ask the Astronomer