Speed of a sunset at different points on the earth

Last Edited By Krjb Donovan
Last Updated: Mar 11, 2014 07:56 PM GMT

Question

I was recently in Kenya quite near the equator. We watched beautiful sunsets over Lake Victoria and were amazed at the speed at which the sun went down - it seemed much faster than at my home in Chicago. I postulated that as the Earth is turning on its axis, the speed of the rotation at the equator is faster than at the higher latitude in Chicago due to the longer distance traveled at the equator at the diameter of the Earth so the sun does appear to go down at a faster rate. Is this correct?

Answer

Hi Sherry,

You nailed it. That is exactly right!

The earth is about 24,000 miles in circumference at the equator. With 24 hours in a day, the line between sunlight and day, called the terminator, speeds along the surface of the earth at 1000 miles per hour. If you were on the top of a mountain or any high place with an unobstructed view into a valley below in the east, you would actually see the darkness rushing over the landscape at 1000 MPH! In the west, there is sunset duration, the time it takes the sun to set from first contact with the horizon until it is completely under.

The sun has an angular diameter of about half a degree. At the equator the rotation of the earth advances objects in the sky in the line between east, zenith and west at 15 degrees per hour, so the sun will hit the horizon and be completely below it in 2 minutes.

To find the approximate value for these figures, terminator speed and sunset duration in other locations, divide 1000 MPH and 2 minutes by the cosine of your latitude. These estimates are more accurate the closer you are to the summer solstice, or June/July in the northern hemisphere or the equatorial region. You are very smart. I am sure you could figure out how to correct for different times of the year. (Hints: cardinal direction, exact point of sunset, north is zero degrees)

Call on me any time.

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