Sundown times: Location, location, location

John Sedlak is a faithful click2pin poster and this is one of his sunsets
John Sedlak is a faithful click2pin poster and this is one of his sunsets

This past weekend, friends of mine spent a solid hour arguing about a rather simple, but perhaps not intuitive, supposition: “Even though the sun sets every night at different times in different places, the time it takes for the sun to actually go down is the same.”

Those who voted true argued that once the sun starts to go down, it goes down and because the earth is spinning equally fast (about 1,000 mph) no matter where you are, then the sunset wouldn’t vary in the time it takes to happen. Those who voted false argued mainly along the lines of experience. After all, if you’ve ever been to Key West, one of the huge tourist attractions is the super-fast sunset from Mallory Square.

Key West Sunset from Mallory Square

The answer is this: the sundown (the actual time it takes for the sun to sink below the horizon) is faster at the equator and takes longer as you go toward the poles. And the time can run from half a minute to a quarter of an hour!

A recent article from EarthSky demonstrated the longest sunsets occur during the solstice and in that article made this point regarding the June summer solstice sundown:

“At more northerly temperate latitudes, the sunset duration is greater; and at latitudes closer to the equator, the sunset duration is less. Near the Arctic Circle (65 degrees north latitude), the duration of a solstice sunset lasts about 15 minutes. At the equator (0 degrees latitude), the solstice sun takes a little over 2 1/4 minutes to set.”

The key is that Earth is tilted (23.44°) on its axis. I took an earth shot below from Wikipedia and drew a red line for the axis to where it meets the Equator. The yellow line is where darkness begins (if you have darkness, you don’t have sun!). With this demonstration, it’s pretty easy to see how at the equator it gets dark first and as you go north will take longer. The angle gives you more sunset!

Red line is the Axis to the equator and the yellow line is there darkness falls

About the Authors: