Earth & Sky

What does 'sunrise time' mean?

Here in Seattle, we have mountains to our east. When they say the sunrise is at 5:30 a.m., does that mean the time the sun rises beyond the horizon, or the time the sun rises above the mountains?

The sunrise time you read in your local newspaper was most likely provided by the U.S. Naval Observatory. Astronomers there calculate sunrise times using the standard laws of planetary motion set down by Isaac Newton and modified by Albert Einstein, plus recent daily observations of the sun.

The calculations are based on the sun’s coming over a flat horizon, so it’s true that objects like mountains and skyscrapers can delay the time you actually see the sun’s rays. What’s more, the U.S. Naval Observatory assumes sea level as the height of all cities, since differences in the elevations of cities don’t change the calculations much.

But the width of a large city could be a factor. The east side of a city could see the sun minutes earlier than the west side. The U.S. Naval Observatory looks at only one sunrise point for each city. These points are calculated using population maps prepared by the Census Bureau.

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