Why do the Chicago sunrise/sunset times provided by the media vary?

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Dear Tom,

Why do the Chicago sunrise/sunset times provided by the media vary?

Thanks,
Nancy Hagberg

Dear Nancy,

The reason for the differences is the location used to determine the exact time of sunrise and sunset. In a large metropolitan area like Chicago, the difference is noticeable. At Chicago’s latitude, the Earth rotates at a speed of about 13 miles a minute, so sunrise/sunset times vary by about a minute for every 13 miles of east-west travel, though north/south distances have little effect. The times at Glenview are about the same as Chicago, while at Crystal Lake, about 25 miles west of Chicago, they are about two minutes later. Chicago’s official times are determined by the U.S. Naval Observatory based on a reference point near the Stevenson Expressway and Western Avenue, a location representing the city’s population weighted center.

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