Is the official time of sunrise in McHenry the same as it is in Chicago?

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Dear Tom,

I love to watch the sunrise but I was wondering, is the official time of sunrise for me in McHenry the same as it is in Chicago?

Sue Shearer

Dear Sue,

It is not. The reason is the location used to determine the time of sunrise and sunset, and in a large metropolitan area like Chicago, the difference is noticeable. At Chicago’s latitude, the Earth rotates at 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. At McHenry, about 45 miles northwest of Chicago, the variance during the year causes sunrise to be as much as four minutes later. Chicago’s official sunrise/sunset 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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