Dear Tom,
When checking different sources, the sunrise/sunset times for Chicago often vary by several minutes. Why is this?
Michael Lee Rogers, Lyons
Claudia Kurianowicz, Hinsdale
Dear Michael and Claudia,
Sunrise and sunset across a large metropolitan area like Chicago can vary by several minutes depending on the location used for the calculation. At ChicagoOs latitude, the Earth rotates at a speed of about 13 miles a minute, so the time of sunrise and sunset will vary by about one minute for every 13 miles of east-west travel. Significantly smaller time differences also result from north-south location variances.
The official sunrise/sunset time for the Chicago area, determined by the U.S. Naval Observatory, is based on a location near the Stevenson Expressway and Western Avenue, representing the cityOs population weighted center.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
Write to: ASK TOM WHY, 2501 Bradley Place, Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
