Why Aren't The Sunrise And Sunset Equal On The Equinox?

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By By Steve Pool

SEATTLE - The first days of spring and fall are a time when there is equal 12-hour daylight across the planet. However, looking at a Seattle sunset chart shows Seattle has 12 hours and 11 minutes of daylight on Sept. 21. Why?

Sunrise and sunset are calculated when the top of the sun is on the horizon. However, equal daylight is measured from the center of the sun -- accounting for a few minutes. The other discrepancy is from when the sun is on the horizon, the Earth's atmosphere bends the sun's light.

As a result, the sun still looks to be above the horizon for a few minutes after it has already set. The U.S. Naval Observatory takes this into consideration when calculating sunrise and sunset times.

For More Information -- www.usatoday.com

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