Length of Day/Night at the Equator

Question: Is the length of day and night equal at the equator?

Answer: There are two answers depending on how you define sunrise and sunset at sea level. The complicating problem is the atmosphere which bends the sun's rays so that we see the sun sphere before it actually is on the horizon.

If you do not consider the atmosphere's influence and calculate sunrise and sunset when the true solar disk is on the horizon, then day and night are equal on the equator. Since the actual difference is small, many models assume 12 hours for each.

However, those who calculate the true times (US Naval Observatory, Greenwich Royal Observatory) add a few minutes to sunset and subtract a few for sunrise to account for the atmospheric refraction. Thus, the equatorial day is 6-8 minutes longer than the night.

The actual difference varies a bit because the refractive properties also change with season and other weather/climate conditions.

The day also lengthens with respect to the night as you ascend from sea level because you can see the horizon at a further distance with altitude.

Learn More From These Books Chosen by The Weather Doctor

• Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.
• Dunlop, Storm: The Weather Identification Handbook: The Ultimate Guide for Weather Watchers,, 2003, The Lyons Press, ISBN 1585748579. (pb).

The Weather Doctor's Weather Almanac Length of Day/Night at the Equator
©2002, Keith C. Heidorn, PhD. All Rights Reserved.
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