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The Longest Day

Week of June 22, 1998

On June 21, 1998 at 10:03 Eastern U.S. time, the Sun hit the highest point above the Celestial Equator in its travel across the sky. We call that the summer solstice, with apologies to those in the southern hemisphere. I won't go into detail here on the mechanics of why this time is special right here, because it takes a while, and besides, I have an explanation elsewhere on my Bad Astronomy pages!

I am often asked, though, why it is that the if the summer solstice is the longest day of the year, why is it we don't have the earliest sunrise and latest sunset on that day? The answer turns out to be an odd one: the Earth orbits the Sun in an ellipse.

If the Earth orbited the Sun in a circle, then the summer solstice would have the earliest/latest sunrise/sunset. But actually, the Earth's orbit is somewhat elliptical. This means that sometimes the Earth is a bit closer to the Sun, and other times it is a bit farther away. When it is closer, it orbits faster, and when it is farther it moves a bit slower in its orbit. This plays havoc with the times of sunrise and sunset. The exact times of sunrise and sunset depend on how fast the Sun appears to move in the sky, and this can change! It so happens that the time of closest approach to the Sun is not at the solstice, so the speed of the Sun in the sky is a bit out of synch with our seasons.

I won't go into details here, because the details are, well, pretty detailed. If you are interested, do a web search on the phrase " equation of time". You'll find lots of links. A short explanation can be found at Nick Strobel's Astronomy Lecture page, for example. Good surfing to you!

©2008 Phil Plait. All Rights Reserved.

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