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Treebeard's Stumper Answer
21 Mar 97

March 21 is the first day of spring. It's also the Vernal Equinox. "Equinox" literally means "equal night." If the Earth's axis weren't tilted, day and night would always be equal. Instead, we have long winter nights and long summer days. On the equinox, we are at the exact moment when day catches up with night and light overcomes darkness. At least in theory. I checked the SB News Press last night, and I find that the sun rises at 6:01 AM and sets at 6:12 PM on the equinox. Why are night and day not equal?

Night and day are almost equal on the equinox, but not quite. This is mostly because of refraction. Just as a straw looks bent in a glass of water, so sunlight is bent as it passes through the Earth's atmosphere. The exact amount of refraction is about 34 minutes of arc on the horizon, a bit more than the apparent diameter of the sun itself. The weird result is that when the sun appears to be sitting on the horizon, the real sun is entirely below it. We only see a virtual sunrise! And so we have a few extra minutes of daylight on the equinox.

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Marc Kummel / mkummel@rain.org