Spring has sprung

Posted by Rich Talcott
on Thursday, March 29, 2007

Spring officially arrived last week at the vernal equinox, when the Sun crossed the celestial equator heading north at 7:07 P.M. CDT March 20. (Unofficially, spring in Wisconsin began last weekend, when the temperature soared to 80° F (27° C) under sunny skies.) As the name implies, the vernal equinox should feature days and nights of equal length. It doesn't. The actual date when day equaled night came a few days before the 20th (with the exact date depending on your latitude).

If Earth were an airless world and the Sun appeared as a point of light in the sky, the equinox would have 12 hours of sunlight and 12 hours of darkness. Our atmosphere bends sunlight so we actually can see the Sun when it lies below the horizon. Under normal atmospheric conditions, the Sun appears about 34 arcminutes (slightly more than the Sun's apparent diameter) higher when it's near the horizon than it truly is.

This accounts for about two-thirds of the discrepancy. The other third comes from the Sun's non-zero diameter. The Sun officially rises and sets when we see its upper limb on the horizon. But because the Sun's apparent radius averages 16' arcminutes, we see the Sun rise about a minute before its center appears on the horizon, and set about a minute later. Both effects combine to make days a few minutes longer and nights a few shorter.

The start of spring is supposed to turn a young man's mind to love. Being closer to geezerdom than to youth, my mind tends to focus on the start of the baseball season. In a few days, major-league baseball teams will be taking the field for games that count. And being an astronomer, I'll be cheering for the team with the most stars. So, if everything works out the way it should, 7 months from now we should be crowning the Yankees as 2007 world champions!

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