Winter is over at 10:49 p.m. Thursday — at least astronomically. That is the exact moment of the vernal equinox, the astronomical beginning of spring. When you were young you were probably taught that on the first day of spring, the days and nights are equal, 12 hours apiece, and from that day on days become longer than nights. Well, that’s just not true. Days actually become longer than nights every year on St. Patrick’s Day! I want to think it’s because of the luck of the Irish, but it’s astronomical refraction that does it.

The Earth’s atmospheric shell bends the incoming rays of the sun in such a way that the sun seems higher in the sky than it actually is. This bending is more severe near the horizon since the atmospheric layer is thicker from our vantage point. The bending of the sun’s light is so extreme near the horizon that when the sun is actually below the horizon it still appears to us to be above the horizon. If our Earth didn’t have an atmosphere, we would indeed have equal days and nights on both the spring and fall equinox dates. Astronomical refraction, however, makes sunrises earlier than they should be and sunsets later, making for a longer day.

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul.

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