Not Quite Equal Night

The March equinox marks the sun’s crossing above the Earth’s equator, moving from south to north. It is also called the vernal equinox in the Northern Hemisphere as it marks the beginning of spring and autumn in the Southern Hemisphere. The March 20, 2023 equinox officially arrives tomorrow at 21:24 UTC. That’s 5:24 p.m. in Paradelle.

Equi + nox in Latin means equal + night and you may have been taught that night and day on the equinox are exactly 12 hours long. That is not entirely accurate. The day is just a bit longer than the night on an equinox.

If we defined sunrise and sunset as the moment when the geometric center of the Sun passes the horizon, then the day and night would be exactly 12 hours long. But we don’t. Sunrise and Sunset are defined as the exact moment the upper edge of the Sun’s disk touches the eastern and the western horizon, respectively. It takes perhaps a few minutes for the Sun to fully set and that makes the day just a bit longer than the night on the equinoxes. Plus, I don’t live near the Atlantic Ocean or have a clear view of the eastern horizon so the Sun “rises” for me later by the time it goes above the First Watchung Mountain.

Do you ever take note of how the arc of the Sun moves in the sky?

I have noticed it since I was a boy based on which windows in the house it appeared in the morning or at sunset. Birds and butterflies notice the change in daylight. Like the Sun, they are moving northward along the Sun’s path. There are earlier sunrises, later sunsets, sprouting plants, and more bird and animal activity this month. The chipmunks have not started scurrying around my backyard yet, but they will soon.

Of course, the Sun isn’t moving at all. This illusion is caused by Earth’s tilt on its axis and constant motion in orbit. 

Even if day and night aren’t exactly equal tomorrow, there is a day when they will be. That day is called equilux (equal+light). The date depends on your latitude and can occur several days to weeks before or after an equinox. For me, it was on March 17 – St. Patrick’s Day.

Here in Paradelle, Daylight Saving Time started on March 12 and that pushed the times for sunrise and sunset ahead an hour. Silly humans.

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A lifelong educator on and off the Internet. Random by design and predictably irrational. It's turtles all the way down. Dolce far niente.

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