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The Eric Factor: What you learned about the equinox was all wrong

Friday marks the end of Winter and the beginning of Spring. Officially, this changes at 5:45 p.m. CDT as the Vernal Equinox takes place. You’ve probably h...
Friday marks the end of Winter and the beginning of Spring. Officially, this changes at 5:45 p.m. CDT as the Vernal Equinox takes place. You've probably heard the term equinox used before. In fact, we talk about it extensively every six months or so!

But the term equinox may be a little deceiving. The word is Latin in origin with "Equi" meaning equal and "Nox" meaning night. But the equinox doesn't necessarily mean we have exactly 12 hours of daylight and 12 hours of night. There are three reasons why this is not true:

  1. Sunrise officially occurs when the first glimpse of light reaches the horizon, not when the center of the sun gets to the horizon. This means that we get a few extra minutes of daylight on the front end of our day.
  2. As our day winds down we gain a few more minutes. That's because sunset technically occurs when the last rays of light go below the horizon. Because that's the top of the sun, and not the center, we gain a few extra minutes.
  3. We could fix the equal day and night problem if we changed the definition of sunrise and sunset to be when the middle of the sun goes above or below the horizon. But that would only work if we were on a planet without an atmosphere. When the sun is high in the sky, the rays of the sun go through less of the atmosphere. But around sunrise and sunset, the light from the sun shines through more of the atmosphere. As this occurs, the gasses and particles within the atmosphere bend the sun's light toward the Earth! This is called refraction. Because the light is bent toward the Earth on both ends of our day, we gain a few extra minutes.

Here in the Quad Cities, our sun rises at 7:06 a.m. on Friday and sets at 7:14 p.m. So we're off by 8 minutes. Those 8 minutes occur mostly because of a technicality in the definition of sunrise and sunset and partly because of the refractive properties of the Earth's atmosphere.

So instead of calling this the "Vernal Equinox," maybe we should rename it "Vernal Claudo-Equinox" as the Latin word for close-to is claudo or clausus.

equinox

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