Why Night Is Not Equal On The Equinox

| March 19, 2017 @ 6:00 am

The March equinox arrives Monday, March 20, at 5:29 am CDT. At that instant, the sun will shine directly on the equator, marking the beginning of spring here in the Northern Hemisphere.

The word “equinox”, comes from Latin from aequus (“equal”) and nox (“night”). With that definition, you’d expect twelve hours of sunlight, or night or both on the day of the equinox.
This would be the case if central Alabama had a level horizon and no atmosphere (which is thankfully not the case) and if we measured the rising and setting based on the Sun’s center (which we don’t).

The difference between sunrise and sunset in central Alabama on Monday will be 12 hours and about 8 minutes, 29 seconds. Those extra moments of sunlight come from atmospheric refraction and the sun’s diameter.

The Earth’s atmosphere bends sunlight as it passes through. This causes the sun appear about half a degree higher than it actually is. This moves sunrises forward and delays sunsets. How much depends on temperature, humidity, and barometric pressure. This is why you’ll rarely see sunrise and sunset times predicted to the second.

Sunrise and sunset are calculated based on the Sun’s first appearance on the eastern horizon and last appearance on the western horizon. But the Sun isn’t a dot in the sky, it spans half a degree. The Sun’s disk spans about a half a degree adding a bit more to the time the Sun appears above the horizon.

So, if day and night aren’t equal on Monday, when are they?

By my calculations, using the 34 arc minute atmospheric refraction constant defined by the United States Naval Observatory, and ignoring the effects of pressure, temperature and : the Sun rose today, March 16 at 6:55:54 a.m. and set 6:56:01.00 p.m. or 12 hours, 0 minutes, 7 seconds.

 

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About the Author ()

Tony Rice is a Solar System Ambassador for NASA/JPL and the voice and brains behinds the weekly Astronomy Report on the WeatherBrains podcast. He grew up in Southern California with Space Shuttle landings and was hooked. He brings weather and space together to communicate the excitement of space exploration and promote a greater appreciation for Earth sciences.

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