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Equal day and night on equinox?

The March 2017 equinox – the Northern Hemisphere’s spring or vernal equinox and the Southern Hemisphere’s autumn equinox – falls at 10:29 Universal Time on Monday, March 20, 2017. For United States’ time zones, that’s 6:29 a.m. EDT, 5:29 a.m. CDT, 4:29 p.m. MDT or 3:29 a.m. PDT. Translate to your time zone. Twice a year – at the March and September equinoxes – everyone worldwide supposedly receives 12 hours of day and 12 hours of night. Is it true? Not exactly. There’s actually more day than night on the day of an equinox. There are two reasons why:

The sun is a disk, not a point

Atmospheric refraction

Keeping reading to understand more …

View larger. | Photo by Abhinav Singhai.

View larger. | Sunset timelapse by Abhinav Singhai.

The sun is a disk, not a point. Watch any sunset, and you know the sun appears in Earth’s sky as a disk.

It’s not pointlike, as stars are, and yet – by definition – most almanacs regard sunrise as when the leading edge of the sun first touches the eastern horizon. They define sunset as when the sun’s trailing edge finally touches the western horizon.

This in itself provides an extra 2.5 to 3 minutes of daylight at mid-temperate latitudes.

Atmospheric refraction raises the sun about 1/2 degree upward at sunrise and sunset. This advances the sunrise yet retards the sunset, adding several minutes of daylight at each end of the day. Image credit: Wikipedia

Atmospheric refraction actually raises the sun about 1/2 degree upward at sunrise and sunset. This advances the sunrise yet retards the sunset, adding several minutes of daylight at each end of the day. Image via Wikipedia

Atmospheric refraction. The Earth’s atmosphere acts like a lens or prism, uplifting the sun about 0.5o from its true geometrical position whenever the sun nears the horizon. Coincidentally, the sun’s angular diameter spans about 0.5o, as well.

In other words, when you see the sun on the horizon, it’s actually just below the horizon.

What does atmospheric refraction mean for the length of daylight? It advances the sunrise and delays the sunset, adding nearly another 6 minutes of daylight at mid-temperate latitudes. Hence, more daylight than night at the equinox.

Astronomical almanacs usually don’t give sunrise or sunset times to the second. That’s because atmospheric refraction varies somewhat, depending on air temperature, humidity and barometric pressure. Lower temperature, higher humidity and higher barometric pressure all increase atmospheric refraction.

On the day of the equinox, the center of the sun would set about 12 hours after rising – given a level horizon, as at sea, and no atmospheric refraction.

Contemplating the sunset on the Philippine island of Leyte.  Photo by Abie Oquias Baybay.

Contemplating the sunset on the Philippine island of Leyte. Photo by Abie Oquias Baybay.

Bottom line: There’s slightly more day than night on the day of an equinox. That’s because sun is a disk, not a point of light, and because Earth’s atmosphere refracts (bends) sunlight. These factors add up to provide an additional 8 or so minutes of daylight on the day of the equinox at mid-temperate latitudes.

Bruce McClure

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