The Equinox Displaced

At the time of the spring equinox (usually March 20) or the fall equinox (September 22 or 23), night and day are supposed to be of equal duration. But according to the almanacs of sunrise and sunset times, on the dates of the equinoxes, daytime is longer by 8 to 10 minutes. How come?

Answer

On the dates of the equinoxes, the day is about seven minutes longer than the night at latitudes up to about 25 degrees, increasing to 10 minutes or more at latitude 50 degrees.

The moment of the equinox occurs when the geometric center of the Sun’s disk crosses the celestial Equator. But the standard definition of sunrise is the time when the Sun’s upper limb is just breaking the horizon, and sunset when the Sun’s upper limb is just disappearing below the horizon. This adds one Sun semidiameter (about 16 arc min.) at both sunrise and sunset, extending the duration of daylight by a little over two minutes.

The other factor is atmospheric refraction, which causes the rays to bend around the horizon. As a result, we see the Sun about 34 arc minutes higher at both sunrise and sunset, adding roughly four minutes to the time that the Sun is above the horizon.

In spring the days get longer as we approach March 20, and the date of equal day and night occurs several days before the March equinox, about March 17 at latitude 40 degrees.

Conversely, in the fall it takes several extra days for the time when the Sun is seen above the horizon to shrink to 12 hours. The date falls on about September 26 at latitude 40 degrees.

On their website, the U.S. Naval Observatory publishes excellent sunrise and sunset tables for any location.

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