The equinox having 12 hours of daylight is a myth

ROCHESTER, NY (WROC) – If you think that the equinox, when the sun’s rays pass over the equator, falls on the same day the northern hemisphere gets 12 hours of light and dark, you would be mistaken.

There are two main reasons why the 12 hour split happens a few days earlier than the vernal equinox and a few days later than the autumnal equinox. One is refraction of light and the other is how we measure time versus the how we measure the equinox.

LIGHT REFRACTION

As light crests over the horizon, it bends a bit and we see light a few seconds or minutes before actual sunrise (and a few seconds or minutes after sunset). The light bends because of earth’s atmosphere. That bend will change with other variables, such as differences in weather.

MEASURING LIGHT AND MEASURING THE EQUINOX

We measure sunrise and sunset times based on when the top of the sun crests over the horizon and then disappears behind the horizon.

We measure the equinox as when the center of the sun’s rays cross over the equator. This is a single point compared to the “top of the disc” that we use with sunrise and sunset. Because of this difference, the equilux will always occur a few days before or after the equinox.

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