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Why are there not exactly 12 hours of daylight on Equinox?

Spring Began on Sunday at 11:33 a.m.
Posted at 11:38 AM, Mar 20, 2022
and last updated 2022-03-21 16:27:36-04

CLEVELAND — Seasons are changing! Spring officially began on Sunday around 11:30 a.m.

We have been gaining daylight rapidly over the last few weeks. The sun came up around 7:31 a.m. on Sunday and will set around 7:40 p.m. in Cleveland.

That is about 12 hours and 8 minutes of daylight. Interestingly, the date when there were nearly equal parts day and night happened a few days before the vernal equinox. The specific dates for this occurrence are different for different latitudes. For our latitude, this happened on St. Patrick's Day with 12 hours and 34 seconds of daylight.


Why is there not equal daylight and nighttime on the equinoxes?

There are only two times of the year when the Earth's axis is tilted neither toward nor away from the sun, resulting in a "nearly" equal amount of daylight and darkness at all latitudes. These events are referred to as Equinoxes. The word equinox is derived from two Latin words - aequus (equal) and nox (night).

At the equator, the sun is directly overhead at noon on these two equinoxes. The "nearly" equal hours of day and night are due to the refraction of sunlight or bending of the light's rays that causes the sun to appear above the horizon when the actual position of the sun is below the horizon. Additionally, the days become a little longer at the higher latitudes (where we live) because it takes the sun longer to rise and set. Therefore, on the equinox and for several days before and after the equinox, the length of day will range from about 12 hours and six minutes at the equator to 12 hours and 8 minutes at 30 degrees latitude to 12 hours and 16 minutes at 60 degrees latitude.

For observers within a couple of degrees of the equator, the period from sunrise to sunset is always several minutes longer than the night. At higher latitudes in the Northern Hemisphere, the date of equal day and night occurs before the March equinox and daytime continues to be longer than nighttime until after a few days after the September equinox.