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Why is there not equal day and night on the equinox?

Posted at 12:01 PM, Sep 20, 2021
and last updated 2021-09-20 12:01:55-04

Seasons are changing! Fall officially begins on Wednesday at around 3:21 pm. We have been losing daylight rapidly over the last few weeks. The sun came up around 7:15 a.m. in Cleveland on Wednesday and will set around 7:23 pm.

But wait a minute...shouldn't the sunrise and sunset be exactly 12 hours apart on the Equinox?

Interestingly, the date when there are equal parts day and night happens a few days after the vernal equinox. The specific dates for this occurrence are different for different latitudes. For our latitude, this will happen on September 25 with 12 hours and 39 seconds of daylight.



On the day of the equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth. However, the Sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. At these times, the center of the disk is already below the horizon.


Additionally, atmospheric refraction (or bending) of the Sun's rays causes the Sun's disk to appear higher in the sky than it would if the Earth had no atmosphere. Thus, in the morning, the upper edge of the disk is visible for several minutes before the geometric edge of the disk reaches the horizon. Similarly, in the evening, the upper edge of the disk disappears several minutes after the geometric disk has passed below the horizon.