In 2023, the first day of Spring is on March 20. According to the Farmer’s Almanac, one question revolving around the vernal equinox concerns the length of day versus night. We have been taught that on the first days of spring and autumn, the day and night are equal to exactly 12 hours all over the world. Yet, if you check the calendar pages in the Almanac, you will find that this is not so. In fact, the tables tell you that on the days of the spring and fall equinox, the length of daylight is actually longer than the darkness by several minutes.
The reason this happens can be attributed to the atmosphere. If Earth was a planet that did not have an atmosphere, then yes, on the equinox days the length of the day and night would be exactly even.
However, our atmosphere acts like a lens and refracts (bends) its light above the edge of the horizon. Put in another way, when you watch the Sun either coming up above the horizon at sunrise, or going down below the horizon at sunset, you are looking at an illusion – the Sun is not really there, but already below the horizon.
As a result, we actually end up seeing the Sun for a few minutes before its disc actually rises and for a few minutes after it has actually set. Thus, thanks to atmospheric refraction, the length of daylight on any given day is increased by approximately six or seven minutes.
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