Do you we have equal amounts of day and night when it’s the equinox?
The answer is…. not quite.
Sunrise on the fall equinox this past Tuesday was 6:52 am, and the sunset was 7:01pm. That equals to 12 hours and 8 minutes of daylight. It’s close to an even 12-hour split, but not quite equal.
Why is that? Two reasons:
1) The sun is a disk and not a point.
2) Atmospheric refraction.
The sun appears in the sky as a disk, it doesn’t come to a point. The official sunrise is when the topmost part of the sun first first touches the eastern horizon. And a sunset is when the topmost part of the sun disappears below the western horizon. This adds about an extra 2 1/2 to 3 minutes of daylight at the mid latitudes, which is central New York is located.
The other reason is atmospheric refraction. Think of the atmosphere like a lens or a prism. When sunlight passes through the atmosphere it makes the sun appear to be a half degree from its true position when it’s near the horizon. This is makes the sunrise time seem earlier and the sunset a little later. It adds about another six minutes of daylight!
6ish minutes+2ish minutes = 8 ish minutes.
So that’s why the amount of daylight on the equinox was off by more than 8 minutes!
We have much more information on the seasons and fun facts about when the daylight does balance out on the vernal and autumn equinoxes if you click here.