If equinox means that day and night are equal, why is there more daytime on March 21?



I learned that the eytmology of equinox is equi (equal) nox (night). So I would assume that the sunrise and sunset of our upcoming vernal equinox (March 21) would be at the same time (e.g. 6:05 am and 6:05 pm), which would equal 12 hours of day and night. But the rise and set on March 21 is 6:58 am and 7:10 pm. That’s about 12 hours and 12 minutes. But on March 16 sunrise and sunset was very close to being precisely 12 hours. Why is the equinox on March 21 and not on March 16?

In: Geology

It depends on the definition of sunrise and sunset.

If you counted a sunrise/sunset when the *center* of the sun goes above/below the horizon, then yes, sunset and sunrise would be (almost) exactly 12 hours apart at the equinox. Instead, the sunrise is when a the *first* bit of the sun becomes visible, and sunset is when the *last* bit of the sun goes down. In other words, when the sun is partially visible and partially occluded, it is still counted as “day” not “night”, so the day is a bit longer than the night at the equinox.

First off the Equinox is actually the 19th or 20th this year (depending on which time zone you’re in).
If you were going with the eytmology you would say that the equinox is the day when day and night are (about) the same length everywhere on the planet.

Scientifically the Equinox is when the plane of the earths equator passes through the center of the sun. The effect of this is that it is the day where the sun rises directly east and sets directly west on the horizon. As for why the day isn’t exactly split between day and night there are several contributing factors to this. Two being the fact that the sun isn’t a point, and that the atmosphere will bend light a little so that where the sun appears to be isn’t quite where it is.

The moment of sunrise and sunset is considered to be when the centre of the sun is 50 minutes of arc (50 sixtieths of a degree) below the horizontal. This is because it’s 16 minutes from the centre of the sun to the edge of the sun and the atmosphere makes things on the horizon appear 34 minutes higher than they really are. This makes the day longer than the night at the equinox.