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.
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