One would think sunrise and sunset would be exactly 12 hours apart on both spring and winter equinoxes, but that hasn’t been my reading of the times on either day. Why?
When the vernal equinox occurred at 10:49 pm on March 19 as the sun crossed the celestial equator into the Northern Hemisphere, just the nights were of equal length at about 11 hours and 53 minutes. In the Northern Hemisphere, approximate 12-hour days and nights occurred a few days before. That’s because sunrise occurs with the top edge of the sun’s disk breaking the horizon and sunset with the top edge falling below. Daylight is further lengthened by the Earth’s atmosphere refracting sunlight at low angles, allowing us to begin to see the sun when it is actually still below the horizon.