Vernal Equinox

Friday was the first day of spring, though with snow on the ground and new snow falling, it looked more like winter. Not even snowdrops, the first of the bulbs to poke through the ground here, are up yet, but the birds are singing their glorious songs of spring and every day the sun shines a bit longer.

I noticed that we didn’t have exactly twelve hours each of day and night on Friday; day was slightly longer. It turns out that that’s because sunrise is counted as the time the top of the sun’s disc first appears on the horizon, whereas sunset is when the trailing edge of the sun disappears below the horizon. That makes “day” slightly longer. In addition, the earth’s atmosphere causes refraction of light, which makes the sun visible to us before it actually rises and for a short time after it actually sets. Here’s a link I found that explained all that clearly and simply:
Equinox: equal day and night, but not quite

 

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