Yesterday on worldmagblog, Chas was wondering why the newspaper listing of sunrise and sunset times showed more than twelve hours of daytime, even though it was the autumnal equinox. After all, everyone knows that day and night are exactly the same length on that day, right?
Apparently not. It’s one of many things that “everyone knows” that isn’t quite accurate. I found out from nationalgeographic.com why it’s not the way everyone thinks it is (including me, until now). It has to do with the size of the sun in the sky, and with bending of light caused by earth’s atmosphere.
The first aspect is that we don’t measure sunrise and sunset from when the center of the sun is even with the horizon, but rather when the “top” of the sun (as we see it) appears over the horizon, or dips below it. That way “daytime” is just a little bit longer than if we measured it from the center of the sun’s disk in the sky. And the bending of light by our atmosphere means that the sun is not actually quite where it appears to our eyes, either.
That probably was not one of your burning questions today, or even yesterday when it was more relevant. But it’s one more little bit of knowledge to take the place of what we thought we knew, that wasn’t quite right. And for me, at least, that is always a good thing.