Why we don’t really have equal day and night on the equinox.

Tonight at 10:29pm EDT we will have the Autumnal Equinox marking the astronomical beginning of fall in the northern hemisphere. Most people think of the equinox as having equal day and night. I mean that’s what equinox actually means in Latin, so why wouldn’t you? 

One small problem though with the whole equal day and equal night thing. It actually doesn’t happen until Friday. Why? Well it has to do with the way we measure when sunrise is and when sunset is marking the length of daylight.

How Sunrise is measured:

This seems straight forward we consider sunrise the exact moment the very top tip of the solar disk breaks the horizon.

Sunset is a bit different:

When it’s time for sunset we don’t exactly do the same thing. If today were to be equal day and night. Then we would have to consider the very bottom tip of the solar disk going below the horizon as sunset. Problem is we don’t!

Sunset is actually measured as the ENTIRE solar disk dipping below the horizon.

This difference iN measuring sunset versus sunrise this way actually ends up being about 8 minutes and 47 seconds. That’s the time it takes for the entire solar disk to go below the horizon at sunset.

This is also true of the solstice’s when we think the longest day(summer solstice) and shortest day(winter solstice).  The longest and shortest days are actually a few days off of those astronomical events as well.