Ask Eric: Why Isn't There Equal Day & Night On The Equinox?

Hi Eric,

I saw on your report this evening that today daylight was 12 hours 6 minutes. But since we're already past autumnal equinox, shouldn't daylight be less than 12 hours? It seems odd. I thought on autumnal equinox, daylight was exactly 12 hours...  What am I missing?

Thanks - Bill Grundmann

Bill - The answer to your question lies in the definition of sunrise and sunset. 'Sunrise' is the instant the edge of the sun's disc is visible above the horizon, and 'sunset' is when the upper edge disappears below the horizon. Neither are measured from the 'center' of the sun.

There's also refraction of the sun's visible light in the atmosphere at work here, which means that the light appears to your eye before the sun's edge is truly there. This adds a couple minutes at the start of the day and then again at the end when you receive light after the sun goes down.

So the end game is that during the equinox, you end up getting a little more daylight than the expected 12 hours even. The day of true equal light is typically 4-5 days after the fact.