Happy equinox!

Tomorrow (or, by the time you read this, today – March 20, in any case) is the equinox. So why is the equinox special?  You’ve heard that it’s the day when day and night are the same length — but that’s really only approximate.  You may also have heard it’s the day the sun stands over the equator — the subsolar point on the equinox will rest on the equator.  And that’s true.  But there’s one more way of looking at it — it’s the day when the terminator between day and night runs directly from pole to pole.

So what does that look like?  This video is comprised of images taken from a Meteosat geosynchronous weather satellite daily, at the same time of day, from equinox to solstice to equinox to solstice to equinox again, September 2010 to September 2011.  Watch the angle of the terminator shift from vertical to tilted and back to vertical again:

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