Equi-Nox means [Day] Equals Night -- Right?

Wrap this around your mental astronomical-phenomena spindle and see if you can make of it more of a skein than a snarl:

Where I live on the west coast of the U.S.A., astronomical equinox is at 1:20AM Pacific Daylight Time on Wednesday 23 September 2015.  The time from sunrise to sunset should be equal to the time from sunset to sunrise, right?

Doesn't work that way.

Wednesday actually has six more minutes of daylight than Friday.  On Friday 25 September we actually get 12 hours zero minutes between sunrise and sunset.


First, the astronomical definition of equinox is at the moment the earth in its orbit around the sun crosses a particular point in reference to what astronomers call the fixed stars.  We know that point wanders with respect to the earth's axis-wobble and the earth's elliptical orbit (precession of the equinox) -- ve-e-e-r-r-y s-slo-o-o-owly!  But can it make a difference of a day or two?

Second, there's the spin of the earth.  Does the spin, added to the round-the-sun motion, make a difference?

I admit I haven't done the research.  But hey, give me some credit for finding the conundrum.

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