[AZ-Observing] Vernal Equinox?

March 20 is when the Vernal Equinox happens, and the sun rises due East and 
sets due West.
I also see that for the Phoenix area, the day that was exactly 12 hours long 
was March 16, when the sun rose at 6:35 am and set at 6:35 pm (according to 
the newspaper and my palm-pilot).

What I am not understanding is why these days don't coincide?  I have looked 
at all my reference materials, and have checked out a number of web sites, 
but can't find an answer.  Is this too easy a question to dignify with an 
answer, or is this a question that anyone can help me with?  Does it have to 
do with our latitude, or the equation of time, or what?

Randy Peterson
EVAC


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