[AZ-Observing] Re: Vernal Equinox?

It's a good (and common) question.  The difference is due to the Sun not 
being a point source and due to atmospheric refraction, so some part of 
the Sun is above the apparent horizon for several minutes more than 12 
hours at the equinoxes.  USNO has a more complete explanation posted at: 
http://aa.usno.navy.mil/faq/docs/equinoxes.html .  Very useful answers to 
similar astronomical questions are posted at the link one step higher, 
i.e.: http://aa.usno.navy.mil/faq/ .

- Brent Archinal
 




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        Subject:        [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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