Sun transit time question May 5, 2004

Posted by E.D.G. May 5, 2004

I am working with a professional computer programmer to develop Basic and
Perl language programs which will generate subsolar and sublunar point
latitudes and longitudes. The first program is now running and is being
"fine tuned." In developing it I noticed something and am curious regarding
what causes it. Perhaps someone reading this note will know the answer.

In theory the sun should be directly above 0 longitude, the Greenwich
longitude, at 12 noon each day. But the longitude numbers for its actual
location at 12 noon that this and other programs generate show that there is
a variation of perhaps as much as 15 minutes in that transit time at
different times of the year. And those variations are reproducible from
year to year.

Q: What is the cause of those variations? I am assuming that I am
interpreting the data correctly.

Might this be due to the fact that the Earth orbit is an ellipse rather than
a true circle. And for that reason the 0 longitude line is directly beneath
the sun at slightly different times during the year because the Earth
rotation does not perfectly match its constantly changing orbital speed
around the sun?