NavList:

All looks good to me until this:

> GMT vs GAT  Variations in the orbit of the earth around the sun.

I used to believe that too.  The eccentricity of Earth's orbit does contribute, but the biggest effect is due to the tilt of the earth's axis.

If you plot GAT vs. GMT you'll see two humps over the course of a year, but if it were solely due to the orbit (Kepler ' Law, etc.) there would be just one hump.

So what's really going on?  Imagine taking a snapshot of the earth every day at the same sidereal time -- say when Sirius is due south of my house.  And plot the Geographic Position of the sun in each snapshot over the course of a year.  You would see the sun on the equator at the spring equinox, rising to cover a big arc up to 23 degrees north over the summer, back to the equator for the fall equinox, and another big arc in the south before joining up where it started.  This path is a great circle, and if the earth's orbit were a perfect circle the sun would move along it at a steady pace of about 1 degree per (sidereal) day.  But look how it crosses the meridians of longitude!  At the equinoxes, the path is tilted and the meridians are well-spaced (since they're near the equator), so the sun's advancement in longitude is less than average.  At the solstices, in contrast, the path is parallel to the equator, and the meridians are (slightly) bunched together, so the sun crosses them faster than average.

If you take your snapshot every mean solar day (i.e. every 24 hours) instead, you get the figure-8-shaped analemma -- the sloping sides represent the slowdowns around the equinoxes, and the top and bottom represent the speedup around the solstices.

  -- Bill N.