After reading this entry, I fired off this half-cocked theory:

At a given location, is the sun found due west at the same time of day every day, and is this the time it sets on the equinox?

Which was incorrect, and launched an ink-cloud precise investigation by me and my staff into the matter, which has apparently gone on for way too long. The search for answers began with tables of sun positions from the US Naval Observatory, mostly-but-not-completely-accurate Wikipedia articles (are there any other kind?), and followed links and queries of very dubious relevance all over the Internet. It can be approximated thusly:

wget -H -r --level=7 -k -p http://en.wikipedia.org/wiki/Time_zone
wget -H -r --level=14 -k -p http://aa.usno.navy.mil/data/docs/RS_OneYear.html

I will compress the result by a factor of lots:

1) The earth rotates once every 23 hours, 56 minutes, and 4 seconds, a sidereal day. It's slowing down, but that's very consistent and accurate.
2) 24 hours is sort-of how long it takes for the sun to cross the same meridian twice, a solar day. This number as a mean is accurate, but it's not consistent throughout a year.
3) It varies because the earth's path around the sun is not a circle, and because of the angle between the equatorial and ecliptical planes (the tilt of the earth). The second one threw me for a loop (ha!). At first I nodded (why not, it effects everything else); on further reflection I shook my head (no, wait, how can the tilt affect the relative position of a meridian?); and finally I nodded again (ignoring rotation completely, the sun passes meridians more quickly on the solstices when its path is parellel to the equator than when it's crossing it at a 23.5 degree angle on the equinoxes.)
4) These differences create 'errors' of seconds that add up over the year to minutes plus or minus at different times of the year. This accumulated difference is called the equation of time, is documented all over the place, and looks like this:


5) That's why apparent solar noon isn't the same time every day, and why my theory that the sun would be at 90 and 270 degrees at the same times every day was wrong.

Between Daylight Savings Time (which as of 2007 is in effect any time of the year when there is daylight) and a difference of 4 minutes per degree of longitude away from the nearest time zone meridian (every 15 degrees), a lot of places will never see solar noon at 12:00. On such a meridian, it would happen only on December 25 (A complete coincidence. The baby Jesus could have been born at any longitude).