I enjoyed reading Bad Astronomy. However, I found a few minor errors and one major error in it that I would like to point out.

First the minor errors:

p53. Change June to July

p56. The ellipse precesses also with a period of 21,000 years.

p107. Change "brighter" to "dimmer".

The major error is on page 55 in the last paragraph, " ... the Earth's north pole will be pointed away from the Sun in June and toward it in December."

This is not true. The calendar is a tropical calendar that accounts for the precession (with leap years). A year in the tropical calendar is the number of days between successive vernal equinox crossings of the sun. Hence, precession is built into the calendar. That actually makes the calendar year slightly longer than the sidereal year. Hence, spring will always be in March. What this does do is to put the summer constellations in the winter and visa versa.

There are several references on the calendar on the web. Here is just one of them that you might want to check out.
http://astro.nmsu.edu/~lhuber/leaphist.html. I know that websites have been known to be wrong, but I check out several references on this topic and I believe this is correct. It makes a lot of sense. Here is quote from the article:

"The tropical year is defined as the mean interval between vernal equinoxes; it corresponds to the cycle of the seasons. The following expression, based on the orbital elements of Laskar (1986), is used for calculating the length of the tropical year:
365.2421896698 - 0.00000615359 T - 7.29E-10 T^2 + 2.64E-10 T^3 [days]
where T = (JD - 2451545.0)/36525 and JD is the Julian day number. However, the interval from a particular vernal equinox to the next may vary from this mean by several minutes. "

When the Romans produced the Julian calendar, they did not have means to measure star positions. There were no accurate time clocks then and the telescope had not been invented. Hence, they adapted a solar calendar, based on the vernal equinox crossing. This relies only on the accuracy of measuring angles. The Gregorian calendar (circa 1532) modified the Julian calendar to correct for errors by adjusting leap years and adding 13 days in October. Interesting enough, the Europeans did not adopt this calendar for 150 years later (mainly because they did not want to accept anything coming out of the Roman Catholic Church). Still the basis for the calendar did not change. Another quote:

"Gregorian calendar. The third type of calendar, the lunisolar calendar, has a sequence of months based on the lunar phase cycle; but every few years a whole month is intercalated to bring the calendar back in phase with the tropical year."

Jim Benet