Today, March 20, "is the Vernal Equinox". This event seems to vary yearly between the 20th and the 22d of March, without any obvious pattern. And I wonder if there is any clear description of the relevant variables that cause the variation. I am not iterested in a formula to calculate it, still less a program or simulation, but some reasoning as to how it varies. I presume it is due to the lineup of the tropical year with the Gregorian calendar date, but just how? I tell people that the equinox happens when the Sun is directly overhead at exactly Noon at some place on the Equator. That acccounts for the odd times of day published but not for the larger scale wanderings.