0 the time in the spring when the sun crosses the equator, and when night and day are of equal length --
The shadow always falls on the winter hemisphere, except when it crosses the equator during the vernal equinox and the autumnal equinox.
However, it is 11 minutes longer than a vernal equinox year.
The reason for this behaviour is (as explained above) that it tracks the observed vernal equinox.
However, the vernal equinox and the full moon were not determined by astronomical observation.
Although the instant of the full moon must occur after the instant of the vernal equinox, it may occur on the same day.
Roughly every 2,150 years the sun's position at the time of the vernal equinox will have moved into a new zodiacal constellation.
In the eighth century it "often" began before the vernal equinox.
It is a solar calendar that is designed to maintain synchrony with the vernal equinox tropical year.
