Which Equinox?

摘要

The reform of the Republican calendar introduced by Julius Caesar in 46 B.C. was carried out to adjust the year and festivities to the seasons. There are varying definitions for equinox, and historians have generally accepted that March 25 was the canonical date for the vernal equinox. We find that the sense of equinox used could have been the day that marked the middle of the interval between the winter and summer solstices. A method of deriving these dates is suggested. This also implies that from the beginning of Caesar's reform, the length of the months, especially February (Februarius) and August (Sextilis), was the same as it is now.