The Babylonian day began and ended with sunset, apparently the disappearance of the rim of the orb. In practice the umu was divided into two, the light, or day, also UD, umu, and dark. Except at the equinoxes, light and dark are unequal in length. In the purely theoretical calendar year of 360 days, certainly never in official use after the beginning of the Third Dynasty of Ur, perhaps never before that, the twelve months of 30 days were given the names of the true lunar months used in the official government calendars which were adjusted periodically to relate 1st Nisan to the spring equinox. In the theoretical calendar 1st Nisan is always the equinox. Thus the night of 1st Adar was theoretically equal to the 1st Iyyar daylight in length. Light and dark, whatever their length, were each divided into three watches; thus the watch, EN.NUN maṣṣartu, also varied seasonally. The routine for reliefs of guards or observers in ancient times can be inferred from Egyptian practice; no precise information seems to be available from cuneiform texts, but it would be surprising if a team for the maṣṣartu was not divided into two, with alternate reliefs in each half-team. The seasonal variations in time-lengths for duty, and other necessities of daily life, required some measurement, by a standard, of seasonal beru and other time units.