Two periods of the same length before and after noon and midnight are distinguished as the first and second dark, the first and second mid-day zyda.

There remain four intervals of three hours each, popularly described as the sleeping, waking, after-sunrise, and fore-sunset zyda respectively.

This is the popular reckoning, and that marked upon the instruments which record time for ordinary purposes, and by these the meals and other industrial and domestic epochs are fixed.

But for purposes of exact calculation, the day, beginning an hour before mean sunrise, is distributed into twelve periods, or antoi, of a little more than two terrestrial hours each.

These again are subdivided by twelve into periods of a little more than 10m., 50s., 2-1/2s., and 5/24s respectively; but of these the second and last are alone employed in common speech.