DIVISIONS OF TIME
WE read in Gn 1: 14 that God created the sun and the moon 'to divide the day from the night and to serve as signs, for feasts and for the days and the years', and time is in practice reckoned by the courses of these two bodies. The day is measured by the apparent revolution of the sun round the earth, the month by the moon's revolution round the earth, the year by the earth's revolution round the sun. The day, the easiest unit to observe, which regulates all life, public and private, has necessarily been taken as the basic unit by all systems, but the lunar month does not equal an integral number of days, and twelve lunar months amount to 354 days, 8 hours and a fraction, whereas a year based on the sun has 365 days, 5 hours and a fraction. The lunar year is therefore nearly eleven days shorter than the solar year. In a primitive society these differences are of little importance and only need to be corrected from time to time by empirical readjustments. But very early in the East, the development of civil and religious institutions, the taxes periodically due to the State, religious festivals, contracts between individuals, all made it necessary to fix past and future dates, in short, to establish an official calendar. These systems varied in different times and places, and the ancient history of the calendar is very complicated.
The Egyptians adopted at first a lunar calendar, adjusted to ensure that the heliacal rising of Sirius ( Sothis)--whose feast had to fall in the last month of the year--should mark the year's end. In order to keep this agreement between the lunar and solar years a lunar month was added from time to time. This calendar regulated the seasonal religious feasts throughout the whole of Egyptian history. At the beginning of the third millennium B.C., to avoid these arbitrary readjustments and to meet the needs of civil life, a solar year was decreed, with twelve months of thirty days each, plus five supernumerary days, making 365 days, starting from the heliacal rising of Sirius. It was the nearest possible number of days to the natural year, but the latter dropped a day behind the civil year every four years. The Egyptians took a long time to deal with this, and the civil year gradually drew apart from the natural year: the first day of the first month could not fall on the heliacal rising of Sirius for another 1460 years (Sothiac period). After a century or two of the 'New'