The earth completes its orbit about the sun in 365 days 5 hr 48 min 46 sec—the length of the solar year. The moon passes through its phases in about 291/2 days; therefore, 12 lunar months (called a lunar year) amount to more than 354 days 8 hr 48 min. The discrepancy between the years is inescapable, and one of the major problems since early days has been to reconcile and harmonize solar and lunar reckonings. Some peoples have simply recorded time by the lunar cycle, but, as skill in calculation developed, the prevailing calculations generally came to depend upon a combination.
The fact that months and years cannot be divided exactly by days and that the years cannot be easily divided into months has led to the device of intercalation (i.e., the insertion of extra days or months into a calendar to make it more accurate). The simplest form of this is shown in ancient calendars which have series of months alternating between 30 and 29 days, thus arriving at mean months of 291/2 days each. Similarly four years of about 3651/4 days each can be approximated by taking three years of 365 days and a fourth year of 366. This fourth year with its intercalary day is the leap year. If calculations are by the lunar cycle, the surplus of the solar over the lunar year (365 over 354) can be somewhat rectified by adding an intercalary month of 33 days every three years.
Reckoning of day and year was considered necessary by many ancient peoples to determine sacred days, to arrange plans for the future, and to keep some intelligible record of the past. There were, therefore, various efforts to reconcile the count in solar, lunar, and semilunar calendars, from the Egyptians and the Greeks to the Chinese and the Maya. The prevailing modern method of constructing a calendar in the Christian West came originally from the Egyptians, who worked out a formula for the solar year (12 months of 30 days each, five extra days a year, and an extra day every four years) that was to be adopted later by the Romans.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.