The True Year.
The base line at Copan yielded accurate data on the exact length of the tropical year, a period varying by a difficult fraction from 365 full days. The tropical year is the time measured by the revolution of the earth around the sun and by the recurring seasons. No agricultural people could neglect this natural time period with its obvious relation to planting and harvest.
Reference has already been made to the notational 360 day year (tun) of the Mayas and to their formal calendar year (haab) of exactly 365 days. The calendar year kept running ahead of the true year by the accumulating amount of the days which we intercalate on leap years but the Mayas wisely made no such intercalations since to have done so would have thrown their day count out of gear with the moon and other planets and the somewhat defective calendar based upon these minor heavenly bodies. Therefore the months of the Mayan year like those of the ancient Egyptian year slowly moved through the seasons. But the Mayas calculated an almost exact correction for the excess of the true year over the vague 365 day year. This excess amounts to about .24 of a day and their correction seems to have been one day in four years for short periods while for long periods they made 29 calendar rounds (1508 calendar years or 550,420 days) equal 1507 tropical years. This is a remarkably accurate adjustment, much closer, in fact, than that of our present Gregorian calendar. This great cycle is comparable to the 1460 year Sothic cycle of the Egyptians in so far as that relates to the flooding of the Nile, but the Egyptian arrangement has an error of about twelve days for the cycle while the Mayan arrangement is accurate to a very small fraction of a day.
In the calendars of various Guatemalan and Mexican tribes the slow shifting of the months is attested by actual statements of early Spanish writers. But the conventional 365 day year was, after all, sufficiently accurate for most purposes since associations between the months and the seasons would hold reasonably true for the average lifetime.