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Leap Year
Nature Bulletin No. 745   February 29, 1964
Forest Preserve District of Cook County
Seymour Simon, President
Roberts Mann, Conservation Editor

Thirty days hath September,
April, June and November;
All the rest have thirty-one
Save February, she alone
Hath eight days and a score
Til leap year gives her one day more.

That's a handy little rhyme we learned at school, many long years ago, but we were never told how come the numbers of days in the respective months are so jumbled up. Nor why we have leap years. Probably our teachers did not know.

It's a long story that really should begin with the Thoth calendar invented in Egypt, 5000 years ago. From it, and the ancient Roman calendar, Julius Caesar developed a better one. He and his nephew, Augustus who succeeded him, were chiefly responsible for the names of the twelve months in our calendar, the numbers of days in those months, and for leap year.

Ours is a solar calendar in which a year is the time required for the earth to complete its annual orbit around the sun. A day is the time required -- 24 hours -- for the earth to make one complete revolution on its axis. The necessity for leap years stem from the fact that there are almost 365-1/4 days in a year. Consequently the common years have 365 days but every fourth one which is exactly divisible by 4, such as 1964, is a leap year with 366 days including 29 in February instead of 28.

Actually, however, there are 365. 2422 days in a solar year -- 365 days, 5 hours, 48 minutes and 46 seconds. That is only 11 minutes and 14 seconds less than 365-1/4 (365. 25) days but in 125 years the accumulated excess in the leap years amounts to a trifle more than one day.

Therefore, in centesimal years not exactly divisible by 400 -- such as 1900, 1800 and 1700 -- there is no leap year. There was one in 1600 and there will be one in the year 2000. Because that slight excess in leap years amounts to a trifle more than one day every 125 years, in AD 4000 there will be no leap year. We thought you might be interested.

The original Roman calendar had a 10-month year of only 304 days. It began in Martius at the time of the vernal equinox. The second month was Aprilis (when leaves and flowers opened), followed by Maius (the time of growth), and Junius (the time of youth's maturity). The fifth month was Quintilis (Latin for five), Sextilis (6), Septembris (7), Octobris (8), Novembris (9) and Decembris (10). Numa Pompilius, successor to the legendary Romulus, took care of the other 61 days by adding Januarius (for Janis, the two-faced god, protector of gateways) and Februarius (the time of sacrifices to atone for sins).

In 46 B. C., Julius Caesar, as Pontitex Maximus, decreed a new calendar. In Egypt he had studied their Thoth calendar based upon a solar year of 365-1/4 days. It included the 7-day week and "leap" years borrowed from the Hebrew calendar. Its year had 12 months, each with 30 days, ending with 5 or 6 days never used in public or private transactions.

Julius Caesar's calendar distributed those surplus days alternately among the 12 months. After his death, January became the first month of a year and Quintilis was renamed Julius in his honor. Augustus Caesar revised the Julian calendar. Sextilis became Augustus, with 31 days so that it would have as many as Julius, and September, October, November and December have 30, 31, 30 and 31 days, respectively, instead of 31, 30, 31 and 30 which is why we learned that handy little rhyme.

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