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Science of Easter and Passover

April 13, 2004

A public radio commentary

This week represents a great triumph of scientific learning. Right now we're between the Jewish Passover Feast and the Christian Easter. To develop a calendar that placed these holidays at the same season every year took the work of the best minds in science over 1500 years to solve.

The solar year, the time it takes for the earth to go around the sun, takes a precise 365.2447 days to complete. The position of the earth relative to the sun determines the seasons, but because of the uneven number of days in the solar year the seasons shifted in early calendars. For example, the weather we associate with February slowly crept into March, spelling disaster for humanity. A farmer, for instance, needed to know when to plant, or the likely date of the first frost.

The Roman Emperor Julius Caesar tried to conquer this problem with a new calendar. He declared the length of a year to be 365 days plus exactly one quarter day. In other words, he rounded that pesky 0.2447 to 0.25. His new calendar, called the Julian Calendar, used three 365-day years, plus a 366-day year every fourth year, a kind of leap year.

To get the whole Roman world up to par with his new calendar, and to put the world back on track so March would come when March should, he made the year 46 B.C. forty-five days long. He called it "Ultimus Annus Confusionis" - the year of confusion.

But Caesar's clock ran too slow by about eleven minutes, so, over a century or so, spring, again, slowly moved into winter.

The next calendar innovation occurred because of religion. The Vernal Equinox - the date when the hours of daylight equal those of darkness - determined the dates of Easter and Passover. The Jewish authorities used a lunar calendar to determine the date of the Vernal Equinox, but the Catholic Pope didn't want to depend on another religion to determine the Christian Easter.

So in the late 16th century Pope Gregory assembled a council to survey the best scientific work of the time and of the previous centuries, including Copernicus's earth shattering observations about the motion of the planets. The Pope charged his council with finding the exact length of the solar year, and then matching a calendar to it. They came up with a calendar based on a year only twenty-six seconds short of the true length of a solar year. They invented a calendar with unequal months and the occasional leap year - the calendar we use today.

In many ways our science today descends from this calendar because the search for a new calendar helped keep alive mathematics and astronomy in a time less than ideal for scientific inquiry. Kept alive not, though, by a love of learning, but to solve an administrative problem of the Pope.

Copyright 2004 William S. Hammack Enterprises