Kenneth Hutson, age 15, of Indianapolis, Ind., for his question:
Does the eclipse have a schedule?
The eclipses of the sun and the moon run on a very strict schedule. This timetable is called a saros from an old Chaldean word meaning repetition, It was the Chaldeans who discovered the orderly schedule of the eclipses, long before the dawn of modern history.
These stargazers lived in ancient Mesopotamia, where the land is flat and the stars are bright in t he desert sky. Star watching was part of their religion. They built tall towers from which they studied and recorded the heavenly parade as it went over the sky night by night, year by year and century by century.
It took these ancient stargazers several thousand years to figure out the schedule of the saros. For though the saros repeats itself every 19 years, the series of eclipses is soon a little further to the nest with each parade and a little further north or south with every third parade. The Chaldeans in their tall towers h ad to watch m any parades, a section at a time, before they could figure out the saros.
The saros schedule is 19 years minus .4.6 of a day. Between the start of one saros and the start of the next, the earth rotates 6585 and 1/3 turns on its axis. In a third of a day, any spot on earth travels 120 longitude degrees further to the west.
After the saros parade has passed three times, the eclipses return to almost the same longitudes. However, the sun and earth meantime have changed slightly in their relative positions. The new saros schedule, after three saroses or 54 years and one month, may repeat a little farther north or south.
The present series of triple saroses seems to have started at the north pole and is gradually sweeping south. A total solar eclipse was visible from the north pole on July 6, 1811. On August 7, 1869, three saroses later, this eclipse was visible across Alaska and Canada. On September 10, 1923,
On September 10, 12_13, it was visible across the southwestern United States. On October 12, 1977 it will be visible south of Panama.
During a complete saros of 19 years minus almost half a day, there are about 29 lunar eclipses and 41 solar eclipses. But only ten of those will be total solar eclipses.
Modern astronomers predict the eclipses based on their knowledge about the movements of the earth, the sun and moon. In the last century, an Austrian astronomer named Oppolzer figured out a long series of past and future eclipses. He charted all those that have occurred since 1207 B.C. and predicted all those‑ that will occur up to the year 2162_ A.D.