Leap Years

Every four years, in years divisible by four, is a leap year, when an extra day is added to the month of February, except years divisible by 100. For example, 1700, 1800, and 1900 were not leap years, yet 2000 was a leap year. Why?

Answers

In years divisible by four, every four years is a leap year except years divisible by 100. If the mean interval between vernal equinoxes, called the tropical year, lasts 365.2422 days, then in 100 years we should experience 36,524.22 days. But there will be 24 leap years in a century normally, so there will be 0.22 day left over. So every 400 years is declared to be a leap year with one extra day to approximate the 0.88 day. The year 2000 was the first such leap year on a year divisible by 100 since the modern calendar began general use in the late 1600s. By the time the British were ready to go along with the rest of Europe in the 1700s, the old Julian calendar required a correction of eleven days! The Gregorian calendar was adopted in Britain in 1752, with Wednesday, September 2, 1752, being followed immediately by Thursday, September 14, 1752.

The famous physicist Isaac Newton was born on Christmas Day, 1642, on the Julian calendar but on January 4, 1643, on the Gregorian calendar in use today. Therefore, Newton was not born in the year of Galileo’s death, 1642!