Matt Schaffer, age 12, of Huntsville, Ala., for his question:
WHAT IS LEAP YEAR?
During the Middle Ages, most European nations considered March 1 or March 25, Annunciation Day, as the first day of a new year. But by 1600, almost every civilized country, with the exception of England, recognized January 1 as the first day of the year. Then in 1752 England adopted the Gregorian calendar and joined the others in calling the first day of January New Year's Day.
As we move into 1980, we move into a leap year.
ILeap year has 366 days instead of the usual 365. The special year happens every year that can be divided evenly by four, except the years that mark the even hundreds, such as 1500. The only century years that are leap years are those that can be divided evenly by 400, such as 1600 and 2000.
One year is the time it takes the earth to make one complete revolution around the sun. There are actually two different kinds of years which are used by astronomers: the solar year and the sidereal year.
The solar year, which is also called the equinoctial or tropical year, is the time between two passages of the sun through the vernal equinox, which happens in March. This type of year lasts 365 days, five hours, 48 minutes and 46 seconds. For all practical and astronomical purposes, this solar year is used and it is the basis of our common or calendar year.
A sidereal year has 365 days, six hours, nine minutes and nine and a half seconds. This year is made up of the time it takes the earth to return to the same place in its orbit, with reference to the fixed stars. The sidereal year is longer than the solar year because of the precession of the equinoxes. The sidereal year is used only in astronomers' calculations.
Since the calendar year is only 365 days long, we have to add an extra day every four years so that we can correct the difference between the solar year and the calendar year. We call this fourth year leap year. The extra day becomes February 29.
Adding an extra day every fourth year makes the average calendar year 11 minutes, 14 seconds too long. For this reason, only century years divisible by 400 become leap years. The years 1700, 1800 and 1900 had only 365 days.
The ancient Greeks used the lunar year, which is made up of 12 lunar months. There were only 354 days in this year.
The Islamic year is established on the changing phases of the moon and lasts for 354 days. The beginning of the Islamic year continually falls earlier in the seasons.
Thirty Islamic years make up a cycle during which there are 11 leap years, each occurring at irregular intervals.