Linda Browning, age 10, of Billings, Mont., for her question:
JUST HOW LONG IS A YEAR?
December 31 is the last day of the year. A 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, equinoctial or tropical year and the sidereal year.
The solar, equinoctial or tropical year is the time between two passages of the sun through the vernal equinox, which occurs in March. This year is 365 days, 5 hours, 48 minutes and 46 seconds long. This year is used for all practical and astronomical purposes. It is the basis of our common or calendar year.
The sidereal year is made up of 365 days, 6 hours, 9 minutes and 9 1/2 seconds. This is 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 just a bit longer than the solar year because of the "precession of the equinoxes," an astronomical term referring to the occurrence of the equinoxes earlier in each sidereal year. The sidereal year is seldom used except in the calculations of astronomers.
Because the calendar year is only 365 days long, we have to add an extra day every four years to correct the difference in time between the calendar year and the solar year. This fourth year is called leap year and the extra day is Feb. 29. There will be leap years in 1992 and 1994.
Adding an extra day every fourth year makes the average calendar year 11 minutes and 14 seconds too long. So the day is not added in the century years, except in those divisible by 400. The years 1700, 1800 and 1900 had only 365 days. The year 2000 will contain 366 days. Thus, the difference between calendar and solar years will vary only one day over a period of several thousand years.
The lunar year is made up of 12 lunar months. The ancient Greeks used this year. It contained 364 days.
In most Christian nations the calendar year starts on Jan. 1.
During the Middle Ages, however, most European nations considered March 12 or March 25, Annunciation Day, as the first day of the calendar year.
By 1600, nearly all civilized countries except England recognized Jan. 1 as the first day of the year. England adopted the Gregorian calendar, which recognized Jan. 1 as the start of the year, on Sept. 14, 1752.
The church calendar, which is used in the Roman Catholic and in most Protestant churches, is regulated partly by the solar and partly by the lunar year. This causes a difference between the fixed feast days which always fall on the same day every year, and movable feasts such as Easter, whose dates vary from year to year.
The fixed feast days are determined by the solar year and the movable feast days by the lunar year.
The Jewish year starts near the time of the autumnal equinox, around Sept. 22. The Islamic year, however, is based on the changing phases of the moon and lasts for 364 days. Therefore, 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.