Jill Bello, age 16, of Asbury Park, N.J., for her question:

WHEN DID WE START USING THE GREGORIAN CALENDAR?

The calendar that is used in almost all of the world today is called the Gregorian calendar. Pope Gregory XIII established it in 1582 to correct the Julian calendar, which Julius Caesar put into effect in 46 B.C.

The Julian calendar year was 11 minutes and 14 seconds longer than the solar year. By A.D. 1580, this difference had accumulated to 10 days.

Pope Gregory dropped 10 days from October to make the calendar year correspond more closely to the solar year. He also decreed that each fourth year would be a leap year, when February would have an extra day. Years marking the century would not be leap years unless divisible by 400. For example, 1600 was a leap year, but 1700, 1800 and 1900 were not. The year 2000 will be a leap year.

At present, the average Gregorian year is 26.3 seconds longer than the solar year. By the year 4316, it will have gained one day on the sun. However, adjustments for this difference are made regularly.