Helen Delaney, age 14, of Columbia, Tenn., for her question:

WHEN WAS THE GREGORIAN CALENDAR ADOPTED?

Most persons in the Western World today use the Gregorian calendar. It was worked out by Pope Gregory XIII in 1582 and was designed to correct the errors of the Julian calendar.

In 1582, on the advice of astronomers, a correction in the difference between the sun and calendar dropped 10 days from October. The day that would have been October 5, 1582, became October 15.

The correction restored the next equinox to its proper date.

To correct the Julian calendars error regularly, the pope decreed that February would have an extra day in century years that could be divided by 400, such as 1600 and 2000, but not in others, such as 1700, 1800 and 1900.

The Gregorian calendar is so accurate that the different between the calendar and solar years is now only about 26.3 seconds. This difference will increase by .53 seconds every 100 years, because the solar year is gradually growing shorter.

The Roman Catholic nations of Europe adopted the Gregorian calendar almost immediately. Various German states kept the Julian calendar until 1700. Great Britain did not change to the Gregorian until 1752, Russia until 1918 and Turkey until 1927.