year

(a or y or yr)

   1. a unit of time, defined to be the period of time required for the Earth to make one revolution around the Sun. To be more precise, the year we use in ordinary life (described in the next entry) is designed to approximate the interval between two arrivals of the Sun at the Tropic of Capricorn, marking the summer solstice in the Southern Hemisphere and the winter solstice in the Northern Hemisphere. Astronomers call this unit the tropical year.

   There are 365.242 199 days in a tropical year, or, to be even more precise, 31 556 925.9747 seconds. Since the symbol yr is specific to English, the symbol for the year often used in scientific writing or other international contexts is a, taken from the Latin word, annus. Thus 1 Ma stands for a million years and 1 Ga for a billion years.

   2. a traditional unit of time usually equal to 365 or 366 days. We need a whole number of days for the calendar year used in ordinary life. Ancient astronomers knew that the year¹ is approximately 365 days long, and we now know the correct figure is approximately 365.242 days. If we use 365 as the number of days in every calendar year, the extra 0.242 day adds up quickly and causes large errors in predicting the seasons. To solve this problem, the Roman emperor Julius Caesar decreed in 46 BC that the calendar year should have 365 days generally, but that every fourth year should have an extra, or 366th, day. The longer year is called a leap year. In this Julian calendar, four years equal exactly 1461 days, so the average Julian year is exactly 365.25 days.

   This was a big step toward accuracy in the calendar, but the Julian year is too long by 0.008 day, or a little over 11 minutes. By the time of the Renaissance, these 11-minute errors had accumulated to a total error of about 10 days (since the Council of Nicaea in 325 AD, which set the rules for deciding when Easter should be celebrated). The spring equinox was occurring near March 11 instead of March 21. In 1582, Pope Gregory XIII decreed that 10 days should be dropped from the calendar: the day after 1582 October 4 was October 15. To reduce future errors, the pope further decreed that years divisible by 100 are not leap years unless they are also divisible by 400. Thus 2000 and 2400 are leap years, but 2100, 2200, and 2300 are not. It took many years, but the Gregorian calendar has now been accepted as the civil calendar in all countries of the world.

   With the Gregorian adjustment, there are exactly 146 097 days in every 400 years, and the average Gregorian year is exactly 365.2425 days. The Gregorian year is still too long, but by less than half a minute. It will take thousands of years for this error to accumulate to 1 day, so the calendar year and the tropical year are in good enough agreement to last us a long time.