Year (a or y or yr) [1]
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
