Measuring Years

The word for the longest measurement of time is kalpa, Hindi for 432 billion years.

  • A year is the time it takes for the earth to revolve around the sun once. A calendar year is 365 days.
  • A solar or tropical year is 365 days, 5 hours, 48 minutes, and 46 seconds. This year is used for most astronomical calculations.
  • A sidereal year is 365 days, 6 hours, and 9 minutes. It is sometimes used by astronomers, because it is the time it takes the earth to return to the same place in its orbit relative to fixed stars.
  • Leap year occurs every 4 years, when all the extra hours, minutes, and seconds of the solar year are added up to make an extra day.
  • B.C. means “before Christ”; A.D. means anno Domini, Latin for “year of our Lord.”
  • A cosmic year is the amount of time it takes the sun to revolve around the center of the Milky Way, about 225 million years.

Groups of Years

  • Olympiad: 4 years
  • Decade: 10 years
  • Score: 20 years
  • Century: 100 years
  • Millennium: 1,000 years

How Often?

  • Annual: Yearly
  • Biannual: Twice a year (not equally spaced)
  • Semiannual: Twice a year equally spaced; every 6 months
  • Semicentennial: Every 50 years
  • Centennial: Every 100 years

The Aztec and Mayan Calendars

The people of these ancient civilizations used two calendars. One was a sacred calendar of 260 days that marked religious feasts. The other was a secular (not religious) calendar of 365 days divided into 18 months of 20 days each. The extra 5 days were added throughout the years, much like the time we add each leap year. Today, many people follow a religious calendar as well as the secular one.

Religious Calendars

  • The Jewish calendar is reckoned from 3761 B.C.
  • The Jewish New Year, called Rosh Hashanah, occurs on the first and second day of the Hebrew month Tishri, which can come in either September or October.
  • The Islamic calendar is based on a lunar (moon) year of 354 days. It is calculated from the Hegira, in A.D. 622, and grouped in 30-year cycles. (The Hegira was the flight from Mecca by Muhammed, the founder of Islam, to escape persecution.)

Seasons for the Northern HemisphereCalendarAncient Calendars

More on Measuring Years from Fact Monster: