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


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