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
- 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.
- 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.)