• 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.)