A year is about 365 days (except in a leap year). It is the time it takes the Earth to go completely around (orbit) the sun once. A year is actually almost 365.25 days long, but a calendar has 365 days, except in a leap year, which has 366 days.^[1]

The year starts on January 1 and ends on December 31 in the Gregorian calendar, but a fiscal year or a school year can start on a different day of the year.

There are several ways used to measure the length of a year.

• a solar year, also called tropical year, is based on the seasons. The Gregorian calendar is based on a tropical year of 365.2425 days. This is the time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice. The word "tropical" in this sense comes from the Greek tropikos meaning "turn".
• a lunar year is based on the moon and is usually 12 lunar months (29 days, 12 hours, 44 minutes each) or 354 days long.
• a sidereal year measures the time between when a selected fixed star is highest in the night sky.
• an anomalistic year is the difference between the times when the Earth gets closest to the sun.
• an eclipse year is the time between node passages. This is when the sun moves through a part of the sky where it is possible for the sun, Earth and moon to be in a line. It is also when eclipses can happen.

Solar and lunar years are used by different calendars for daily life. The other measurements are used by astronomers.

• Ma (for meganum) — a unit of time equal to one million years. The suffix "Ma" is often used in scientific disciplines such as geology, paleontology, and astronomy to signify very long time periods into the past or future. The simpler term "mya" for "million years ago" is generally preferred on this wiki as being intuitively more simple for non-technical readers.

A specific calendar is provided for the liturgical year.