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One year is about 365 days long (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 365.2422 days long, but a calendar has 365 days, except in a leap year.
There are several ways used to measure the length of a year.
- a solar year is based on the seasons. The Gregorian calendar is based on the solar year. The solar year is 365 days long.
- a tropical year is a solar year as measured between two vernal equinoxes, sometimes called the first day of spring.
- 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.
- There was no year numbered "year zero" in a normal system of counting, because it would mean there is a year earlier than the first year, which was the year AD one in the Anno Domini system, also called 1 CE in the Common Era, used with our Gregorian calendar. However, some astronomers call the year 1 BC (or BCE) "year 0" to make it easier for them to count leap years before that year.
- Ma (for megaannum) — 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.
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