Year

A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, "Martian year".

Seasonal year

A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, or the flowering of a species of plant.

Astronomical year

An astronomical year is the time between two recurrences of an astronomical event.

The sidereal year is the time for the Earth to complete one revolution of its orbit, as measured in a fixed frame of reference (such as the fixed stars, Latin sidus). Its duration is on average:

365.256363051 days (365d 6h 9m 9s) (at the epoch J2000.0 = January 1, 2000 12:00 TT).

A tropical year is the time for the Earth to complete one revolution with respect to the framework provided by the intersection of the ecliptic (the plane of the orbit of the Earth) and the plane of the equator (the plane perpendicular to the rotation axis of the Earth). Because of the precession of the equinoxes, this framework moves slowly backwards along the ecliptic with respect to the fixed stars (with a period of about 26,000 years); as a consequence, the Earth completes this year before it completes a full orbit as measured in a fixed reference frame. Therefore a tropical year is shorter than the sidereal year.

The exact length of a tropical year depends on the chosen starting point: for example the vernal equinoctial year is the time between successive vernal equinoxes, and is the basis for the Gregorian calendar, which is the international standard civil calendar.

The mean tropical year (averaged over all tropical ecliptic points) is:

365.24218967 days (365d 5h 48m 45s) (at the epoch J2000.0).

Calendar year

A calendar year is the time between two dates with the same name in a calendar.

Calendars usually aim to predict the seasons, but because it is hard to determine the length of the seasonal year, they instead use an astronomical year as a surrogate for the seasonal year. For example, the ancient Egyptians used the heliacal rising of Sirius to predict the flooding of the Nile.

The Gregorian calendar aims to keep the vernal equinox on or close to March 21; hence it follows the vernal equinox tropical year.

No astronomical year has an integer number of days, so any calendar that follows an astronomical year must have a system of intercalation such as leap years.

A Julian year is exactly 365.25 days, the average length of the year in the Julian calendar. It is still used in ephemerides because of the very simple conversion between Julian dates and Julian years: 100 Julian years is just another way of saying 36525 days.

Summary of various kinds of year

353, 354 or 355 days -- the lengths of regular years in some lunisolar calendars
354.37 days -- 12 lunar months - the average length of a year in lunar calendars
365 days - a regular year in many solar calendars - ~31.53 million seconds
365.242199 days - a mean Tropical year
365.2424 days - a vernal equinox Tropical year.
365.2425 days - the average length of a year in the Gregorian calendar
365.25 days - the average length of a year in the Julian calendar; the light year is based on it; it is 31,557,600 seconds
365.2564 days - a sidereal year
366 days = a leap year in many solar calendars - 31.62 million seconds
383, 384 or 385 days -- the lengths of leap years in some lunisolar calendars
383.9 days -- 13 lunar months - leap year in some lunisolar calendars

An average year is 365.2425 days = 52.1775 weeks, 8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds.

A common year is 365 days = 8,760 hours = 525,600 minutes = 31,536,000 seconds.

A leap year is 366 days = 8,784 hours = 527,040 minutes = 31,622,400 seconds.

The 400-year cycle of the Gregorian calendar has 146097 days and hence exactly 20871 weeks.

This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia and from ShiningRise.com

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