# Definition of a year

Is there an acceptable definition of a year (in number of days)?

What is the rationale behind choosing 365.242 as the number of days in an year? Is it defined by SI?

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This question is a better fit with the Earth Sciences sister site. –  David Hammen Aug 2 at 14:12
That value seems to be quite close to the current mean tropical year. With the caveat that this value is changing. –  celtschk Aug 2 at 18:35
It's frequently "close enough" to use the conversion $1\,\mathrm{year} = \pi\times10^7\,\mathrm s$. –  rob Aug 3 at 12:53
And don't forget leap years and leap seconds. –  jinawee Aug 3 at 14:13
Protip: anything related to the calendar has $P\geq0.98$ of being a complete mess. –  Matteo Italia Aug 3 at 14:24

A year is defined by the time earth takes to circle the sun once: http://en.wikipedia.org/wiki/Year
This does however not correspond to an even number of days. The SI unit of time is the second and $1\,\mathrm{day} = 24\cdot 3600\,\mathrm s$

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right. How did I forget that. –  TJ- Aug 2 at 13:41
Since the orbit of the Earth precesses, I am unsure of this definition, and what "circling the sun" precisely means. There exists, for instance, the concept of sidereal year, where the orbit of the Earth is described "with respect to the fixed stars". See also this answer to another question, which includes the definition of a Julian year. –  J-T Aug 2 at 14:18
Well I think it depends on what you term an acceptable definition of a year. If you want just a short answer then I think the above definition is enough. However, if you are to set up an internationally acceptable calender then I guess you have to consider the differences between sidereal, or Julian years. –  maze-cooperation Aug 2 at 18:46
This is an acceptable definition of a year. So although it's not a complete answer, and certainly not the best answer here, it's not as bad as the vote total of -3 would suggest. –  David Z Aug 3 at 5:12

Which year? The sidereal year? The tropical year? The anomalistic year? The calendar year (and whose calendar)?

The sidereal year is the average amount of time it takes the Earth to make one complete orbit about the Sun with respect to the fixed stars. The tropical year is the amount of average amount of time between successive spring equinoxes. The tropical year is 20.4 minutes shorter than the sidereal year thanks to axial precession. The anomalistic year is the average amount of time between successive perihelion passages. The anomalistic year is 4.7 minutes longer than the sidereal year thanks to apsidal precession.

The tropical year, 365.24219 days, is what drives our seasons. The Julian year was an attempt to fix problems with the Roman calendar and make it more or less stay in sync with the seasons. The Romans added the concept of a leap year every four years, making the Julian year 365.25 days long. Astronomers still use the Julian year for measuring time.

The Julian calendar has a problem: It's a tiny bit too long. The Julian calendar was off by almost half a month by the 18th century, which is when many countries switched to the Gregorian calendar with its slightly more complex leap year rule. The Gregorian calendar has 97 leap days every 400 years, resulting in a year of 365.2425 days (on average). That's still a bit longer than the tropical year. It will take about 3200 years for the Gregorian calendar to be off by one day, assuming that a tropical year remains a constant length.

The Iranian calendar is a bit better than the Gregorian calendar, at least currently. The Iranian calendar has a complex formula that yields 8 leap days every 33 years, resulting in a year of 365.242424... days (on average). It will take 4300 years before the Iranian calendar is off by a day, assuming the tropical year keeps its constant length during that 4300 year interval.

Those calculations assumed a constant length tropical year. That is not the case. Perturbations by other planets make the tropical year vary. The variance works in the favor of the Gregorian calendar. It will take about 4000 years before the Gregorian calendar is off by a day, and by then the Iranian calendar will be off by a bit more than a day.

There are other years as well. Some countries use a lunar based year rather than a solar year.

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Could you give us a reference that discusses all these subjects? –  metacompactness Aug 3 at 8:21
@metacompactness - The wikipedia article cited in maze-cooperation's answer is good place to start. I'll add that and other references to my answer. Don't hold your breath, though; this is looking to be a busy week. –  David Hammen Aug 5 at 15:30

When "year" is used as a unit for things unrelated to Earth's orbit, such as distances in light years or the age of the universe in years, it is the Julian year of exactly 365.25·86400 SI seconds.

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That is only one of many definitions of a year, that, according to the link, does not "correspond to the many other ways of defining a year" and does not answer the question of why a year is 365.242 days. In short, this should be a side note, a comment. –  Mr Lister Aug 3 at 7:13
@Mr Lister: I may have read the question too narrowly. It did ask if there is an SI definition of the year, and the Julian year is essentially the "SI year", although it is not an official SI unit. I don't think there is any other widely used definition of a year as an exact number of seconds or days. Figures like 365.242 days are approximations of a year defined by astronomical observations. –  benrg Aug 3 at 8:13
@benrg - The Julian year is not the "SI year". Ironically, even though the General Conference on Weights and Measures officially meets every four years, the General Conference on Weights and Measures has not established a definition of what "year" means. There is no SI unit called "year" (the same goes for "month", "day", "hour", and "minute"). It's the International Astronomical Union (IAU) that recognizes the "year" (Julian year) as a unit. With regard to what a "day" is, that's the job of the International Earth Rotation and Reference Systems Service (IERS). –  David Hammen Aug 3 at 9:45
@DavidHammen That's interesting. While I can certainly understand the ambiguity of what a day is, with all the leap seconds and all, not to mention the fact that some days are only 23 hours long and others 25 hours, I'd certainly expect the minute and the hour to be as firmly defined as the second. –  Mr Lister Aug 3 at 14:54
@DavidHammen Or did you simply mean "neither the minute, nor the hour, is the SI unit of time, because there is only one SI unit of time, the second"? –  Mr Lister Aug 3 at 14:55

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