# What is a Year? **URL:** \ **Category:** Factual Questions\ **Created:** [October 3, 2002, 9:04pm UTC](https://boards.straightdope.com/t/what-is-a-year/130735 "2002-10-03T21:04:03Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![BZ00000](https://avatars.discourse-cdn.com/v4/letter/b/47e85d/32.png) [@BZ00000](https://boards.straightdope.com/u/BZ00000)\ **Post date:** [October 3, 2002, 9:04pm UTC](https://boards.straightdope.com/t/what-is-a-year/130735/1 "2002-10-03T21:04:03Z")
Is it how long it takes the Earth to go around the Sun once or what the calendar says? ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [October 3, 2002, 9:33pm UTC](https://boards.straightdope.com/t/what-is-a-year/130735/2 "2002-10-03T21:33:03Z")
The former. ---
**Author:** ![zev\_steinhardt](https://avatars.discourse-cdn.com/v4/letter/z/97f17d/32.png) [@zev\_steinhardt](https://boards.straightdope.com/u/zev_steinhardt)\ **Post date:** [October 3, 2002, 9:35pm UTC](https://boards.straightdope.com/t/what-is-a-year/130735/3 "2002-10-03T21:35:56Z")
It depends… are you talking about a calendar year or an astronomical year? A calendar year is 365 (or 366) days. No more and no less. The earth, however, revolves around the sun every 365 1/4 days yeah, I know that’s not exact… That is an astronomical year. Zev Steinhardt ---
**Author:** ![h.sapiens](https://avatars.discourse-cdn.com/v4/letter/h/e0b2c6/32.png) [@h.sapiens](https://boards.straightdope.com/u/h.sapiens)\ **Post date:** [October 3, 2002, 11:56pm UTC](https://boards.straightdope.com/t/what-is-a-year/130735/4 "2002-10-03T23:56:25Z")
It takes the earth 365 days, 6 hours, 9 minutes and 10 seconds to make a complete orbit around the sun, coming back to the exact point where it started, relative to the positions of distant stars. This is known as a sidereal year. A tropical year, or the amount of time from vernal equinox to vernal equinox, is 365 days, 5 hours, 48 minutes, and 46 seconds. This is due to the wobble of the earth’s axis and the precession (shift) of the equinox. ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [October 4, 2002, 12:09am UTC](https://boards.straightdope.com/t/what-is-a-year/130735/5 "2002-10-04T00:09:10Z")
Yes, there are many different “years” to an astronomer. In addition to the two that **h.sapiens** lists, there is the “anomalistic” year and the “eclipse” year. Don’t worry, though. It’s not as bad as it sounds. These specialized years are only used for specific purposes. It’s not like if you ask an astronomer how long a year is, she’ll get confused and ask for more information. ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [October 4, 2002, 12:13am UTC](https://boards.straightdope.com/t/what-is-a-year/130735/6 "2002-10-04T00:13:40Z")
Zev: Depends on which calendar you’re using. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [October 4, 2002, 12:48am UTC](https://boards.straightdope.com/t/what-is-a-year/130735/7 "2002-10-04T00:48:17Z")
[This site](http://scienceworld.wolfram.com/astronomy/Year.html) gives information on the various definitions of year: > [@](#): > > There are a number of different types of “years” defined for astronomical and civil use. A Gregorian year (or “calendrical year”) is the number of days in a given year of the Gregorian calendar (which 365 days in non-leap years and 366 days in a leap year). A tropical year is the time from one equinox to the next. A sidereal year is the time between maximum elevation passages of a fixed star. The anomalistic year is the time from one perigee to the next. The eclipse year is the time from node to node passage of the Moon. Take your pick. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [October 4, 2002, 12:49am UTC](https://boards.straightdope.com/t/what-is-a-year/130735/8 "2002-10-04T00:49:12Z")
This question made me wonder. About the astronomical year, that is. Is it the galactic year or the universal year? Just as the sidereal day differs by about 4 minutes from the average solar day because the earth revolves around the sun and therefore the appears in a different position against the “fixed” stars at the end of each solar day, so the solar system revolves around the center of the galaxy (with a period of 50 million years, IIRC) and so faces the “fixed” galaxies from a slightly different vantage each year. I figure the difference is about 3/5 of a second a year, but that is the sort of correction that the astronomers make when they insert a leap second every year or two. So what is it? The teeming one wants to know. ---
**Author:** ![kniz](https://avatars.discourse-cdn.com/v4/letter/k/c37758/32.png) [@kniz](https://boards.straightdope.com/u/kniz)\ **Post date:** [October 4, 2002, 6:00am UTC](https://boards.straightdope.com/t/what-is-a-year/130735/9 "2002-10-04T06:00:23Z")
> [@](#): > > \*Originally posted by Hari Seldon \* > **This question made me wonder. About the astronomical year, that is. Is it the galactic year or the universal year?** It is an “Earth year”, not even a “solar year”, since it is simply measuring how long it takes the earth to circle the sun. A “Venus year” would be shorter and a “Pluto year” much longer, but as far as I know we would still measure them in earthly terms. 😕 ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [October 4, 2002, 6:16am UTC](https://boards.straightdope.com/t/what-is-a-year/130735/10 "2002-10-04T06:16:44Z")
> [@](#): > > It is an “Earth year”, not even a “solar year”, since it is simply measuring how long it takes the earth to circle the sun The question was, how do you know it’s made one complete circle of the sun? When it’s come back to the same position (angle) relative to the Galaxy, or relative to the Universe? As **Hari Seldon** said there is a measureble difference between the two. I’m not 100% certain but I believe it’s relative to the Universe. Quasars are often used as reference frames. They are farther away than anything else we can observe, and they are point sources so position measurement is not ambiguous. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [October 4, 2002, 11:46am UTC](https://boards.straightdope.com/t/what-is-a-year/130735/11 "2002-10-04T11:46:54Z")
It occurs to me that there is yet another year, the most important for biological activity, and that is the year from one winter solstice to the next. That year presumably differs from the others by some relatively insignificant, but still measurable, amount. That is the one whose drift caused the Gregorian calendar reform, which was caused by the difference between this solstitice year and 365.25 day year, which made planting dates wrong. ---
**Author:** ![UDS](https://avatars.discourse-cdn.com/v4/letter/u/ecae2f/32.png) [@UDS](https://boards.straightdope.com/u/UDS)\ **Post date:** [October 4, 2002, 12:10pm UTC](https://boards.straightdope.com/t/what-is-a-year/130735/12 "2002-10-04T12:10:59Z")
The year from solstice to solstice is presumably the same as the year from equinox to equinox (the tropical year). Or am I being too simplistic here? ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [October 4, 2002, 8:43pm UTC](https://boards.straightdope.com/t/what-is-a-year/130735/13 "2002-10-04T20:43:06Z")
Yes, UDS, you are. The time from one vernal equinox to the next is 365.242374 days. The time from one winter solstice to the next is 365.242740 days. The difference is caused by regression of the Earth’s perihelion. See “Marking Time” by Duncan Steel for a thorough discussion of the many different types of astronomical and calendrical years.