<p>A distiction has to be made between the length of a year, and the period of the orbit.</p>

<p>For human purposes, a year is the time it takes for one full cycle of the seasons.  It is the time between one vernal Equinox and the next.  Seasons are caused by the tilt of the Earth, and if the tilt was fixed, then this time period would be the the time it took for the Earth to orbit the sun: 365.2564 days (in which each day is 86400 seconds)</p>

<p>However the direction of the Earth's axis (relative to the stars) isn't fixed. It precesses in a circle. This means that the moment of the vernal equinox isn't at the same point in the orbit each year. In fact we reach the vernal equinox in slightly less time than it takes to orbit the sun.  The time between successive equinoxes is 365.24219 days.</p>

<p>One  mean <a href="https://en.wikipedia.org/wiki/Tropical_year" rel="noreferrer">tropical year</a> is 365.24219 days. One <a href="https://en.wikipedia.org/wiki/Sidereal_year" rel="noreferrer">sidereal year</a> is 365.256363 days.</p>

CC BY-SA 4.0
2020-03-20T19:51:08.040Z
FWIW, that's a mean tropical year, which is slightly different to a Vernal Equinox year (~365.242374 days). See https://www.timezone.com/2002/09/25/on-the-units-of-time-part-iii-the-year/
PM 2Ring
CC BY-SA 4.0
2020-03-20T20:20:36.090Z
Another FWIW, when I used to like to remember useless stuff, it was 365.24219878 days
Dr Chuck
CC BY-SA 4.0
2020-03-21T01:59:41.740Z
Very nice, thank you! It looks like $$\frac{365.25636}{365.256363-365.24219} = 25771.2808$$

which is about the same as the [period of Earths axial precession](https://en.wikipedia.org/wiki/Axial_precession).
uhoh
CC BY-SA 4.0
2020-03-21T08:15:32.453Z
@PM2Ring fyi I've just asked [So, what, exactly, is a tropical period?](https://astronomy.stackexchange.com/questions/35553/so-what-exactly-is-a-tropical-period)
uhoh
CC BY-SA 4.0
2020-03-20T19:38:14.817Z
2020-03-20T19:54:55.390Z
2020-03-20T19:54:55.390Z
James K
<p>Both are correct.</p>

<p>The <a href="https://en.wikipedia.org/wiki/Sidereal_year" rel="noreferrer">Sidereal Year</a> is the length of time it takes Earth to complete an orbit around the sun, relative to the fixed stars. It is 365.2564 days.</p>

<p>The <a href="https://en.wikipedia.org/wiki/Tropical_year" rel="noreferrer">Tropical Year</a> is the length of time it takes for the Sun to complete a cycle around the Ecliptic and return to the position in the cycle of seasons; e.g. from Vernal Equinox to Vernal Equinox.  It's about 365.24217 days, about 20 minutes shorter than the Sidereal year because of the precession of the equinoxes. This is the year that the Gregorian Calendar is attempting to emulate.</p>

CC BY-SA 4.0
2020-03-20T19:36:45.477Z
2020-03-20T19:36:45.477Z
notovny
<p>The NPR News item and podcast <a href="https://www.npr.org/2020/03/19/817237429/spring-starts-today-all-over-america-which-is-weird" rel="nofollow noreferrer">Spring Starts Today All Over America, Which Is Weird</a> includes the following:</p>
<blockquote>
<p>But why isn't the time of the equinox the same each year?</p>
<p>The short answer is that the time and the date are imperfect human constructs that we use to keep track of our planet's movements.</p>
<p>The longer answer involves leap years.</p>
<p>All of this is caused simply by the fact that the spin of the Earth doesn't divide evenly into one year,&quot; <strong>says <a href="https://solarsystem.nasa.gov/people/1040/michelle-thaller/" rel="nofollow noreferrer">Michelle Thaller</a>, an astrophysicist turned space communications expert at NASA.</strong></p>
<p>One spin of the Earth around its axis is one day. <strong>&quot;The problem is we're happily spinning on our axis, and the Earth is going around the Sun, but one year — one complete path around the Sun — isn't an even, exact number of days. In fact, it's 365.24 [days].&quot;</strong></p>
</blockquote>
<p><a href="https://en.wikipedia.org/wiki/Earth" rel="nofollow noreferrer">Wikipedia gives</a> Earth's orbital period as 365.256363004 days and I have always remembered it to be 365.2564 which is the same value as Wikipedia, just to fewer digits.</p>
<p>So is a year really about 365.24 days or closer to 365.2564 days?</p>

CC BY-SA 4.0
2020-03-21T15:24:43.333Z
One way to look at it: If a "year" was 365.2564 days long, our calendar would typically need a leap year every four years that contain one extra day, but would occasionally need double leap years, years in which two extra days are added. Instead we typically have single day leap years every four years, but occasionally skip leap years (e.g., 1700, 1800, and 1900 were not leap years), making the mean calendar year a bit less than 365.25 days long.
David Hammen
<solar-system><earth><orbital-mechanics><time>
Is a year really 365.24 days, or is it 365.2564 days like I remember?
uhoh
# Is a year really 365.24 days, or is it 365.2564 days like I remember?

(Asked by: username_0 on 2020-03-20)

The NPR News item and podcast [Spring Starts Today All Over America, Which Is Weird](https://www.npr.org/2020/03/19/817237429/spring-starts-today-all-over-america-which-is-weird) includes the following:

> But why isn't the time of the equinox the same each year?
>
> The short answer is that the time and the date are imperfect human constructs that we use to keep track of our planet's movements.
>
> The longer answer involves leap years.
>
> All of this is caused simply by the fact that the spin of the Earth doesn't divide evenly into one year," **says [Michelle Thaller](https://solarsystem.nasa.gov/people/1040/michelle-thaller/), an astrophysicist turned space communications expert at NASA.**
>
> One spin of the Earth around its axis is one day. **"The problem is we're happily spinning on our axis, and the Earth is going around the Sun, but one year — one complete path around the Sun — isn't an even, exact number of days. In fact, it's 365.24 [days]."**

[Wikipedia gives](https://en.wikipedia.org/wiki/Earth) Earth's orbital period as 365.256363004 days and I have always remembered it to be 365.2564 which is the same value as Wikipedia, just to fewer digits.

So is a year really about 365.24 days or closer to 365.2564 days?

**Tags:** solar-system, earth, orbital-mechanics, time
> **Comments:**
> **username_1**: One way to look at it: If a "year" was 365.2564 days long, our calendar would typically need a leap year every four years that contain one extra day, but would occasionally need double leap years, years in which two extra days are added. Instead we typically have single day leap years every four years, but occasionally skip leap years (e.g., 1700, 1800, and 1900 were not leap years), making the mean calendar year a bit less than 365.25 days long.  

---

## 2 answers

### Answer by username_5 on 2020-03-20. Score: 9 | Accepted answer

Both are correct.

The [Sidereal Year](https://en.wikipedia.org/wiki/Sidereal_year) is the length of time it takes Earth to complete an orbit around the sun, relative to the fixed stars. It is 365.2564 days.

The [Tropical Year](https://en.wikipedia.org/wiki/Tropical_year) is the length of time it takes for the Sun to complete a cycle around the Ecliptic and return to the position in the cycle of seasons; e.g. from Vernal Equinox to Vernal Equinox. It's about 365.24217 days, about 20 minutes shorter than the Sidereal year because of the precession of the equinoxes. This is the year that the Gregorian Calendar is attempting to emulate.


---

### Answer by username_2 on 2020-03-20. Score: 6

A distiction has to be made between the length of a year, and the period of the orbit.

For human purposes, a year is the time it takes for one full cycle of the seasons. It is the time between one vernal Equinox and the next. Seasons are caused by the tilt of the Earth, and if the tilt was fixed, then this time period would be the the time it took for the Earth to orbit the sun: 365.2564 days (in which each day is 86400 seconds)

However the direction of the Earth's axis (relative to the stars) isn't fixed. It precesses in a circle. This means that the moment of the vernal equinox isn't at the same point in the orbit each year. In fact we reach the vernal equinox in slightly less time than it takes to orbit the sun. The time between successive equinoxes is 365.24219 days.

One mean [tropical year](https://en.wikipedia.org/wiki/Tropical_year) is 365.24219 days. One [sidereal year](https://en.wikipedia.org/wiki/Sidereal_year) is 365.256363 days.

> **Comments:**
> **username_3**: FWIW, that's a mean tropical year, which is slightly different to a Vernal Equinox year (~365.242374 days). See https://www.timezone.com/2002/09/25/on-the-units-of-time-part-iii-the-year/  
> **username_4**: Another FWIW, when I used to like to remember useless stuff, it was 365.24219878 days  
> **username_0**: Very nice, thank you! It looks like $$\frac{365.25636}{365.256363-365.24219} = 25771.2808$$  which is about the same as the [period of Earths axial precession](https://en.wikipedia.org/wiki/Axial_precession).  
> **username_0**: @PM2Ring fyi I've just asked [So, what, exactly, is a tropical period?](https://astronomy.stackexchange.com/questions/35553/so-what-exactly-is-a-tropical-period)  

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