<blockquote>
  <p>Why does a year last the same time as the four seasons?</p>
</blockquote>

<p>Because that is how a year is defined. Our year is based on the time from one March equinox to the next. The yearly cycle of equinoxes and solstices is in turn driven by the Earth's axial tilt. This axial tilt is what drives the seasons.</p>

<p>Multiple ancient societies independently discovered the relationship between how these cycles and when was the best time to plant and time to harvest. They also discovered that one year is a bit more than 365 days long, and curiously, that the stars do not quite follow the same cycle. Over the course of 70.5 years, the date on which a star rose at a certain location shifted by one day. We base our year on the Sun, not on the stars, and because we base it on the Sun, a year stays in sync with the seasons.</p>

<p>There is an issue here, which is that a year is a bit more than 365 days long. Our civil calendar is based on days rather than years. The Romans addressed this issue by adding a leap day every fourth year. This made a year 365.25 days long, on average. The correct value needed to stay in sync with the seasons is a year that is 365.2421875 days long, on average.</p>

<p>This became a problem (rather than just an issue) during the Renaissance. The discrepancy between 365.25 days and 365.2421875 days had made the seasons fall out of sync with the calendar by 10 days at the time of Pope Gregory XIII. Most importantly (to the Roman Catholic church, that is), properly calculating the date on which Easter fell was in jeopardy. The Julian calendar had 100 leap years for every 400 years. The Gregorian reform made 10 days vanish (or more, depending on when a country adopted the Gregorian calendar), and made only 97 years out of 400 be leap years. This makes the Gregorian year be 365.2425 days long, on average. This is considerably closer to the ideal of 365.2421875 days than is the 365.25 days used in Roman times.</p>

CC BY-SA 3.0
2017-01-10T13:31:40.727Z
2017-01-10T13:31:40.727Z
David Hammen
<blockquote>
  <p>Why does a year last the same time as the four seasons ?</p>
</blockquote>

<p>Long before we had any serious science of astronomy, we had seasons.  We defined a year as being four seasons.  All you need to do that is long term memory.</p>

<p>Once you can count and record events  in some way, you start counting days.</p>

<p>As we accumulated knowledge of the stars, we noticed that the changes of positions of the stars in the heavens followed very regular cycles.  That gave us a way to relate a year to an exact number of days ( and in fact to a fraction of a day ).</p>

<p>When we developed physics enough to be able to associate the yearly cycle with an orbital cycle we changed our definition of year to be exactly that cycle ( and we even allow for slight changes in that's cycle's length ).</p>

<p>So we invented the approximate year first and then refined it to match our understanding of physics.</p>

<blockquote>
  <p>The seasons could take 100 days or 200 or 20 years (aprox. like in GoT)... but exactly (exactly???) 1 year. Why?</p>
</blockquote>

<p>Because a year happens to be the period of time that matches the changes in seasons and heavens we first noticed and first gave that name.</p>

<p>We've just refined the definition a bit more.</p>

<p>After some thought I've edited my original to include a reference to the <a href="https://en.wikipedia.org/wiki/Egyptian_calendar" rel="nofollow noreferrer">Egyptian Calendar</a> which has three seasons ( winter, summer and inundation ), so four seasons doesn't have to be the number.  It is notable that the approximate year length seems to be the same.  I dare say other similar examples are out there and unknown to me.</p>

CC BY-SA 3.0
2017-01-09T19:19:05.477Z
"We defined a year as being four seasons." Reference needed for this. Even [prehistoric societies](https://en.wikipedia.org/wiki/History_of_calendars#Prehistory) tracked the sun. Unless you are defining season purely astronomically, seasons will not always line up with a solar year.
called2voyage
CC BY-SA 3.0
2017-01-10T12:57:02.583Z
Stars are not a good metric of what constitutes a year. Over the course of 71 years, the date on which a star rises at a certain azimuth will shift by over a day due to the difference between a sidereal year and a tropical year. The Sun provides a much better metric.
David Hammen
CC BY-SA 3.0
2017-01-09T18:57:50.010Z
2017-01-09T19:40:01.947Z
2017-01-09T19:40:01.947Z
StephenG - Help Ukraine
<p>I suspect that this question may get closed as too trivial, but to put it simply a year is more or less defined as the time that it takes for the Earth to complete an orbit around the Sun. Because the <a href="https://en.wikipedia.org/wiki/Axial_tilt#Earth" rel="nofollow noreferrer">Earth rotates on an axis that is tilted relative to its orbit around the Sun</a>, rather than perpendicular, the Northern and Southern hemispheres alternate being inclined toward or away from the Sun. The hemisphere that is facing the Sun experiences Summer and the hemisphere that is facing away from the Sun experiences Winter. So a cycle of seasons is roughly equivalent to the time it takes for the Earth to complete its orbit.</p>

<p><a href="https://i.sstatic.net/qbxdb.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/qbxdb.jpg" alt="Earth&#39;s orbit plus axial tilt creates seasons"></a></p>

<p><sup>Image Credit: <a href="http://scijinks.jpl.nasa.gov/review/solstice/seasons-earth-orbit_large.jpg" rel="nofollow noreferrer">NASA</a></sup></p>

CC BY-SA 3.0
2017-01-10T23:34:49.393Z
I understand now. In [link](https://en.wikipedia.org/wiki/Earth's_orbit) there is an image with the key, the earth goes "up and down" respect the sun. The traslation movement makes the seasons itself. No sense with my question then. Thanks.
Nicolás Cerrejón
CC BY-SA 3.0
2017-01-11T12:15:15.350Z
Good image, also note it's summer in south hemisphere while it's winter in north and vice-versa
jean
CC BY-SA 3.0
2017-01-09T16:42:53.523Z
2017-01-09T16:42:53.523Z
called2voyage
<p>Why does the traslation movement of the earth last the same time as the four seasons? The seasons could take 100 days or 200 or 20 years (aprox. like in GoT)... but exactly (exactly???) 1 year. Why?</p>

<p>Edit: I think that I need to redefine the question.<br>
I define a year as the full traslation movement of the earth around the sun.<br>
And the four seasons as the full movement of the perpendicular axis from 0 to 23.5 to 0 again to -23.5 to 0 again.<br>
Why do these two movements take the same time?</p>

CC BY-SA 3.0
2017-01-09T17:22:59.417Z
Have you done any research before asking this question? For example the first line of the wikipedia article on seasons: easons result from the yearly orbit of the Earth around the Sun. In other words, the seasons are caused by the yearly orbit of the sun by the Earth.
James K
CC BY-SA 3.0
2017-01-10T14:21:29.963Z
Nicolas, I saw your edit. I think you still have some misunderstandings. The axial tilt varies, but not that much. It is not the change in tilt that causes the seasons. Earth's axial tilt varies between 22.1 and 24.5 degrees, currently it is about 23.5 degrees. What causes the seasons is that Earth's tilt presents a different angle of Earth toward the Sun depending on its position in orbit, see the diagram in my answer. If you want, I can include this comment in my answer as well.
called2voyage
CC BY-SA 3.0
2017-01-10T15:05:51.940Z
Note that the change to which @called2voyage referred to is quite slow, varying between 22.1 and 24.5 degrees and back over the course of 41000 years.
David Hammen
CC BY-SA 3.0
2017-01-10T15:55:56.500Z
Your new edit makes your title match your previous clarification, but it is still answered by what David and I have posted in answers and comments thus far. If you are confused about something, respond directly to the answer or comment that you have an issue with and let us know how your question has not been addressed.
called2voyage
<earth>
Why does the traslation movement of the earth last the same time as the four seasons?
Nicolás Cerrejón
# Why does the traslation movement of the earth last the same time as the four seasons?

(Asked by: username_0 on 2017-01-09)

Why does the traslation movement of the earth last the same time as the four seasons? The seasons could take 100 days or 200 or 20 years (aprox. like in GoT)... but exactly (exactly???) 1 year. Why?

Edit: I think that I need to redefine the question.  
I define a year as the full traslation movement of the earth around the sun.  
And the four seasons as the full movement of the perpendicular axis from 0 to 23.5 to 0 again to -23.5 to 0 again.  
Why do these two movements take the same time?

**Tags:** earth
> **Comments:**
> **username_1**: Have you done any research before asking this question? For example the first line of the wikipedia article on seasons: easons result from the yearly orbit of the Earth around the Sun. In other words, the seasons are caused by the yearly orbit of the sun by the Earth.  
> **username_2**: Nicolas, I saw your edit. I think you still have some misunderstandings. The axial tilt varies, but not that much. It is not the change in tilt that causes the seasons. Earth's axial tilt varies between 22.1 and 24.5 degrees, currently it is about 23.5 degrees. What causes the seasons is that Earth's tilt presents a different angle of Earth toward the Sun depending on its position in orbit, see the diagram in my answer. If you want, I can include this comment in my answer as well.  
> **username_3**: Note that the change to which @called2voyage referred to is quite slow, varying between 22.1 and 24.5 degrees and back over the course of 41000 years.  
> **username_2**: Your new edit makes your title match your previous clarification, but it is still answered by what David and I have posted in answers and comments thus far. If you are confused about something, respond directly to the answer or comment that you have an issue with and let us know how your question has not been addressed.  

---

## 3 answers

### Answer by username_2 on 2017-01-09. Score: 6 | Accepted answer

I suspect that this question may get closed as too trivial, but to put it simply a year is more or less defined as the time that it takes for the Earth to complete an orbit around the Sun. Because the [Earth rotates on an axis that is tilted relative to its orbit around the Sun](https://en.wikipedia.org/wiki/Axial_tilt#Earth), rather than perpendicular, the Northern and Southern hemispheres alternate being inclined toward or away from the Sun. The hemisphere that is facing the Sun experiences Summer and the hemisphere that is facing away from the Sun experiences Winter. So a cycle of seasons is roughly equivalent to the time it takes for the Earth to complete its orbit.

[![Earth's orbit plus axial tilt creates seasons](https://i.sstatic.net/qbxdb.jpg)](https://i.sstatic.net/qbxdb.jpg)

Image Credit: [NASA](http://scijinks.jpl.nasa.gov/review/solstice/seasons-earth-orbit_large.jpg)

> **Comments:**
> **username_0**: I understand now. In [link](https://en.wikipedia.org/wiki/Earth's_orbit) there is an image with the key, the earth goes "up and down" respect the sun. The traslation movement makes the seasons itself. No sense with my question then. Thanks.  
> **username_5**: Good image, also note it's summer in south hemisphere while it's winter in north and vice-versa  

---

### Answer by username_3 on 2017-01-10. Score: 1

> Why does a year last the same time as the four seasons?

Because that is how a year is defined. Our year is based on the time from one March equinox to the next. The yearly cycle of equinoxes and solstices is in turn driven by the Earth's axial tilt. This axial tilt is what drives the seasons.

Multiple ancient societies independently discovered the relationship between how these cycles and when was the best time to plant and time to harvest. They also discovered that one year is a bit more than 365 days long, and curiously, that the stars do not quite follow the same cycle. Over the course of 70.5 years, the date on which a star rose at a certain location shifted by one day. We base our year on the Sun, not on the stars, and because we base it on the Sun, a year stays in sync with the seasons.

There is an issue here, which is that a year is a bit more than 365 days long. Our civil calendar is based on days rather than years. The Romans addressed this issue by adding a leap day every fourth year. This made a year 365.25 days long, on average. The correct value needed to stay in sync with the seasons is a year that is 365.2421875 days long, on average.

This became a problem (rather than just an issue) during the Renaissance. The discrepancy between 365.25 days and 365.2421875 days had made the seasons fall out of sync with the calendar by 10 days at the time of Pope Gregory XIII. Most importantly (to the Roman Catholic church, that is), properly calculating the date on which Easter fell was in jeopardy. The Julian calendar had 100 leap years for every 400 years. The Gregorian reform made 10 days vanish (or more, depending on when a country adopted the Gregorian calendar), and made only 97 years out of 400 be leap years. This makes the Gregorian year be 365.2425 days long, on average. This is considerably closer to the ideal of 365.2421875 days than is the 365.25 days used in Roman times.


---

### Answer by username_4 on 2017-01-09. Score: 0

> Why does a year last the same time as the four seasons ?

Long before we had any serious science of astronomy, we had seasons. We defined a year as being four seasons. All you need to do that is long term memory.

Once you can count and record events in some way, you start counting days.

As we accumulated knowledge of the stars, we noticed that the changes of positions of the stars in the heavens followed very regular cycles. That gave us a way to relate a year to an exact number of days ( and in fact to a fraction of a day ).

When we developed physics enough to be able to associate the yearly cycle with an orbital cycle we changed our definition of year to be exactly that cycle ( and we even allow for slight changes in that's cycle's length ).

So we invented the approximate year first and then refined it to match our understanding of physics.

> The seasons could take 100 days or 200 or 20 years (aprox. like in GoT)... but exactly (exactly???) 1 year. Why?

Because a year happens to be the period of time that matches the changes in seasons and heavens we first noticed and first gave that name.

We've just refined the definition a bit more.

After some thought I've edited my original to include a reference to the [Egyptian Calendar](https://en.wikipedia.org/wiki/Egyptian_calendar) which has three seasons ( winter, summer and inundation ), so four seasons doesn't have to be the number. It is notable that the approximate year length seems to be the same. I dare say other similar examples are out there and unknown to me.

> **Comments:**
> **username_2**: "We defined a year as being four seasons." Reference needed for this. Even [prehistoric societies](https://en.wikipedia.org/wiki/History_of_calendars#Prehistory) tracked the sun. Unless you are defining season purely astronomically, seasons will not always line up with a solar year.  
> **username_3**: Stars are not a good metric of what constitutes a year. Over the course of 71 years, the date on which a star rises at a certain azimuth will shift by over a day due to the difference between a sidereal year and a tropical year. The Sun provides a much better metric.  

---

