<p>The two coincided about 800 years ago. The December solstice and perihelion date <a href="https://www.timeanddate.com/astronomy/perihelion-aphelion-solstice.html" rel="nofollow noreferrer">coincided in 1246</a>, 773 years ago.</p>

<p>There are many different concepts of what qualifies as a <a href="https://en.wikipedia.org/wiki/Year#Astronomical_years" rel="nofollow noreferrer">"year"</a>. Three of them are the sidereal, tropical, and anomalistic years. The sidereal year measures how long it takes for the Earth to complete one orbit about the Sun with respect to the remote stars.</p>

<p>The tropical year measures how long it takes for the Earth to pass from one March equinox to the next. The tropical year is about 20 minutes shorter than the sidereal year due to the slow changes in the Earth's rotation axis caused by the Moon and the Sun.</p>

<p>The anomalistic year measures how long it takes for the Earth to pass from one perihelion to the next (or one aphelion to the next). The anomalistic year is about 7.8 minutes longer than the sidereal year due to gravitational perturbations from the other planets, with Jupiter and Venus dominating over all the others. The tropical year is thus shorter than the anomalistic year by about 28 minutes. That the tropical year and anomalistic year have slightly different frequencies means that there is a very slow <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/beat.html" rel="nofollow noreferrer">beat frequency</a> with a beat period of about 26000 years.</p>

<p>Human civilization arose about half of this period ago (about 12000 years ago) when the last glaciation ("ice age") ended. At that time, the anomalistic and tropical years were almost 180 degrees out of phase with respect to where they are now. The end of a glaciation typically occurs when northern hemisphere summers coincide with perihelion. That makes summers in far northern latitudes short in duration but rather hot.</p>

<p>One of the key proxies regarding whether glaciations start or end is the mean July temperature at far northern latitudes. When perihelion occurs a few weeks after the June solstice, the short but hot summers that result makes snow from the previous winter melt rather than build up. The melting that results from such short but hot summers triggers a positive feedback that makes the end of a glaciation very rapid, in a geological context of "rapid".</p>

<p><br>
To clarify things,</p>

<blockquote>
  <p>So, there is no particular reason in orbital mechanics; it just happens that we live in an era when they are close together?</p>
</blockquote>

<p>Exactly.</p>

<p>Or maybe not. The end of the last glaciation, which occurred about 12000 years ago, massively spurred human development. The cycle then was almost exactly opposite what it is now. It took about 10000 years for humanity to develop to a somewhat advanced level (e.g., Roman Empire level), and then only another couple of thousand to develop to the stage where we are now. Whether humanity can survive for another 13000 years without wiping itself out will determine whether this was happenstance.</p>

CC BY-SA 4.0
2019-06-29T18:13:23.847Z
Nice answer (as usual). FWIW, due to the precession of the equinox, there are a few options for the length of the tropical year, depending on which equinox (or solstice) you start the year on. The usual figure quoted is for the mean tropical year. Astronomer Duncan Steel argues that the Gregorian calendar was adopted to stabilize the date of Easter, so the length of the Gregorian year ought to approximate the March equinox tropical year, not the mean tropical year.
PM 2Ring
CC BY-SA 4.0
2019-06-29T20:20:22.053Z
Thanks.  Very interesting but not quite what I was after.  I am interested in why the solstice and aphelion are close but not what effect that have on the weather.  Nor am I asking about the length or definition of the year.
badjohn
CC BY-SA 4.0
2019-06-29T20:33:27.940Z
@badjohn - They're close now because the solstices and perihelion/aphelion happened to be coincide the about 800 years ago. The very small difference in the length of the tropical and anomalistic year means that they remain fairly close to this day. That will not be the case 10000 years from now.
David Hammen
CC BY-SA 4.0
2019-06-29T20:38:16.560Z
So, there is no particular reason in orbital mechanics; it just happens that we live in an era when they are close together?
badjohn
CC BY-SA 4.0
2019-06-29T21:57:35.523Z
@badjohn - Exactly.
David Hammen
CC BY-SA 4.0
2019-06-29T22:32:22.537Z
Thanks again.  If you add that to the answer, I'll accept it.
badjohn
CC BY-SA 4.0
2019-07-04T08:29:20.067Z
An interesting final point.  The anhropic principle: why does the universe happen to suit us?  Well, if it didn't then we wouldn't be here saying that it didn't.
badjohn
CC BY-SA 4.0
2021-12-30T22:32:23.130Z
Maybe it's a reading comprehension problem, but this doesn't seem to answer the question, UNLESS I don't understand what a solstice is.  If a solstice is when the sun is the most south or most north in the sky, based on the Earth's tilt toward/away from the sun, wouldn't that occur the same day as aphelion or perihelion?
scott.korin
CC BY-SA 4.0
2021-12-31T11:05:33.517Z
@scott.korin I don't know why you would that that is the case. The December solstice and Earth aphelion are currently separated by about two weeks, and the separation between the two is increasing. That's the point of the answer.
David Hammen
CC BY-SA 4.0
2022-01-13T17:40:11.927Z
@DavidHammen I think it's a basic misunderstanding I have about alignment of the Earth's axis.  Simple orbital models in school that I was shown made it appear that the earth's tilt toward/away from the sun was the most extreme on the solstices but that's just not the case.
scott.korin
CC BY-SA 4.0
2019-06-29T17:19:57.513Z
2019-06-30T20:40:49.040Z
2019-06-30T20:40:49.040Z
David Hammen
<p>We have just passed one of the solstices and are approaching aphelion.  The two events are close but not simultaneous.  There is no very obvious (to me) reason why the two should coincide or be close.  Is there a less obvious reason why they are close?  If so, is the gap closing, growing, or remaining approximately constant?</p>

<orbit><solstice>
Aphelion and the solstice
badjohn
# Aphelion and the solstice

(Asked by: username_0 on 2019-06-29)

We have just passed one of the solstices and are approaching aphelion. The two events are close but not simultaneous. There is no very obvious (to me) reason why the two should coincide or be close. Is there a less obvious reason why they are close? If so, is the gap closing, growing, or remaining approximately constant?

**Tags:** orbit, solstice

---

## 1 answers

### Answer by username_1 on 2019-06-29. Score: 5 | Accepted answer

The two coincided about 800 years ago. The December solstice and perihelion date [coincided in 1246](https://www.timeanddate.com/astronomy/perihelion-aphelion-solstice.html), 773 years ago.

There are many different concepts of what qualifies as a ["year"](https://en.wikipedia.org/wiki/Year#Astronomical_years). Three of them are the sidereal, tropical, and anomalistic years. The sidereal year measures how long it takes for the Earth to complete one orbit about the Sun with respect to the remote stars.

The tropical year measures how long it takes for the Earth to pass from one March equinox to the next. The tropical year is about 20 minutes shorter than the sidereal year due to the slow changes in the Earth's rotation axis caused by the Moon and the Sun.

The anomalistic year measures how long it takes for the Earth to pass from one perihelion to the next (or one aphelion to the next). The anomalistic year is about 7.8 minutes longer than the sidereal year due to gravitational perturbations from the other planets, with Jupiter and Venus dominating over all the others. The tropical year is thus shorter than the anomalistic year by about 28 minutes. That the tropical year and anomalistic year have slightly different frequencies means that there is a very slow [beat frequency](http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/beat.html) with a beat period of about 26000 years.

Human civilization arose about half of this period ago (about 12000 years ago) when the last glaciation ("ice age") ended. At that time, the anomalistic and tropical years were almost 180 degrees out of phase with respect to where they are now. The end of a glaciation typically occurs when northern hemisphere summers coincide with perihelion. That makes summers in far northern latitudes short in duration but rather hot.

One of the key proxies regarding whether glaciations start or end is the mean July temperature at far northern latitudes. When perihelion occurs a few weeks after the June solstice, the short but hot summers that result makes snow from the previous winter melt rather than build up. The melting that results from such short but hot summers triggers a positive feedback that makes the end of a glaciation very rapid, in a geological context of "rapid".

To clarify things,

> So, there is no particular reason in orbital mechanics; it just happens that we live in an era when they are close together?

Exactly.

Or maybe not. The end of the last glaciation, which occurred about 12000 years ago, massively spurred human development. The cycle then was almost exactly opposite what it is now. It took about 10000 years for humanity to develop to a somewhat advanced level (e.g., Roman Empire level), and then only another couple of thousand to develop to the stage where we are now. Whether humanity can survive for another 13000 years without wiping itself out will determine whether this was happenstance.

> **Comments:**
> **username_2**: Nice answer (as usual). FWIW, due to the precession of the equinox, there are a few options for the length of the tropical year, depending on which equinox (or solstice) you start the year on. The usual figure quoted is for the mean tropical year. Astronomer Duncan Steel argues that the Gregorian calendar was adopted to stabilize the date of Easter, so the length of the Gregorian year ought to approximate the March equinox tropical year, not the mean tropical year.  
> **username_0**: Thanks.  Very interesting but not quite what I was after.  I am interested in why the solstice and aphelion are close but not what effect that have on the weather.  Nor am I asking about the length or definition of the year.  
> **username_1**: @badjohn - They're close now because the solstices and perihelion/aphelion happened to be coincide the about 800 years ago. The very small difference in the length of the tropical and anomalistic year means that they remain fairly close to this day. That will not be the case 10000 years from now.  
> **username_0**: So, there is no particular reason in orbital mechanics; it just happens that we live in an era when they are close together?  
> **username_1**: @badjohn - Exactly.  
> **username_0**: Thanks again.  If you add that to the answer, I'll accept it.  
> **username_0**: An interesting final point.  The anhropic principle: why does the universe happen to suit us?  Well, if it didn't then we wouldn't be here saying that it didn't.  
> **username_3**: Maybe it's a reading comprehension problem, but this doesn't seem to answer the question, UNLESS I don't understand what a solstice is.  If a solstice is when the sun is the most south or most north in the sky, based on the Earth's tilt toward/away from the sun, wouldn't that occur the same day as aphelion or perihelion?  
> **username_1**: @scott.korin I don't know why you would that that is the case. The December solstice and Earth aphelion are currently separated by about two weeks, and the separation between the two is increasing. That's the point of the answer.  
> **username_3**: @DavidHammen I think it's a basic misunderstanding I have about alignment of the Earth's axis.  Simple orbital models in school that I was shown made it appear that the earth's tilt toward/away from the sun was the most extreme on the solstices but that's just not the case.  

---

