<p>The moon is so big that the processes that circularize and reduce the equatorial inclination would take much longer. The moon is big because of how it formed: a huge collision in the early solar system. (Unlike, say the Galilean moons that probably formed along with Jupiter, or Triton, that looks like a captured TNO)</p>

<p>The other fact that makes its orbit complex is that the moon is the closest moon to the sun, so solar perturbation is more significant than for other moons.  Solar perturbation is the main cause of the periodic variation in inclination and the various other forms of precession seen in the moon's orbit.</p>

<p>The sun also pulls the moon out of a equatorial orbit and into a roughly ecliptic orbit as the perturbations of the sun become more significant than those of the equatorial bulge. As noted <a href="https://astronomy.stackexchange.com/a/35407/16685">elsewhere</a>  at lunar distance, the sun provides more than 50% of the gravitational field.</p>

<p>So the reason the moon's orbit is complicated is that it does follow the laws of physics, and with three (or more) bodies, those laws have complex effects.</p>

CC BY-SA 4.0
2020-06-13T04:11:52.697Z
@PM2Ring Surely that doesn't matter because the Moon's orbit around the Sun is almost the same as the Earth's orbit around the Sun. What matters is the difference in gravity when the Moon is on the Sun side of the Earth and when it's opposite the Sun side. That difference would be much smaller than the effect of the Earth's gravity.
CJ Dennis
CC BY-SA 4.0
2020-06-13T05:15:09.120Z
@PM2Ring "the Sun pulls roughly twice as strongly". Since the Sun is "pulling" the Earth as well as the Moon, this statement has less relevance than it appears.
CJ Dennis
CC BY-SA 4.0
2020-06-13T06:25:24.863Z
@PM2Ring Yes, that is what I was getting at.
CJ Dennis
CC BY-SA 4.0
2020-06-13T07:16:39.600Z
Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/109287/discussion-between-pm-2ring-and-cj-dennis).
PM 2Ring
CC BY-SA 4.0
2020-06-12T08:40:02.810Z
2020-06-13T06:23:45.647Z
2020-06-13T06:23:45.647Z
James K
<p>In addition to what @JamesK said, I would like to point out that the statement that the moon's inclination &quot;somehow changes over time from 18° to 28°&quot; is rather misleading.</p>
<p>Even if the moon orbited the Earth in a perfect circle, in exactly the same plane as the Earth orbits the Sun (known as the <a href="https://en.wikipedia.org/wiki/Ecliptic" rel="nofollow noreferrer">ecliptic plane</a>), you would seem to see the moon sometimes traveling north of the equator and sometimes south, from the Earth's perspective, just as the noontime <a href="https://en.wikipedia.org/wiki/Sun_path" rel="nofollow noreferrer">Sun seems to move north and south from day to day</a> over the course of a year, for the simple reason that the Earth's north pole does not point perfectly perpendicular to the plane in which the Earth orbits the sun. As the spinning Earth travels around the sun, its axis of spin keeps pointing in essentially the same direction (roughly towards Polaris, the &quot;North Star&quot;, at least for the next few thousand years or so until <a href="https://en.wikipedia.org/wiki/Axial_precession" rel="nofollow noreferrer">precession</a> gradually moves it), while the Sun's (and solar system's) axis points in a different direction, with the result that at opposite points in its orbit, the Earth's axis effectively tilts about 23.5 degrees towards the sun's axis (at the summer solstice) and similarly 23.5 degrees away from it (at the winter solstice).</p>
<p>The plane of the Moon's orbit around the Earth is only tilted about 5 degrees or so from the ecliptic plane. The pair of points at which the Moon's orbit crosses the ecliptic plane (the <a href="https://en.wikipedia.org/wiki/Lunar_node" rel="nofollow noreferrer">orbital nodes</a>) slowly rotate around the earth every 18.6 years or so. (When one of the nodes passes between the Earth and the Sun and the moon passes through either of the nodes, then you get a <a href="https://en.wikipedia.org/wiki/Eclipse" rel="nofollow noreferrer">solar or lunar eclipse</a>.) The Moon will be at maximum angular deviation from the ecliptic plane at times when it is at a one of the points in its orbit exactly between its two nodes. At such times, the Moon's inclination with respect to the ecliptic plane will be at a maximum: either +5 or -5 degrees.</p>
<p>The Earth's axial tilt (and hence also the tilt of the equator) with respect to the ecliptic plane will be a fairly constant 23.5 degrees, but the direction it faces with respect to the moon will vary over time, causing the effective tilt to vary between ± 23.5 degrees.</p>
<p>Therefore, at times when the moon is inline with the earth axial tilt, and the Moon is midway between its two nodes, the total angular deviation between the equator and the moon will be one of the values calculable from ± 23.5 ± 5 degrees.</p>
<p>Another way to look at it is to consider the Earth's equatorial tilt (with respect to the ecliptic plane) in the direction of the moon to be a sine wave ranging from ± 23.5 degrees, and the moon's tilt (also with respect to the ecliptic plane) in the direction of the Earth to be a different sine wave (at a different frequency!) ranging from ± 5 degrees. The angle between the equator and the Moon is then the sum of these two waves.</p>

CC BY-SA 4.0
2020-06-17T10:03:24.407Z
Another Wikipedia article related to this topic is titled [Lunar standstill](https://en.wikipedia.org/wiki/Lunar_standstill).
PM 2Ring
CC BY-SA 4.0
2020-06-13T11:21:47.843Z
2022-02-26T09:32:22.717Z
2022-02-26T09:32:22.717Z
Some Guy
<p>I like to think I have a good enough understanding of astrophysics but there's still one thing that I just can't explain. </p>

<p>Why does the Moon orbit the Earth the way it does?</p>

<p>Many other moons in the Solar System orbit their planet along the equator and have a nearly circular orbit. After researching, it seems to boil down to two main causes:</p>

<ul>
<li><p>Since every planet rotates on its axis, it has an equatorial bulge, and that extra mass around its equator creates an unequal gravitational field that over millions of years aligns the satellite's orbit around it.</p></li>
<li><p>There's a phenomenon called tidal circularization, which on long timescales, acts to dampen the satellite's eccentricity and make its orbit nearly circular. </p></li>
</ul>

<p>Okay, that makes sense for most moons. But our own Moon has an orbit that is notably eccentric and it's also inclined to Earth's equator by an amount that somehow changes over time from 18° to 28°. Why is the Moon's orbit so complicated?</p>

CC BY-SA 4.0
2020-06-12T08:30:52.630Z
I don't have enough confidence or detail to make this an answer, but I think the main things are that the Moon is big compared to the Earth, far from the Earth compared to the size of the Earth and the system as a whole is close to the Sun.
Steve Linton
CC BY-SA 4.0
2020-06-12T08:45:48.000Z
As I've said before, [Lunar theory](https://en.wikipedia.org/wiki/Lunar_theory) is complicated. ;) Regarding the nodal precession, it helps to think of the Moon's orbit being slightly inclined to the ecliptic and perturbed by the Earth's equatorial bulge. I suppose I should put that in a proper answer... Ah, James beat me to it.
PM 2Ring
CC BY-SA 4.0
2020-06-13T04:42:44.063Z
Also related: https://astronomy.stackexchange.com/q/10946/16685
PM 2Ring
<the-moon><astrophysics><orbital-mechanics>
Why is the Moon's orbit so complicated?
ChristieToWin
# Why is the Moon's orbit so complicated?

(Asked by: username_0 on 2020-06-12)

I like to think I have a good enough understanding of astrophysics but there's still one thing that I just can't explain.

Why does the Moon orbit the Earth the way it does?

Many other moons in the Solar System orbit their planet along the equator and have a nearly circular orbit. After researching, it seems to boil down to two main causes:

* Since every planet rotates on its axis, it has an equatorial bulge, and that extra mass around its equator creates an unequal gravitational field that over millions of years aligns the satellite's orbit around it.
* There's a phenomenon called tidal circularization, which on long timescales, acts to dampen the satellite's eccentricity and make its orbit nearly circular.

Okay, that makes sense for most moons. But our own Moon has an orbit that is notably eccentric and it's also inclined to Earth's equator by an amount that somehow changes over time from 18° to 28°. Why is the Moon's orbit so complicated?

**Tags:** the-moon, astrophysics, orbital-mechanics

> **Comments:**
> **username_1**: I don't have enough confidence or detail to make this an answer, but I think the main things are that the Moon is big compared to the Earth, far from the Earth compared to the size of the Earth and the system as a whole is close to the Sun.  
> **username_2**: As I've said before, [Lunar theory](https://en.wikipedia.org/wiki/Lunar_theory) is complicated. ;) Regarding the nodal precession, it helps to think of the Moon's orbit being slightly inclined to the ecliptic and perturbed by the Earth's equatorial bulge. I suppose I should put that in a proper answer... Ah, username_3 beat me to it.  
> **username_2**: Also related: https://astronomy.stackexchange.com/q/10946/16685  

---

## 2 answers

### Answer by username_3 on 2020-06-12. Score: 35

The moon is so big that the processes that circularize and reduce the equatorial inclination would take much longer. The moon is big because of how it formed: a huge collision in the early solar system. (Unlike, say the Galilean moons that probably formed along with Jupiter, or Triton, that looks like a captured TNO)

The other fact that makes its orbit complex is that the moon is the closest moon to the sun, so solar perturbation is more significant than for other moons. Solar perturbation is the main cause of the periodic variation in inclination and the various other forms of precession seen in the moon's orbit.

The sun also pulls the moon out of a equatorial orbit and into a roughly ecliptic orbit as the perturbations of the sun become more significant than those of the equatorial bulge. As noted [elsewhere](https://astronomy.stackexchange.com/a/35407/16685) at lunar distance, the sun provides more than 50% of the gravitational field.

So the reason the moon's orbit is complicated is that it does follow the laws of physics, and with three (or more) bodies, those laws have complex effects.

> **Comments:**
> **username_4**: @username_2 Surely that doesn't matter because the Moon's orbit around the Sun is almost the same as the Earth's orbit around the Sun. What matters is the difference in gravity when the Moon is on the Sun side of the Earth and when it's opposite the Sun side. That difference would be much smaller than the effect of the Earth's gravity.  
> **username_4**: @username_2 "the Sun pulls roughly twice as strongly". Since the Sun is "pulling" the Earth as well as the Moon, this statement has less relevance than it appears.  
> **username_4**: @username_2 Yes, that is what I was getting at.  
> **username_2**: Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/109287/discussion-between-username_2-and-username_4).  

---

### Answer by username_5 on 2020-06-13. Score: 14

In addition to what @username_3 said, I would like to point out that the statement that the moon's inclination "somehow changes over time from 18° to 28°" is rather misleading.

Even if the moon orbited the Earth in a perfect circle, in exactly the same plane as the Earth orbits the Sun (known as the [ecliptic plane](https://en.wikipedia.org/wiki/Ecliptic)), you would seem to see the moon sometimes traveling north of the equator and sometimes south, from the Earth's perspective, just as the noontime [Sun seems to move north and south from day to day](https://en.wikipedia.org/wiki/Sun_path) over the course of a year, for the simple reason that the Earth's north pole does not point perfectly perpendicular to the plane in which the Earth orbits the sun. As the spinning Earth travels around the sun, its axis of spin keeps pointing in essentially the same direction (roughly towards Polaris, the "North Star", at least for the next few thousand years or so until [precession](https://en.wikipedia.org/wiki/Axial_precession) gradually moves it), while the Sun's (and solar system's) axis points in a different direction, with the result that at opposite points in its orbit, the Earth's axis effectively tilts about 23.5 degrees towards the sun's axis (at the summer solstice) and similarly 23.5 degrees away from it (at the winter solstice).

The plane of the Moon's orbit around the Earth is only tilted about 5 degrees or so from the ecliptic plane. The pair of points at which the Moon's orbit crosses the ecliptic plane (the [orbital nodes](https://en.wikipedia.org/wiki/Lunar_node)) slowly rotate around the earth every 18.6 years or so. (When one of the nodes passes between the Earth and the Sun and the moon passes through either of the nodes, then you get a [solar or lunar eclipse](https://en.wikipedia.org/wiki/Eclipse).) The Moon will be at maximum angular deviation from the ecliptic plane at times when it is at a one of the points in its orbit exactly between its two nodes. At such times, the Moon's inclination with respect to the ecliptic plane will be at a maximum: either +5 or -5 degrees.

The Earth's axial tilt (and hence also the tilt of the equator) with respect to the ecliptic plane will be a fairly constant 23.5 degrees, but the direction it faces with respect to the moon will vary over time, causing the effective tilt to vary between ± 23.5 degrees.

Therefore, at times when the moon is inline with the earth axial tilt, and the Moon is midway between its two nodes, the total angular deviation between the equator and the moon will be one of the values calculable from ± 23.5 ± 5 degrees.

Another way to look at it is to consider the Earth's equatorial tilt (with respect to the ecliptic plane) in the direction of the moon to be a sine wave ranging from ± 23.5 degrees, and the moon's tilt (also with respect to the ecliptic plane) in the direction of the Earth to be a different sine wave (at a different frequency!) ranging from ± 5 degrees. The angle between the equator and the Moon is then the sum of these two waves.

> **Comments:**
> **username_2**: Another Wikipedia article related to this topic is titled [Lunar standstill](https://en.wikipedia.org/wiki/Lunar_standstill).  

---

