<p>The plane of the Moon's orbit experiences an 18.6 year Nodal Precession, so <a href="https://en.wikipedia.org/wiki/Orbit_of_the_Moon#Inclination_to_the_equator_and_lunar_standstill" rel="nofollow noreferrer">the angle between its orbital plane and the equatorial plane varies</a>. Even if the planes coincided at some time, it would be momentary.</p>
<p><a href="https://en.wikipedia.org/wiki/Orbit_of_the_Moon#Inclination" rel="nofollow noreferrer">Wikipedia</a> discusses the origin of the orbital inclination:</p>
<blockquote>
<p>The mean inclination of the lunar orbit to the ecliptic plane is
5.145°. Theoretical considerations show that the present inclination relative to the ecliptic plane arose by tidal evolution from an
earlier near-Earth orbit with a fairly constant inclination relative
to Earth's equator.[11] It would require an inclination of this
earlier orbit of about 10° to the equator to produce a present
inclination of 5° to the ecliptic. It is thought that originally the
inclination to the equator was near zero, but it could have been
increased to 10° through the influence of planetesimals passing near
the Moon while falling to the Earth.[12] If this had not happened, the
Moon would now lie much closer to the ecliptic and eclipses would be
much more frequent.[13]</p>
</blockquote>

CC BY-SA 4.0
2020-12-06T15:35:10.737Z
It seems that the OP misinterpreted  the diagram linked in the question to indicate that the lunar orbit is inclined at 5.4° to the celestial equator rather than the ecliptic.
PM 2Ring
CC BY-SA 4.0
2020-12-06T05:30:38.127Z
2020-12-06T18:04:27.510Z
2020-12-06T18:04:27.510Z
Keith McClary
<p>Or was it always at about 5.14 degrees inclination or has the inclination changed over time?</p>

<p><a href="https://en.wikipedia.org/wiki/Orbit_of_the_Moon#/media/File:Earth-Moon.PNG" rel="noreferrer">See diagram</a>.</p>

<p>James K's answer to <a href="https://astronomy.stackexchange.com/questions/22830/view-of-moon-from-south-pole">this question</a> got me thinking about this, and I don't mean to call him out, but this part of his answer doesn't seem quite right when he wrote:</p>

<blockquote>
  <p>Our moon is unique in being close to the plane of the ecliptic, and
  not in the plane of the equator, which suggests its formation was not
  like that of other moons in the solar system.</p>
</blockquote>

<p>For starters, I imagine the physics of the initial orbit of an impact moon would be somewhat complicated, but it seems likely, given an angled impact like the one that's believed to have formed our moon (and hitting at an angle is statistically more common than a direct impact anyway), </p>

<p>So, in this scenario, the planet gets significant angular momentum and this angular momentum should dictate the planet's new equator and axial tilt relative to it's orbit.  I would think the moon should form roughly along that same equatorial plane, but I'm just guessing.   Perhaps it could form several degrees off - not sure.</p>

<p>The 2nd point, is, a rapidly rotating planet with a large equatorial bulge and a close moon, if the Moon formed off the equator, would it's orbit migrate over the equator where the gravitation was greatest and would that happen relatively quickly or not at all?</p>

<p>That's basically the question.   Was the Moon always at roughly 6 degrees off the Earth's equator or has it only moved off an orbit over Earth's equator more recently, perhaps due to the gravitational effect of the Sun?    </p>

<p>Or are there other factors.   Mars' axis is thought to have changed rather significantly due to Jupiter's gravitational effects perhaps 100,000 years agao and it's Moons orbit over Mars' equator which suggests that Mars' equatorial bulge dragged the Moons with it? - or is my thinking way off on that?</p>

<p>My thinking is that a planet's equatorial bulge would drive moons towards a 0 degree inclination around it's equatorial bulge and our Moon is different because of proximity to the sun which also has a strong gravitational effect.   The 5.14 degrees of inclination is a balance between the Earth's equator and the solar gravity.</p>

CC BY-SA 4.0
2020-12-06T15:32:00.427Z
Note that the linked diagram shows the angle between the Moon's orbital plane and the *ecliptic*, the Earth's orbital plane, *not* the Earth's equatorial plane. The obliquity of the ecliptic is ~ 23.5° and the Moon's orbital plane inclination to the ecliptic is ~5.4°, so its inclination to the equatorial plane can vary from 18.1° to 28.9°, and it precesses between those limits over an 18.6 year cycle. See https://en.wikipedia.org/wiki/Lunar_precession particularly the diagram near the top of that article, and the section on nodal precession.
PM 2Ring
<the-moon><orbital-mechanics><orbital-migration>
When the Moon formed and early on, did it orbit over the Earth's equator?
userLTK
# When the Moon formed and early on, did it orbit over the Earth's equator?

(Asked by: username_0 on 2017-09-25)

Or was it always at about 5.14 degrees inclination or has the inclination changed over time?

[See diagram](https://en.wikipedia.org/wiki/Orbit_of_the_Moon#/media/File:Earth-Moon.PNG).

James K's answer to [this question](https://astronomy.stackexchange.com/questions/22830/view-of-moon-from-south-pole) got me thinking about this, and I don't mean to call him out, but this part of his answer doesn't seem quite right when he wrote:

> Our moon is unique in being close to the plane of the ecliptic, and
> not in the plane of the equator, which suggests its formation was not
> like that of other moons in the solar system.

For starters, I imagine the physics of the initial orbit of an impact moon would be somewhat complicated, but it seems likely, given an angled impact like the one that's believed to have formed our moon (and hitting at an angle is statistically more common than a direct impact anyway),

So, in this scenario, the planet gets significant angular momentum and this angular momentum should dictate the planet's new equator and axial tilt relative to it's orbit. I would think the moon should form roughly along that same equatorial plane, but I'm just guessing. Perhaps it could form several degrees off - not sure.

The 2nd point, is, a rapidly rotating planet with a large equatorial bulge and a close moon, if the Moon formed off the equator, would it's orbit migrate over the equator where the gravitation was greatest and would that happen relatively quickly or not at all?

That's basically the question. Was the Moon always at roughly 6 degrees off the Earth's equator or has it only moved off an orbit over Earth's equator more recently, perhaps due to the gravitational effect of the Sun?

Or are there other factors. Mars' axis is thought to have changed rather significantly due to Jupiter's gravitational effects perhaps 100,000 years agao and it's Moons orbit over Mars' equator which suggests that Mars' equatorial bulge dragged the Moons with it? - or is my thinking way off on that?

My thinking is that a planet's equatorial bulge would drive moons towards a 0 degree inclination around it's equatorial bulge and our Moon is different because of proximity to the sun which also has a strong gravitational effect. The 5.14 degrees of inclination is a balance between the Earth's equator and the solar gravity.

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

> **Comments:**
> **username_1**: Note that the linked diagram shows the angle between the Moon's orbital plane and the *ecliptic*, the Earth's orbital plane, *not* the Earth's equatorial plane. The obliquity of the ecliptic is ~ 23.5° and the Moon's orbital plane inclination to the ecliptic is ~5.4°, so its inclination to the equatorial plane can vary from 18.1° to 28.9°, and it precesses between those limits over an 18.6 year cycle. See https://en.wikipedia.org/wiki/Lunar_precession particularly the diagram near the top of that article, and the section on nodal precession.  

---

## 1 answers

### Answer by username_2 on 2020-12-06. Score: 1

The plane of the Moon's orbit experiences an 18.6 year Nodal Precession, so [the angle between its orbital plane and the equatorial plane varies](https://en.wikipedia.org/wiki/Orbit_of_the_Moon#Inclination_to_the_equator_and_lunar_standstill). Even if the planes coincided at some time, it would be momentary.

[Wikipedia](https://en.wikipedia.org/wiki/Orbit_of_the_Moon#Inclination) discusses the origin of the orbital inclination:

> The mean inclination of the lunar orbit to the ecliptic plane is
> 5.145°. Theoretical considerations show that the present inclination relative to the ecliptic plane arose by tidal evolution from an
> earlier near-Earth orbit with a fairly constant inclination relative
> to Earth's equator.[11] It would require an inclination of this
> earlier orbit of about 10° to the equator to produce a present
> inclination of 5° to the ecliptic. It is thought that originally the
> inclination to the equator was near zero, but it could have been
> increased to 10° through the influence of planetesimals passing near
> the Moon while falling to the Earth.[12] If this had not happened, the
> Moon would now lie much closer to the ecliptic and eclipses would be
> much more frequent.[13]

> **Comments:**
> **username_1**: It seems that the OP misinterpreted  the diagram linked in the question to indicate that the lunar orbit is inclined at 5.4° to the celestial equator rather than the ecliptic.  

---

