<p>Indeed, the moon's orbital plane is inclined with respect to the earth's equatorial plane, and this inclination changes over time: the angle between both planes varies between 28°36′ and 18°20' with a period of approximately <strong>18.6 years</strong>.</p>
<p>This is the combination of the 23°27′ inclination of Earth's equatorial plane with respect to the ecliptic (precession period ~26000 years), and the 5°09' inclination of the Moon's orbit with respect to the ecliptic (precession period ~18.6 years).</p>
<p>Since the precession period of Earth's equatorial plane can be considered negligible compared to that of the Moon's orbital plane, we can just say that the angle between the Moon's orbital plane and Earth's equatorial plane changes with a period of approximately 18.6 years.</p>
<p>You can find more find more information on this topic in Wikipedia's <a href="https://en.wikipedia.org/wiki/Orbit_of_the_Moon#Inclination_to_the_equator_and_lunar_standstill" rel="nofollow noreferrer">Orbit of the Moon; Inclination to the equator and lunar standstill</a>.</p>
<hr />
<p><a href="https://i.sstatic.net/DdYJ8.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/DdYJ8.png" alt=":Lunar Orbit and Orientation with respect to the Ecliptic" /></a></p>
<p><strong>Above:</strong> Lunar orbit and orientation with respect to the Ecliptic, showing the values of all relevant angles. (<a href="https://commons.wikimedia.org/wiki/File:Lunar_Orbit_and_Orientation_with_respect_to_the_Ecliptic.svg" rel="nofollow noreferrer">source</a>)</p>
<p><strong>Below:</strong> The two extremes, referred to as major and minor <a href="https://en.wikipedia.org/wiki/Lunar_standstill" rel="nofollow noreferrer">Lunar standstill</a>. (<a href="https://commons.wikimedia.org/wiki/File:Lunar_standstill.GIF" rel="nofollow noreferrer">source</a>)</p>
<p><a href="https://i.sstatic.net/8ZhQ8.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/8ZhQ8.gif" alt="Lunar_standstill" /></a></p>

CC BY-SA 4.0
2022-09-13T22:39:03.557Z
Excellent answer, +1. I already had these drawings in my now-deleted answer so I just copy/pasted them here for the benefit of future readers. Feel free to edit further.
uhoh
CC BY-SA 4.0
2022-09-14T07:31:03.463Z
@OrOrg: Thanks for the excellent answer. Brings about a lot of clarity. One supplementary question if you don't mind. I understand (may be I am wrong) that the earth's axis of rotation (which is in itself inclined - or "obliquity" by 23.44 degrees), is also not stable, but "VOBBLING" around an imaginary vertical axis. If this is true, earth's equatorial plane will also change its position. In such a case, what happens to the angle of Moon's plane of orbit? Does it remain in sync with equatorial plane, or is it independent?
Niranjan
CC BY-SA 4.0
2022-09-14T07:53:23.627Z
@Niranjan Everything you say is correct. The "wobbling" (called 'precession') of the Earth's axis of rotation is the 26000 year cycle. The Moon's plane of orbit also precesses around this same imaginary vertical axis, which is the 18.6 years cycle. However, the Moon's plane of orbit and the Earth's equatorial plane are (at least to very good approximation) independent of each other.
OrOrg
CC BY-SA 4.0
2022-09-14T22:59:23.313Z
@Niranjan See https://www.iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html for some info on the irregularities of the Earth's motion.
PM 2Ring
CC BY-SA 4.0
2022-09-15T09:48:20.727Z
@OrOrg: Thanks a lot.
Niranjan
CC BY-SA 4.0
2022-09-15T09:55:35.420Z
@PM 2Ring: Sure I will visit the link. Thanks for providing the link. Such info. is exactly what I was looking for. While Google throws up many pages based on the 'CUE WORD", many times the correct cue word itself is an "UNKNOWN"... If I do not know where to look for the information, (such as iers.org, in this case) it is difficult to do some homework. Once the basics are known, specific queries carry some weight. Thanks again.
Niranjan
CC BY-SA 4.0
2022-09-13T15:37:04.553Z
2022-09-14T08:07:05.867Z
2022-09-14T08:07:05.867Z
OrOrg
<p>Is the plane in which Moon orbits around earth, inclined with respect to earth's equatorial plane? If yes, is it constant or does it change with time? If it changes, how much time does it take to make a complete circle around earth?</p>

CC BY-SA 4.0
2022-09-12T12:11:13.033Z
https://en.wikipedia.org/wiki/Orbit_of_the_Moon
planetmaker
CC BY-SA 4.0
2022-09-12T12:20:47.400Z
I’m voting to close this question because there is no evidence that OP has done any prior research for this question. The Wikipedia article has clearly explained the topic.
Nilay Ghosh
CC BY-SA 4.0
2022-09-12T13:13:14.507Z
Please read https://astronomy.stackexchange.com/help/how-to-ask - we can expect that you tell us about your own endeavours to find an answer and why or how, for instance, the wiki article does NOT answer it.
planetmaker
CC BY-SA 4.0
2022-09-12T22:08:12.990Z
**voting to leave open** because lack of prior research is not a close reason. [The new and increasingly popular "close for insufficient research" close reason, are we headed in the wrong direction here? Is there a way to reduce?](https://astronomy.meta.stackexchange.com/q/876/7982)
uhoh
CC BY-SA 4.0
2022-09-12T22:33:15.917Z
The diagrams in the answers here may be helpful: https://astronomy.stackexchange.com/q/47502/16685
PM 2Ring
CC BY-SA 4.0
2022-09-13T05:47:24.293Z
It's important to note that the Earth's equatorial plane changes due to precession and nutation.  So the moon's orbit would appear to change, relative to the equator, just due to that.
Greg Miller
CC BY-SA 4.0
2022-09-13T07:56:46.573Z
@Nilay Ghosh: A knowledgeable person tends to guide first, by way of sharing his knowledge and sharing information about the source of the same and then advice about doing homework. Empty drums make a lot of noise.
Niranjan
CC BY-SA 4.0
2022-09-13T08:00:45.510Z
@Greg Miller: Does that mean that the Moon's orbital plane is inclined w.r.t earth's equatorial plane - at whatever angle be it, but that angle is constant, and appears to change only because it follows earth's equatorial plane?
Niranjan
CC BY-SA 4.0
2022-09-13T08:09:42.343Z
@uhoh: Thanks for your support. The purpose of asking a question is to get answers quickly and to the point. (Though it may sound selfish "shortcut"). Doing research is certainly helpful, but it requires lot of time, and perhaps some amount of interpretation. My experience  is that sometimes we have to read 1000 pages (for example), to find an indirect answer in 23rd line on page no. 783 of the thousand pages. JUST FYI ... Apollo 11 crew did depend on knowledge gained by earlier space flight crew. They did not repeat the tasks done earlier. Thanks.
Niranjan
CC BY-SA 4.0
2022-09-13T11:34:20.553Z
@Niranjan that might be your reason for asking a question, it certainly isn’t the function of SO to act as  help desk for people who don’t fancy doing their own research. I don’t think your question should be closed, but it’s perfectly valid to downvote a question which shows a lack of effort.
user438383
CC BY-SA 4.0
2022-09-13T22:38:17.493Z
@Niranjan I understand your reasoning. But as you know (I see you're quite an active SE user in multiple sites) Stack Exchange is different than other Q&A sites and there is an expectation that some effort is demonstrated. It probably makes things worse saying "it may sound selfish 'shortcut'"! Adding evidence of prior research can be as little as a link to a relevant Wikipedia article and short explanation why it isn't sufficiently helpful to you "...but what is still leaving me wondering is..." for example. Forget the "... indirect answer in 23rd line on page no. 783 of the thousand pages.."
uhoh
CC BY-SA 4.0
2022-09-14T07:41:26.373Z
@Uhoh: I accept your response.  I agree this time I did not do any homework before asking the question. Without any intention to offend, I think, if sufficient amount of Net surfing/homework on Wiki etc is done, one would get the answer anyway, because after all you or anyone who knows the answer has sourced it from some article / paper / site etc. unless its his own experience (such as that in case of astronauts). In such a case, there is no need to "ASK" a question on this forum. Despite this, I do agree that I should have tried to get some knowledge beforehand. Sorry for that. Will improve.
Niranjan
<the-moon><orbital-elements>
Help me better understand how the plane of Moon's orbit around Earth behaves
Niranjan
# Help me better understand how the plane of Moon's orbit around Earth behaves

(Asked by: username_0 on 2022-09-12)

Is the plane in which Moon orbits around earth, inclined with respect to earth's equatorial plane? If yes, is it constant or does it change with time? If it changes, how much time does it take to make a complete circle around earth?

**Tags:** the-moon, orbital-elements

> **Comments:**
> **username_1**: https://en.wikipedia.org/wiki/Orbit_of_the_Moon  
> **username_2**: I’m voting to close this question because there is no evidence that OP has done any prior research for this question. The Wikipedia article has clearly explained the topic.  
> **username_1**: Please read https://astronomy.stackexchange.com/help/how-to-ask - we can expect that you tell us about your own endeavours to find an answer and why or how, for instance, the wiki article does NOT answer it.  
> **username_3**: **voting to leave open** because lack of prior research is not a close reason. [The new and increasingly popular "close for insufficient research" close reason, are we headed in the wrong direction here? Is there a way to reduce?](https://astronomy.meta.stackexchange.com/q/876/7982)  
> **username_4**: The diagrams in the answers here may be helpful: https://astronomy.stackexchange.com/q/47502/16685  
> **username_5**: It's important to note that the Earth's equatorial plane changes due to precession and nutation.  So the moon's orbit would appear to change, relative to the equator, just due to that.  
> **username_0**: @username_2: A knowledgeable person tends to guide first, by way of sharing his knowledge and sharing information about the source of the same and then advice about doing homework. Empty drums make a lot of noise.  
> **username_0**: @username_5: Does that mean that the Moon's orbital plane is inclined w.r.t earth's equatorial plane - at whatever angle be it, but that angle is constant, and appears to change only because it follows earth's equatorial plane?  
> **username_0**: @username_3: Thanks for your support. The purpose of asking a question is to get answers quickly and to the point. (Though it may sound selfish "shortcut"). Doing research is certainly helpful, but it requires lot of time, and perhaps some amount of interpretation. My experience  is that sometimes we have to read 1000 pages (for example), to find an indirect answer in 23rd line on page no. 783 of the thousand pages. JUST FYI ... Apollo 11 crew did depend on knowledge gained by earlier space flight crew. They did not repeat the tasks done earlier. Thanks.  
> **username_6**: @username_0 that might be your reason for asking a question, it certainly isn’t the function of SO to act as  help desk for people who don’t fancy doing their own research. I don’t think your question should be closed, but it’s perfectly valid to downvote a question which shows a lack of effort.  
> **username_3**: @username_0 I understand your reasoning. But as you know (I see you're quite an active SE user in multiple sites) Stack Exchange is different than other Q&A sites and there is an expectation that some effort is demonstrated. It probably makes things worse saying "it may sound selfish 'shortcut'"! Adding evidence of prior research can be as little as a link to a relevant Wikipedia article and short explanation why it isn't sufficiently helpful to you "...but what is still leaving me wondering is..." for example. Forget the "... indirect answer in 23rd line on page no. 783 of the thousand pages.."  
> **username_0**: @username_3: I accept your response.  I agree this time I did not do any homework before asking the question. Without any intention to offend, I think, if sufficient amount of Net surfing/homework on Wiki etc is done, one would get the answer anyway, because after all you or anyone who knows the answer has sourced it from some article / paper / site etc. unless its his own experience (such as that in case of astronauts). In such a case, there is no need to "ASK" a question on this forum. Despite this, I do agree that I should have tried to get some knowledge beforehand. Sorry for that. Will improve.  

---

## 1 answers

### Answer by username_7 on 2022-09-13. Score: 4 | Accepted answer

Indeed, the moon's orbital plane is inclined with respect to the earth's equatorial plane, and this inclination changes over time: the angle between both planes varies between 28°36′ and 18°20' with a period of approximately **18.6 years**.

This is the combination of the 23°27′ inclination of Earth's equatorial plane with respect to the ecliptic (precession period ~26000 years), and the 5°09' inclination of the Moon's orbit with respect to the ecliptic (precession period ~18.6 years).

Since the precession period of Earth's equatorial plane can be considered negligible compared to that of the Moon's orbital plane, we can just say that the angle between the Moon's orbital plane and Earth's equatorial plane changes with a period of approximately 18.6 years.

You can find more find more information on this topic in Wikipedia's [Orbit of the Moon; Inclination to the equator and lunar standstill](https://en.wikipedia.org/wiki/Orbit_of_the_Moon#Inclination_to_the_equator_and_lunar_standstill).

---

[![:Lunar Orbit and Orientation with respect to the Ecliptic](https://i.sstatic.net/DdYJ8.png)](https://i.sstatic.net/DdYJ8.png)

**Above:** Lunar orbit and orientation with respect to the Ecliptic, showing the values of all relevant angles. ([source](https://commons.wikimedia.org/wiki/File:Lunar_Orbit_and_Orientation_with_respect_to_the_Ecliptic.svg))

**Below:** The two extremes, referred to as major and minor [Lunar standstill](https://en.wikipedia.org/wiki/Lunar_standstill). ([source](https://commons.wikimedia.org/wiki/File:Lunar_standstill.GIF))

[![Lunar_standstill](https://i.sstatic.net/8ZhQ8.gif)](https://i.sstatic.net/8ZhQ8.gif)

> **Comments:**
> **username_3**: Excellent answer, +1. I already had these drawings in my now-deleted answer so I just copy/pasted them here for the benefit of future readers. Feel free to edit further.  
> **username_0**: @username_7: Thanks for the excellent answer. Brings about a lot of clarity. One supplementary question if you don't mind. I understand (may be I am wrong) that the earth's axis of rotation (which is in itself inclined - or "obliquity" by 23.44 degrees), is also not stable, but "VOBBLING" around an imaginary vertical axis. If this is true, earth's equatorial plane will also change its position. In such a case, what happens to the angle of Moon's plane of orbit? Does it remain in sync with equatorial plane, or is it independent?  
> **username_7**: @username_0 Everything you say is correct. The "wobbling" (called 'precession') of the Earth's axis of rotation is the 26000 year cycle. The Moon's plane of orbit also precesses around this same imaginary vertical axis, which is the 18.6 years cycle. However, the Moon's plane of orbit and the Earth's equatorial plane are (at least to very good approximation) independent of each other.  
> **username_4**: @username_0 See https://www.iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html for some info on the irregularities of the Earth's motion.  
> **username_0**: @username_7: Thanks a lot.  
> **username_0**: @username_4: Sure I will visit the link. Thanks for providing the link. Such info. is exactly what I was looking for. While Google throws up many pages based on the 'CUE WORD", many times the correct cue word itself is an "UNKNOWN"... If I do not know where to look for the information, (such as iers.org, in this case) it is difficult to do some homework. Once the basics are known, specific queries carry some weight. Thanks again.  

---

