Talk:Lunar month

28 days
It is popularly believed that a lunar month is equal to 28 days, which is obviously complete rubbish. Does anyone know how this was arrived at? (I assume it is an approximate average between the synodic month and orbital period, although obviously I am not about to include my own wild assumptions in the article). --Lezek 03:42, 5 October 2005 (UTC)


 * You have 4 phases (new/cresent, first quarter, full, and last quarter), and each phase lasts ~1 week. Generic street talk (slang) often treats it as a 28 day cycle. -- Kheider (talk) 10:34, 15 June 2010 (UTC)
 * In what country? I was taught the real deal before I thirteen,in what used to be called Science. 28 days refers to the Sidereal period,from star to star, & the Anomalistic: neither are easily observed & noted.AptitudeDesign (talk) 12:26, 18 April 2014 (UTC)

The phases of the Moon are a separate thing from the orbit of the Moon. The orbital period of the Moon affects the phases, but the phases do not affect the orbital period. The orbital period is between 27 and 28 days. It is rounded up to 28 days so we do not start the first day of a new month in the middle of the last day of the previous month, for those of us who use the Moon's orbital period as the basis for the length of the month. I have never understood why most cultures reckon the length of months based on the phase cycle of the Moon (the synodic month), because it makes no sense to do it that way. Perhaps they prefer a visible, though less accurate, way of estimating the months. Thibeinn (talk) 20:28, 22 June 2020 (UTC)

Comment from article
This line removed - it had been in article for 12 months without comment. Saltmarsh 11:22, 31 December 2005 (UTC)
 * This should perhaps be merged with Month.

Synodic month
235 arcsine [1/38]equals the average length, while the real length depends upon the Earth's distance from the Sun. 29d 06h 32m to 29d 19h 59m. Therefore,it does not "vary by up to seven hours", but plus or minus seven hours. The range is 13 hours & 37 minutes,not half of that.AptitudeDesign (talk) 12:20, 18 April 2014 (UTC)

If this is the "main article"...
Right now, the Month article and this article about the Lunar month are contradicting as to the question which of these is the main article where it concerns the astronomical definitions. Under https://en.wikipedia.org/wiki/Synodic_month#Types_of_months_in_astronomy, it links to this article as the main article. But then it doesn't really make sense that the most complete explanation of the different definitions is not in this article, but in the article Month.

It becomes extra confusing when this article, supposedly the main one about astronomical definitions of a month, links the names of the several different definitions back to the Month article if you click on them. That would make sense if the Month article was the main article about the astronomical definitions.

I think it would make more sense to switch the texts, i.e. put the longer explanations, that are now in the Month article, in this article instead. And put the slightly shorter (more summarized) texts that are now here, in the Month article instead, which after all also includes other non-astronomical definitions, and links to this article as being the main article about astronomical definitions. As it stands now, with the most complete explanations being in the Month article, a separate article about the Lunar Month (astronomical definitions) becomes almost obsolete, since it adds nothing extra to the information that is already in the bigger article. Greetings, RagingR2 (talk) 13:25, 20 August 2014 (UTC)


 * It looks like this article could be merged with month with some of it transferred to lunar calendar or lunisolar calendar. Karl (talk) 11:38, 22 August 2014 (UTC)

Table of Month Lengths
I've been trying to trace down the source for the Table of Month Lengths in this article. It is based on a calculation from (Chapront-Touzé and Chapront 1991) but the book does not give the month lengths in the form provided here. The values are apparently from an independent calculation by editor. The table has existed since the creation of the article Month in 2001 and its later move to this article in 2014. At no point in this creation or move is there an explanation of the underlying calculation.

In 2001 Wikipedia was not as rigorous as it presently is about original research, but the table seems to go beyond the currently accepted limits on Routine Calculations. I'd appreciate an explanation of the origins of these parameters if one is available. --SteveMcCluskey (talk) 22:27, 5 August 2019 (UTC)


 * Ephemeride theories like in the book of the Chapronts (p.12 table 4) give expressions for the fundamental arguments, like mean anomaly, in the form of a polynomial. The linear term provides the number of degrees d per Julian century (36525 days). The period is then 360*36525/d. I do not know or care if this meets Wikipedias current limits on routine calculations. It is a useful parameter to have listed somewhere accessible like Wikipedia (and more useful to most people than an expression for the mean anomaly): but I will never get a paper published in a peer-reviewed journal deriving these values, because it is too trivial - anyone who had calculus in secondary school should be able to do it. See New_moon for a detailed derivation of a polynomial expression based on updated values. Tom Peters (talk) 06:27, 20 August 2019 (UTC)


 * Thanks for the reply. I suspected it had something to do with the Chapronts' Table 4, which should be cited specifically in the footnote. However, it's still not clear to this reader how the time term Y in the month lengths, e.g., tropical month = $27.322$ + $0$ × Y, relates to specific elements in the Chapronts' Table 4. On another point, the authors use symbols (L, D, l', l, F) which imply a familiarity with how these elements of celestial mechanics relate to the various months; we can't assume that for a typical Wikipedia reader. Could you draft an explanatory footnote to show what you did here?  It would be very helpful to show the reader the basis of this table; I agree that it is very useful but I feel uncomfortable citing it without such a clear demonstration of its basis. --SteveMcCluskey (talk) 20:11, 20 August 2019 (UTC)


 * See my edits. Tom Peters (talk) 22:01, 21 August 2019 (UTC)

Generalizations are NOT facts
From the article...

Cycle lengths​[edit source]

Regardless of the culture, all lunar calendar months approximate the mean length of the synodic month, the average period the Moon takes to cycle through its phases (new, first quarter, full, last quarter) and back again: 29–30[9] days.

From me...

That statement is a generalization, not a fact. In most cultures it is true. In some cultures it is not true.

Thibeinn (talk) 19:44, 22 June 2020 (UTC)

Thibeinn, could you give an example of a culture where a lunar calendar month does not approximate the mean length of the synodic month? — Preceding unsigned comment added by Evan W Morton (talk • contribs) 20:49, 3 July 2020 (UTC) Apologies for failing to sign the previous comment. I'm a newbie. I'll put four tildes and see if it works.Evan W Morton (talk) 20:54, 3 July 2020 (UTC)

Ambiguous pronoun
In the phrase affecting its motion , it's not obvious what "it" refers to. Please change it to "affecting the Moon's motion" or "affecting the Earth's motion" or whatever is accurate.Evan W Morton (talk) 20:57, 3 July 2020 (UTC)

Edits to section #Synodic month
If anyone is concerned/interested by my edits to the section on the synodic month, it arises from a discussion at talk:Lunar calendar. That would be the best place for comments right now. --John Maynard Friedman (talk) 11:27, 30 April 2021 (UTC)

What kind of days?
The length of a sidereal month is given as "27.321661 days (27 d 7 h 43 min 11.6 s)". I can only get this to be correct if "day" is interpreted as a day of 86,400.002 s, while the "d" in "27 d" is interpreted as 86,400 s. The latter is indeed the accepted standard interpretation of the time unit d. The former – interpreting the common noun "day" as meaning "day of 86,400.002 s" – is not. Shouldn't it be made explicit that the meaning here is 27.321661 days of 86,400.002 s (and likewise for the other types of months)? --Lambiam 11:56, 23 September 2021 (UTC)


 * The reference (Lang) gives only 27.321661 days - the value 27d 7h 43m 11.6s appears to have been added by someone else who did a poor job in converting this to hms. It should be 27d 7h 43m 11.5s. Nobody uses days of 86,400.002 seconds. AstroLynx (talk) 12:12, 23 September 2021 (UTC)

Lunation - other meaning
“Lunation” redirects to this article, which defines it as an average duration, along with “”. However, there appears to be a different meaning, as e.g. used in https://www.timeanddate.com/moon/phases: On that page, “lunation” refers to a specific event (or a specific lunar month, to stick with the title of this article) which is consecutively numbered with a number currently in the 12hundreds.

Now it may justly be said that we're all used to such ambiguity from the word “year”, which also can refer to an average duration as well as to a certain moment in time, such as the year 2021 or the Year of the Four Emperors. But for “year”, we can disambiguate the two meanings, such as by adding “AD”. There seems to be no such disambiguation for “lunation”, so we need some extra clarity here.

It would also be good to include when the count of lunations started, and maybe a fragment of a calendar showing lunation #1 and some lunations around our time. ◅ Sebastian 10:51, 10 November 2021 (UTC)


 * The various numberings of calendrical lunations is discussed here. AstroLynx (talk) 12:15, 10 November 2021 (UTC)


 * Thanks! I added the appropriate Redirect template to the section here. ◅ Sebastian 15:49, 13 January 2022 (UTC)

FALSE STATEMENT (I *think*!) in this sentence: "the ascending node, where the Moon enters the Northern Celestial Hemisphere, and the descending node, where the Moon moves into the Southern."
At the time of typing this, under the "Draconic month" section, the article states, "...: the ascending node, where the Moon enters the Northern Celestial Hemisphere, and the descending node, where the Moon moves into the Southern."

The words "Northern Celestial Hemisphere" in this sentence link to a page about the northern celestial hemisphere as defined by the equatorial plane. However, when the moon passes through the ascending node, it enters a celestial hemisphere defined by the ecliptic plane, not the equatorial plane.

Similarly, the word "Southern" at the end of the above sentence also links to an article about a celestial hemisphere defined by the equatorial plane and not the ecliptic plane.

The lunar nodes relate to the ecliptic plane, not the equatorial plane.

I'd correct this myself, but (i) I'm not 100% certain that I'm correct in the above, and (ii) I'm not very experienced with Wikipedia editing (putting links in etc).

51.219.141.160 (talk) 14:52, 12 April 2022 (UTC)

Tropical month
I inserted the following: Just as the tropical year is based on the amount of time between "turnings" of the sun (based on the Greek word τροπή meaning “turn, solstice”), the tropical month is the average time between corresponding lunistices, when the moon is furthest north, or furthest south. It is also the average time between successive moments when the moon crosses from the southern celestial hemisphere to the northern (or vice versa), or successive crossing of a given right ascension or ecliptic longitude. The moon rises at the North Pole once every tropical month, and likewise at the South Pole.

AstroLynx reverted this, saying that it was not an improvement. I object. It adds a lot of informative content that was not there before. I have restored this paragraph. I think the next paragraph should be edited (as I did at the time), but I'm leaving it now. I would remind AstroLynx to follow the policies as spelled out for example in BOLD, revert, discuss cycle, especially being courteous, not reverting without giving good reasons, and being open to discussion. Eric Kvaalen (talk) 19:02, 26 September 2023 (UTC)


 * I reverted your recent change as it mixes up the primary definition of the tropical month ('returns in ecliptic longitude') with secondary consequences. The tropical month is traditionally defined by astronomers as the average period between successive returns of the moon to the same ecliptic longitude (such as equinox to equinox). It's true that the same period also applies to the interval between 'turnings' of the moon or lunistices but those are secondary to the primary astronomical definition.


 * My proposal is to first give the primary definition and then state the other periods related to it. AstroLynx (talk) 22:41, 26 September 2023 (UTC)

The recently added alternative definition for the length of the tropical month is unsourced and appears to be based on original research. As has been noted by an anonymous editor, it is also contradicts the traditional definition given in the next paragraph which can also be found in online sources such the Britannica, Collins Dictionary or the Merriam-Webster Dictionary. Unless the recently added paragraph is properly sourced, I propose to delete it at the end of this month. AstroLynx (talk) 15:40, 15 October 2023 (UTC)