Talk:Lunar month

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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)[reply]

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)[reply]

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)[reply]

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)[reply]

Comment from article

This should perhaps be merged with Month.

This line removed - it had been in article for 12 months without comment. Saltmarsh 11:22, 31 December 2005 (UTC)[reply]

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)[reply]

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)[reply]

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)[reply]

Table of Month Lengths

@TomPeters: 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 Tom Peters (talk · contribs). 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)[reply]

@SteveMcCluskey: 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#Explanation_of_the_formula for a detailed derivation of a polynomial expression based on updated values. Tom Peters (talk) 06:27, 20 August 2019 (UTC)[reply]

@TomPeters: 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 = 27.321582241 + 0.000000001506 × 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)[reply]

@SteveMcCluskey: See my edits. Tom Peters (talk) 22:01, 21 August 2019 (UTC)[reply]

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)[reply]

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)[reply]