Talk:Lunar distance (navigation)

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Tried to explain this back in November but had it deleted by someone who simply proclaimed that they couldn't understand it. Hopefully this works better!

Apologies. I know how you feal - I've had it done to me!
After I read the article it took me a while to work out what it must have been saying, and when I finally worked it out, the explanation was so different I couldn't just edit it!
I see you didn't massacre my own contribution, so hopefully it has helped.
I've read what you've added ... I'll reword a bit about the 15 degree west because it isn't immediately obvious that this is an hour unless you already know the subject! It's really quite a difficult subject to put in a few words. It really needs a few people to read it and see if it makes sense!

Mike 11:48, 8 February 2007 (UTC)

PS. I've remembered why I came here - it was to do with the error - having worked out the rough error I put in that section, which I thought would be useful, but I'm not too concerned if it disappears!Mike 12:11, 8 February 2007 (UTC)

Additions to article (was thanks)[edit]

PPS. Thanks for the article, I remember a TV program on "Longitude", where it simple asked the audience to laugh at another approach to working out position. Now that I understand the technique it explains not only what the other approach was, but a lot of history such as why they built observatories on remote islands - it was to create a realiable and accurate local fixed reference point from which they could then work out the exact position of other local points using much less accurate equipment so long as it was calibrated to the known reference point!

I presume that is also why they were so interested in solar eclipses. I assume they could calculate the time of the eclipse much more accurately than their time pieces could measure time, therefore using an eclipse, they would have a very accurate local time reference from which, using the moon's position they could then very accurately determine the longitude of the observatory .... it all makes sense because of this article! Mike 12:11, 8 February 2007 (UTC)

After being a little agrieved about having my original article being removed, I realised that it was too complicated. Sometimes criticism is the best compliment. The result of the many contributions since then has been a great improvement to the article.


Using the method of lunar distances to find longitude and time is tricky business. There are a lot of corrections applied to get the required accuracy.

Here are some of the esoterica:

  • The sun very rarely reaches its highest point exactly at noon. "Local apparant noon" is the time when the sun passes the observer's meridian. Because the sun's declination is always changing, the sun will reach its highest point just before or just after this time. Only at the precise moment of solstice does the sun's declination "pause". Near the times of the equinoxes, the difference between the time of the sun's maximum height and its crossing of the meridian is greatest (perhaps as much as 40 to 50 seconds). Thanks to GH for reminding us of this one. --SV Resolution(Talk) 16:47, 25 July 2007 (UTC)

Sources of information[edit]

In my updates to this page, I am relying on the expertise of several individuals knowledgeable in the current (yes, current!) and historical practice of using lunar distances for longitude. I expect a lot (for this article) of activity in the next month or so, and hope to get all the facts properly cited. --SV Resolution(Talk) 12:33, 31 July 2007 (UTC)

The era of longitude by Lunars[edit]

When exactly did HMNAO stop publishing the lunars? 1852 or 1906? says that radio signals were used for setting chronometers at sea by 1905, the last year the HMNAO published lunars. --SV Resolution(Talk) 04:36, 3 August 2007 (UTC)

Thank you, anonymous editor[edit]

For correcting the dates of the Norie references. I cannot believe I didn't spot that mistake. --SV Resolution(Talk) 18:10, 6 August 2007 (UTC)


In the method section, The passage which used to read (152803345):

Knowing Greenwich time and local time, 
the navigator can work out longitude.<ref name="Norie 1828 pg 222"/> 

now reads (153036652):

Knowing Greenwich time and the altitudes of the moon and the other body, 
the navigator can apply the [[intercept method]] to find his latitude and longitude. 
Alternately, the navigator can [[longitude by chronometer|first determine local time, 
and then longitude]].<ref name="Norie 1828 pg 222"/> 

Note the reference to longitude by chronometer. If you've got the chronometer, why bother with lunars?

As has been noted in the Google NavList group, the intercept method was not used during the "golden age of lunars". Further, the almanacs originally only gave the moon's GHA and dec at noon and midnight, GMT, so the moon could not originally have been used in the intercept method, in any case. (I do hope the experts at NavList can provide helpful references for this information, so we can include it in the article).

I propose going back to the simpler statement, in order to avoid having to explain that, in the age before chronometers, navigators actually didn't do it that way.

I think that, in the history section, it would be appropriate to list the different approaches taken at different times. Up to and including the present, of course. I don't feel qualified to write this history myself, but I do feel strongly that the confusing statement in Method ought to be undone.

--SV Resolution(Talk) 00:53, 23 August 2007 (UTC)

I don't mind removing the reference to the intercept method, which came later.

The reference to longitude by chronometer was really to the concept that knowing the (Greenwich) time and one altitude lets you calculate the longitude. If you have a chronometer it can tell you the time. The lunar is another source of time. That's why I changed the wording of the reference. Jrvz 00:32, 24 August 2007 (UTC)

I think I know what you were getting at. I made a reference to Celestial Navigation, and separated much of your detail from the summary of the method. At what point do you make the atmospheric corrections? Before parallax, or do you plug it all into the same complicated formula? -- 01:08, 24 August 2007 (UTC)

The Diagram[edit]

The diagram appears to show the constellation Orion (vaguely), but the Moon cannot be in that position relative to Orion. Setting that aside, since the resemblance to Orion is rather weak, you'll notice that the Moon is a crescent. That means that the Sun would be in the sky to the right of the Moon in a direction perpendicular to the points on the end of the crescent. That would be above the horizon as the diagram is drawn. In other words, the Moon can only appear as a crescent in that orientation in daytime and so the stars would be invisible. Other than that, it's a nice artistic rendering. I like the boat! 02:56, 4 September 2007 (UTC)

I've replaced the image with one that has a more realistic star field. The distance is shown relative to Regulus in Leo. I should have rotated the star field clockwise a bit, but didn't notice the problem until nearly done. At least the relative positions of Regulus and the Moon are realistic. --Michael Daly (talk) 20:54, 10 December 2007 (UTC)

Lunar rate needs citation?[edit]

In the first sentence of the Errors section, someone wants a citation for the statement that a lunar distance changes about 1/2 degree per hour. Is that really necessary? It's well known that the Moon makes an orbit in about 30 days. That's 12 degrees per day, or 1/2 degree per hour. I don't think a citation is necessary. Paul Hirose 23:50, 15 September 2007 (UTC)

I agree. On the other hand, it is easy to find such citations, and that's an opportunity to link another book (or web site) on navigation which could only benefit the readers of this page. 23:50, 20 September 2007 (UTC)

It just repeats what is written in the previous section (which is referenced), so I just added the same reference in the Errors section. Michael Daly 05:53, 13 October 2007 (UTC)


In a few recent edits, it's been stated that radio time signals "replaced" the chronomter or superceded other methods of getting GMT. This is misleading. Radio time signals were used to verify the chronometers. Also, it's worth noting that vessels at sea with radio could compare values for GMT even before official time signal broadcasts. This was nothing more than an extension of the old system of "speaking other ships". Before radio, it was quite common for two vessels passing each other at sea to signal their longitudes (or equivalently GMT). Radio extended the range at which this could be done from a few hundred yards to thousands of miles. —Preceding unsigned comment added by (talk) 00:44, 18 October 2007 (UTC)

I read that there was a significant decline in the use of chronometers in the merchant marine after radio time signals became common. Navies continued to use the chronometer with no change, since radio was not sufficiently reliable (wartime jamming, etc.). However, I've been trying to find that reference for a couple of days and can't locate it in my various books. I have not been able to find any other online reference that documents anything like this. There is documentation for what you describe - the radio time signals were used to verify chronometers. If I can't find any reference to support the decline, I'll remove that from the article - I may have read it in one of the books at a reference library I frequent and I'll be in there next Wed (Oct 24) so I'll have verified by then. If you feel it should be removed until I can verify, feel free to do so.
The changes I just put in are in response to the recent change that claimed that the radio replaced the chronometer in WWI, which is definitely not true. As I had written in my prior changes, WWI was the timeframe for the start of significant radio use in shipping. Michael Daly 05:15, 18 October 2007 (UTC)
I checked all my sources and can't find the reference I need. I've removed the reference to wireless telegraphy time signals replacing the chronometer and added a note that it speeded up the obsolescence of lunars. --Michael Daly 23:54, 24 October 2007 (UTC)

Additional discussion[edit]

Note that there was some significant additional discussion leading to edits of this Wikipedia page on the Navigation List (traditional navigation discussion group). Most of the direct discussion took place in August, 2007. Visit the group's archive [1] and search on Wikipedia. There are also numerous discussions of lunar distances generally in this archive. Membership is open to anyone. 22:52, 22 October 2007 (UTC)

History section removed and replaced with link to History of longitude[edit]

Due to the redundant and sometimes contradictory information in various articles on navigation history, I amalgamated all the history sections into a single History of longitude article.

All the articles, including this one, have been changed to refer to that article instead of trying to rehash the same information.

Please restrict information in this article to be specific to the use of the method. --Michael Daly 18:27, 30 October 2007 (UTC)

I think it was a mistake to blow away the whole history section. A "typical" encylopedia article would include history specific to the topic. I agree that the earlier history section was a bit of a mess, but I think it would be best to restore appropriate material to this article. I'll start a new section sometime soon. (talk) 03:42, 28 November 2007 (UTC)

I think it's a mistake to have several articles with the same history sections that are inconsistent.
There is no requirement to have a specific history section on any page. This whole article is about a historically significant technique - no one but hobbyist navigators use the technique any more.
Referring to a single history article that covers the entire topic of longitude from all angles makes much more sense. --Michael Daly (talk) 06:53, 28 November 2007 (UTC)

Well, I certainly understand your concerns. Nonetheless, the article is better with a History section. I took the previous one, trimmed out a lot of extraneous material, cleaned up some language, and re-instated it. (talk) 12:57, 28 November 2007 (UTC)

You also reinstated the same kinds of misconceptions, errors and redundancy which caused me to remove the section in the first place. You clearly do not understand my concerns.
The Shovell disaster was not a significant motivating factor in either the development of the lunar distance method or the establishment of the Board of Longitude. May has shown that the disaster was not clearly a problem in longitude. The effort in developing the lunar distance method commenced prior to the disaster and the establishment of the Board occurred 7 years later. The petition for the establishment of a prize was independent of the disaster.
Mayer's work was presented to the Board in 1755 - the middle of the century. Stating that it was in the latter half makes it sound much later than it was. The data used by Maskelyne was based on Mayer's original tables but the Almanac used more sophisticated information based on the work of Flamsteed, Halley, Bradley and others and used the Newtonian theory. The article as you posted it makes it sound like Mayer's work was the eventual solution.
Harrison's award is irrelevant to the history of the lunars. The fact that chronometers exist is sufficient.
The statement that lunars were used for decades is immediately followed by a statement of the years lunars were significant. This is redundant and the prior statement contributes nothing.
The final statements on the relative role of lunars and chronometers duplicate what is in the history of longitude article and are not useful here. --Michael Daly 20:45, 30 November 2007 (UTC)

MichaelDaly: please refrain from summary deletion of material you do not like. It is poor behavior on Wikipedia.

To your points: 1) you claim that the Shovell disaster was not connected to the story, citing May. The article by W.E. May is interesting, of course, but it in no way constitutes proof of the revisionist hypothesis that this was a latitude error rather than a longitude error. I can quote a half-dozen sources that DO agree this was an important longitude story. May HIMSELF shows a spread of some 300 minutes of arc in relative longitude among the vessels whose logbooks survived and only 20 minutes of arc in relative latitude error. He carefully masks the equally large error in absolute longitude error with his diagram displaying the vessel tracks as if the absolute longitude has been determined. Note also that one of May's principal points in his article was that the latitude of the Scillies was not known. And yet this very error which he considers so important was so widely known that it had even been written up in the Transactions of the Royal Society. And finally let's remember that Wikipedia is about citations --not personal opinions. 2) You say that Mayer's work was mid-century rather than "latter half". That's a minor point, but sure, change it to read "mid-century". No problem. Your claim that Mayer's tables were not the key is seriously misleading. The only significant contribution from Flamsteed to all of this were the exact positions of the nine/ten lunars stars, which were widely known by this period in any case. Halley has no significant contribution. 3) The minor redundancy in the text is no reason to delete that section. Frequently, good prose has some degree of redundancy. That's how points are made clear. 4) The peripheral reference to chronometers explains something very specific about the end of the lunars era. Lunars were NOT killed off by radio time signals, as you have written elsewhere --they were long over and done with by then. They were killed off by cheap chronometers by c.1850/55. Only the end of the highly conservative continued publications of the tables in the almanacs coincides with the proliferation of radio time signals. 01:30, 1 December 2007 (UTC)

As opposed to your summary deletion of my changes?
You want citations, not opinions - well it is your opinion that a history section is required and your opinion that your additions are perfect and do not need any changes.
1) I did not say that the Shovell disaster is not connected to the story. I said that the disaster is not such a significant factor that it needs to be identified separately. The motivation for determining longitude was established well before the disaster and the establishment of the BoL was well after it. Hence - the significance of that specific disaster is overstated and much of what is written about it is based on legends and myths, not facts.
2)I did not say that Mayer's tables were not the key - I said they were used by Maskelyne initially but that the tables were not the be-all-and-end-all of the lunar distance method. Nautical Almanac was not just a re-hashing of Mayer's tables and your inclusion does not make that clear.
4)Where specifically have I ever written that lunars were killed off by radio signals? Provide a reference or retract your ridiculous claim.
This entire history section is redundant and will provide a point where two articles (this and the history of longitude) will diverge with more contributors adding different bits of information. There is no requirement whatsoever that there be a history section in this article. For example, the Sextant article contains no history section; the history of the sextant is dealt with elsewhere. The sextant article is about the function and use of the sextant. So with this article - it should be about the use and practice of lunars; the history is adequately covered, with a broader context, elsewhere. --Michael Daly 20:09, 1 December 2007 (UTC)

Need more discussion on determining/using local time[edit]

I think the subject of how to 1) determine local time and 2) how to get from local time vs. GMT to longitude should be discussed more explicitly. Determining local time might warrant its own entry.--icedtea1954 (talk) 01:26, 23 January 2008 (UTC)

Equation of time[edit]

When computing the difference between GMT and local time, one has to take the Equation of time into account. It amounts up to a quarter of an hour! Rgds --Boobarkee (talk) 12:39, 18 August 2009 (UTC)

Longitude before the Telescope[edit]

I`m in search for data on the accuracy of astronomic derived longitude (land based, I assume lunar eclipse or occulation) before the time of telescopes and marine dead reckoning accuracy (all times, all ships) -- Portolanero (talk) 14:36, 16 June 2011 (UTC)

Is 'citation needed' really needed? IN PRACTICE section[edit]

In Practice ... Step One – Preliminaries

   ... other body (see any nautical almanac from 1767 to c.1900).[citation needed]

To my way of thinking, 'see any nautical almanac' is all the citation needed, considering that a number of them are available. That fact ought to be known to most readers of this article. (talk) 19:55, 30 September 2012 (UTC)Mac Hayes

... Also in Step One – Preliminaries

   ...Since the Moon's apparent size varies with its varying distance from the Earth.[citation needed]

Is a citation needed for such an obvious statement? Does someone wonder if perhaps some believe that the moon stays the same apparent size despite its varying distance from earth and therefore the statement requires proof? This too is a fact that ought to be know to anyone with a rudimentary knowledge of astronomy. — Preceding unsigned comment added by (talk) 07:21, 9 April 2014 (UTC)

I agree these are questionable requests for citation. However, I've added one for the second, specifically referring to an example of corrections for lunar semidiameter given in Dutton's. Adding a citation for a source that is rather old (i.e. for the first item) is a bit more of a challenge. (talk) 09:00, 23 June 2014 (UTC)

Relative costs[edit]

I read somewhere that the cost of sextants could exceed the cost of chronometers, and if I find it again, will post the reference here. --Pawyilee (talk) 10:28, 7 December 2012 (UTC)

27day intersection sun moon at same lattitude longitude[edit]

This "fun fact" observable by most quickly, helps explain sun moon angle (and indirectly, how celestial plane aziumuth intersection is a possible basis for releasing observed angle charts, error not incl)

Using sunclock(1) software animated at 1 day intervals one can easily see the sun yearly period rise and fall between the tropic lines of cancer and capricorn. The moon does the same every 27 days.

At 12/1 pm every 27 days they always intersect, have roughly the same latitude and longitude ! (roughly) This means if it is 1/2 way between summer and winter (sun is on equator) then 1/2 between 27 days, and stand at (0,80) deg. the sun and moon are the same place in the sky directly overhead, every 27 days.

An observer at different (lat,lon) can get a rough position fix with a single (1pm) sighting. The sun gives rough latitude just by angle (due to time of year) and any angle between sun an moon observed is due to now known latitude difference and longitude difference (ie, at 80 deg). Every 27 days. Elsewise (between days) one would need a chart for time and day and do some extra work.

The sun-moon angle published in nav. books may refer to something else that i'm unsure of i don't have one.

Similarly one can use Orion belt for azimuth being overhead directly at night, but the angles are so slight it is too difficult without expensive equipment.

What is the USS Peacock section for?[edit]

This little anecdote is interesting to some, but provides nothing of significance to the topic of the lunar distance method. Can someone suggest a valid reason for its inclusion (as opposed to any number of other anecdotes about lunars)? (talk) 08:57, 23 June 2014 (UTC)

William Baffin[edit]

I've removed mention of it for the time being since it was uncited, unmentioned by Britannica, and centuries off the first use listed here, but William Baffin's page formerly claimed he had practiced lunar distance. Kindly restore it with citation there and add it here if that's an accurate claim. Worth noting the error if it turns out to have been one. — LlywelynII 12:57, 4 December 2015 (UTC)