Talk:Lunar distance (navigation)

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)[reply]

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)[reply]

Additions to article (was thanks)

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)[reply]

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.

Esoterica

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)[reply]

Sources of information

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)[reply]

The era of longitude by Lunars

When exactly did HMNAO stop publishing the lunars? 1852 or 1906? http://www.math.uu.nl/people/wepster/ldtab.html 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)[reply]

Thank you, anonymous editor

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)[reply]

Method

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)[reply]

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)[reply]

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? --72.94.157.91 01:08, 24 August 2007 (UTC)[reply]

The Diagram

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!24.136.6.69 02:56, 4 September 2007 (UTC)[reply]

Lunar rate needs citation?

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. 24.136.6.69 23:50, 20 September 2007 (UTC)[reply]

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)[reply]

History

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.