# How do you determine longitude from the stars? **URL:** \ **Category:** Factual Questions\ **Created:** [August 21, 2003, 1:11pm UTC](https://boards.straightdope.com/t/how-do-you-determine-longitude-from-the-stars/196646 "2003-08-21T13:11:17Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [August 21, 2003, 6:32pm UTC](https://boards.straightdope.com/t/how-do-you-determine-longitude-from-the-stars/196646/21 "2003-08-21T18:32:03Z")
> [@](#): > > \*Originally posted by G. Odoreida \* > What I was trying to find out was whether there was a method that does not rely on chronometers One method was the use of “lunars”. With respect to the fixed stars, the moon has enough apparent motion to be useable as a sort of heavenly clock. The idea was to measure the angular distance between its edge and some known stars. The drawbacks are that it doesn’t move rapidly enough to give great acuracy, and it’s only visible part of the time. To fix the location of a point on land really well, you’d need to average the results of many observations. Also, the moon is close enough that there is some real work to be done dealing with issues of parallax. Googling “Lunars Longitude” gives some interesting links. Impractical as may be, there is obviously a lot of current interest in this. ---
**Author:** ![bryanmcc](https://avatars.discourse-cdn.com/v4/letter/b/eada6e/32.png) [@bryanmcc](https://boards.straightdope.com/u/bryanmcc)\ **Post date:** [August 21, 2003, 7:50pm UTC](https://boards.straightdope.com/t/how-do-you-determine-longitude-from-the-stars/196646/22 "2003-08-21T19:50:34Z")
> [@](#): > > \*Originally posted by sailor \* > \*\*Well, there is a non-answer if I ever saw one. the question is " How do you determine longitude from the stars?" and I don’t see how your post answers that. > > Furthermore, your post is _wrong_. You need to know Universal Time (UT, GMT for practical purposes), not local time. If you know Local Legal time and the hours difference with UT you can calculate UT. Local solar time would be useless. \*\* The OP was “How do you determine longitude from the stars?” (presumably _only_ from the stars), and my answer was that you can’t. You must have a clock as well (even if the clock is an astronomical event). Of course, the fact that two more thorough posts snuck through before mine rendered that answer fairly redundant. Additionally, the OP sounded like a person with little knowledge of the subject looking for a simple answer. I now see that s/he is quite knowledgeable, and that s/he was looking for a more thorough answer. Also, you are of course correct: _local_ time is precisely what you do not need. You need the time of reference location, such as the time in Greenwich, England, to which you can compare your local astronomical observations. Please excuse the typo. ---
**Author:** ![The\_Stafford\_Cripps](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_stafford_cripps/32/7167_2.png) [@The\_Stafford\_Cripps](https://boards.straightdope.com/u/The_Stafford_Cripps)\ **Post date:** [August 21, 2003, 10:19pm UTC](https://boards.straightdope.com/t/how-do-you-determine-longitude-from-the-stars/196646/23 "2003-08-21T22:19:56Z")
> [@](#): > > \*Originally posted by sailor \* > \*\*As I said, the observation of celestial events, like the occultation of the moons of Jupiter were used but chronometers were also of use. It was (is) not a matter of either/or but a matter of continuously revising and improving previous estimates and all information was taken into account and given the appropriate weight. There was no way to determine longitude with a precission of a cm though because that was beyond the precission of the instruments they had. You would need to determine the meridian passage with an precision of 1/200 second which was only possible by averaging thousands of readings and even then you weren’t too sure. \*\* Thanks, **sailor** , this clears things up for me, I now realise why they spent so many years making their observations when determining the longitude of Madras and other places. Incidently, your previous post to that one deserves recognition as a straight dope phenomenon I hadn’t previously experienced. On my computer screen it is just the right length so that when I press Pg Dn your name in the next post appears in almost exactly the same place. Wooh! ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [August 21, 2003, 10:54pm UTC](https://boards.straightdope.com/t/how-do-you-determine-longitude-from-the-stars/196646/24 "2003-08-21T22:54:03Z")
When surveying India you do not care so much where Mt. Everest is in relation to Buckingham Palace but in relation to other parts of India. You should realize that maps and charts are usually referred, not to Greenwich but to some closer point with assumed coordinates and this is called the [map datum](http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=map+datums) so you have different maps of the same area based on different datums. [Here](http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html) you can see the transition from the 1927 North American Datum to the newer 1983 datum. > [@](#): > > The readjustment of the North American Datum of 1927 (NAD 27), Old Hawaiian Datum and Puerto Rico Datum to the North American Datum of 1983 (NAD 83 (1986)) in July 1986 was both a change in reference ellipsoid and a “clean up” of nearly 200 years of surveying data held by NGS. Based on this readjustment and redefinition, positions of points can change between 10 - 100 meters, in the conterminous United States, more than 200 meters in Alaska, Puerto Rico and the Virgin Island and in excess of 400 meters in Hawaii. Consequently, the shift between the various datums are not uniform across the United States and there is no single value that can be applied to latitudes or to longitudes based on old datums to convert them to NAD 83. NADCON was developed in order to facilitate conversion between the datums,. The grids used by the program are based on more than 150,000 horizontal control points whose coordinates reside in NGS’ data base, and provide transformed positions based on the shifts of the control nearest to the input position. It is just not possible to get 1cm precission in cartography due to the limitations of instruments etc. A precision of 1 cm in 80,000 km (=8,000,000 cm) is just not so easy but you also have to take into account that the earth is not a solid sphere but it has tides even within the solid parts and the continents drift etc. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [August 22, 2003, 11:37am UTC](https://boards.straightdope.com/t/how-do-you-determine-longitude-from-the-stars/196646/25 "2003-08-22T11:37:01Z")
Citing myself from [a page about the history of celestial navigation](http://sailor.teemingmillions.com/cnhist.htm): > [@](#): > > With the advent of the chronometer, longitude could be determined. This was done in the following manner. The navigator would determine latitude by the same method he had used for the last few hundred years: the noon sight. He would later take a sight of a body, still generally the Sun, on or close to the prime vertical, that is to say, due East or West. This was called the ‘time sight’ because time had to be noted. With his dead reckoned latitude from the noon sight (today we would say ‘advancing this LOP’) he would then calculate longitude. > > The time sight was a complement to the noon sight and the next logical step. It was more easily accepted by navigators than a radically new system would have been and it became the standard of the day. > > Aside from the central topic of navigation at sea, it is interesting to note that land surveyors and map makers of the time had the same problem of determining their position from celestial observations but they came up with completely different solutions. The mariner has a clear view of the horizon and can measure H with his sextant, which is not the case on land. On the other hand the mariner has no means of measuring accurately the bearing (azimuth) of a body or the precise time of its meridian passage. An astronomical observatory on the contrary, has a meridian telescope which serves to time with great precision the meridian passage of stars and consequently determine longitude. ---
**Author:** ![The\_Stafford\_Cripps](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_stafford_cripps/32/7167_2.png) [@The\_Stafford\_Cripps](https://boards.straightdope.com/u/The_Stafford_Cripps)\ **Post date:** [August 22, 2003, 11:45am UTC](https://boards.straightdope.com/t/how-do-you-determine-longitude-from-the-stars/196646/26 "2003-08-22T11:45:24Z")
Yeah, I knew that, I wasn’t trying to imply that the project to triangulate India was the same as the one determining the longitude of Madras. The triangulation was first instituted by the government of Madras, and its first baseline was there. I suppose it made sense for the goverment or some other organisation to try and find a figure for the longitude of that base, but the book I read didn’t go into any detail about it. Part of the justification for the triangulation was to determine the shape of the earth over the 78th meridian; these people were pioneers of geodesy and they did make some important advances, such as the realisation that the earth is not uniformly dense. It’s interesting you mention Mt Everest, as the triangulation teams were not allowed into Nepal, so the huge mountains on the northern border of Nepal (such as what became known as Everest) had to be surveyed from inside India, up to 200 miles away. ---
**Author:** ![The\_Stafford\_Cripps](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_stafford_cripps/32/7167_2.png) [@The\_Stafford\_Cripps](https://boards.straightdope.com/u/The_Stafford_Cripps)\ **Post date:** [August 22, 2003, 11:47am UTC](https://boards.straightdope.com/t/how-do-you-determine-longitude-from-the-stars/196646/27 "2003-08-22T11:47:32Z")
**Sailor** Thanks for the further information. My last post was referring to your 11.54pm post. ---
**Author:** ![carver42](https://avatars.discourse-cdn.com/v4/letter/c/bc8723/32.png) [@carver42](https://boards.straightdope.com/u/carver42)\ **Post date:** [August 22, 2003, 12:51pm UTC](https://boards.straightdope.com/t/how-do-you-determine-longitude-from-the-stars/196646/28 "2003-08-22T12:51:50Z")
> [@](#): > > \*Originally posted by rodent \* > \*\*Here is an excellent historical book on how it was solved: > > [http://www.amazon.com/exec/obidos/tg/detail/-/0802713122/qid=1061482246/sr=1-1/ref=sr\_1\_1/102-3108755-5673708?v=glance&s=books](http://www.amazon.com/exec/obidos/tg/detail/-/0802713122/qid=1061482246/sr=1-1/ref=sr_1_1/102-3108755-5673708?v=glance&s=books) \*\* I have also heard it on Books on tape. It is very interesting. Talks about development of clocks!!😃 ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [August 22, 2003, 1:16pm UTC](https://boards.straightdope.com/t/how-do-you-determine-longitude-from-the-stars/196646/29 "2003-08-22T13:16:45Z")
Sailor, incredibly minor nitpicky correction: 80,000 km is 8,000,000,000 (that’s eight billion!) cm. You missed a few zeros in your post. Of course this only makes your point stronger. There is almost nothing in science known to one part in eight billion. [Previous page](https://boards.straightdope.com/t/how-do-you-determine-longitude-from-the-stars/196646.md?page=1)