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.
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.
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!
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 so you have different maps of the same area based on different datums.
Here you can see the transition from the 1927 North American Datum to the newer 1983 datum.
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.
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.
