Determine longitude with a watch and a stick - Page 1
Forum section: electronics / general

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 06, 2018, 11:18:02 pm
content: Of course determining Longitude requires precise knowledge of the time.And that prompted the English Parilament to offer the "Longitude Prize" in 1714.The prize was £20,000, (equivalent of £2 million today).So John Harrison set to work creating a timepiece that would keep accurate time on board a pitching ship.His first three models (Called "H1, "H2", and "H3") were large brass contraptions.They sent H3 out on a trial voyage, but they had no way of determining the accuracy of H3.So Harrison developed "H4" which was a pocket-watch and turned out to be better than H3.It rather reminded me of the volt-nuts here on these forums and the issue of how do you measure things.A photo of H1, H2, H3, H4, and H5

author: helius
author_rank: Super Contributor
timestamp: September 07, 2018, 12:15:28 am
content: "Longitude", starring Michael Gambon and Jeremy Irons:

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 07, 2018, 12:24:09 am
content: A shame they never went with the chain of barges all across the atlantic firing signal rockets at the same time. Impractical but spectacular!

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 07, 2018, 03:59:03 am
content: So how much do you think you could have got for your cheap digital watch in the early 1700s? We take an awful lot for granted these days.

author: ivaylo
author_rank: Frequent Contributor
timestamp: September 07, 2018, 06:56:20 am
content: Quote from: CatalinaWOW on September 07, 2018, 03:59:03 amYou’ll be surprised. A contemporary digital watch would be more practical to handle for sure, but even John Harrison‘s H4 from the 18th century was about 10 times more accurate than pretty much every watch you can put on your wrist today (with a few notable exceptions costing pretty much the same as the navigation clocks of olden times). And for navigation that’s key, an error of one second in time puts you something like four nautical miles away from your actual location.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 07, 2018, 07:45:27 am
content: Quote from: CatalinaWOW on September 07, 2018, 03:59:03 amNot as much as the guy selling the batteries for it in 1708...

author: Homer J Simpson
author_rank: Super Contributor
timestamp: September 07, 2018, 07:53:15 am
content: The Clock That Changed the World

author: Ian.M
author_rank: Super Contributor
timestamp: September 07, 2018, 08:53:18 am
content: Quote from: ivaylo on September 07, 2018, 06:56:20 amNo. The Earth rotates 360° in 24H, which is 15° per hour or 1° every four minutes. Therefore 1 minute is 15' (of longitude), which is 15 nautical miles at the equator. Convert that to nautical miles by multiplying by the cosine of the latitude.A 1 minute error in one's chronometer is probably tolerable when making a landfall, but you'll need to keep a very sharp lookout and avoid approaching low-lying coasts with off-shore reefs.Its possible to determine longitude to an accuracy only limited by ones observing instruments and the stability of the platform one is observing from by theMethod of Lunar Distances.Once you've got a method of determining Greenwich time by observation, you only need a clock of sufficient accuracy to carry that time forward for a week or so, to allow for bad weather when the opportunity for a sun or star sight may be very limited, and to cover the period over the new moon, when the moon is too low in the sky, and only visible at dawn or dusk, so its impossible to get a fix.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 07, 2018, 09:07:36 am
content: Quote from: ivaylo on September 07, 2018, 06:56:20 amWhile cheap digital watches may not spec out to meet the performance you require, my personal experience is that errors are commonly only off less than minutes per year. And with seconds at midlatitudes being a smallish fraction of a nautical mile, and most voyages being only a few months between landfalls I suspect they would have been happy with the performance. Even happier if it we're ones with a solar cell booster. Don't know how long one of them lasts since mine is still going strong after a dozen years.

author: rstofer
author_rank: Super Contributor
timestamp: September 07, 2018, 02:05:39 pm
content: My Garmin Forerunner 235 should have time pretty much nailed!The trick with lesser watches is to log the error per day (month) and account for it. I don't know whether mechanical or electronic would have less drift considering the environment.WWV was always handy for logging error or resetting.I bought a 100+ year old copy of Bowditch just to learn about Lunar Distance. It seems simple enough but I never did get around to trying it.

author: ivaylo
author_rank: Frequent Contributor
timestamp: September 07, 2018, 08:26:36 pm
content: Thanks for the Method of Lunar Distances reference, @Ian.M, haven’t heard about it. And for correcting my mental math errorDo you sail or something?