Time, Sextants, Tables, Longitude, Latitude

CaptainBob

CaptainBob

Well-Known Member
Joined
7 Nov 2007
Messages
1,477
Location
North Yorkshire
www.yacht-forum.co.uk
Was having a chat with my dad in the pub, and couldn't decide between us if you need to accurately know the time in order to make use of a sextant and tables to work out your position.

I seem to remember reading a book about harrison and clocks etc, which seemed to imply that you can find position using time and something - and the alternative astronomer dudes were attempting to solve the problem using only stars/moon/sun...

So what's the deal. Can you find your position with a sextant and tables without a clock? Or is that just tables of old and new ones need a clock plus sextant?

Short version of answer please /forums/images/graemlins/smile.gif

Ta!
 
I saw that programe several years ago. The conclusion was that it was possible to determine longitude by observation but it took several hours of calculations by hand. The usual method is with sextant, tables, and clock. Every 4 seconds of time error will give 1 minute ( 1 mile) of longitude error.
 
All commonly used methods of astro-navigation do require an accurate time, IIRC 4 seconds error in time can equate to 1 mile error.

There are ways of plotting your position without having accurate time, but you might struggle to get within 100 miles of your real position.

The issue of course is longitude, latitude is easy to measure.
 
he probably did what we did sailing to indonesia in the 70's after 7 days of heavy overcast and no chance of a sight. You sail slowly at night looking for a light and during the day you hope you can match the land profile to the little drawings on the chart.
 
I believe Slocum used the method of 'Lunar distances' - see here:

http://en.wikipedia.org/wiki/Lunar_distance_%28navigation%29

Francis Chichester was a fan of this method also.

Today you can buy a watch that is accurate to a second a day for £25 (I have two..) so why bother ?
Unless you are bored at sea and love the maths - too many chances of making mistakes for me - I would stick with the Casio and the Air tables, especially for a beginner.

Good luck
 
you can get accurate latitude without time but you need tables, I tried it and as long as you can plot a good curve an hour or so before and after estimated local mid day, then from the graph curve even if your actual peak was a poor sight then the graph will show up the true altitude. This was hoe they used to do crossings before H4, keep a good watch and look for the clouds or other signs of land approaching.

This means running East West is kind of OK but traveling north or south and approaching land you were never sure quite which bit of land you were going to hit! Longitude was always the problem.
 
But if I recall correctly Jack makes frequent reference to the quality of his clocks. I think it was the norm in his day for captains to buy their own chronometers as they were better than those supplied by the admiralty
 
[ QUOTE ]
I believe Slocum used the method of 'Lunar distances'
Francis Chichester was a fan of this method also.


[/ QUOTE ]

Captain James Cook had no chronometer on his first Pacific voyage and used lunar distances to find his longitude. For his 2nd and 3rd voyages he had a Kendall chronometer but still used lunar distances occasionally to check the chronometer, which was found to lose just over 2 minutes a year, at a regular rate. Not bad for an eighteenth-century, hand-made watch!

Ref: "Captain Cook" by Alan Villiers [A good £2-worth, from the remaindered bookshop in Greenwich!]
 
[ QUOTE ]
still used lunar distances occasionally to check the chronometer, which was found to lose just over 2 minutes a year, at a regular rate. Not bad for an eighteenth-century, hand-made watch!


[/ QUOTE ]
That of course is the critical thing about a chronometer, not that it keeps exact time but that the error is predictable - so a chronometer that regularly loses 5 secs a day is much better than one that is accurate to within a couple of seconds a day, but may gain some days and lose on others.
 
In his book about his first circum-navigation, when faced with possible time errors, Robin Knox Johnson refers to Slocum's use of lunar observations, but adds that the necessary tables are no longer available, that in 1969.
 
You must log in or register to reply here.

Share this page

Top