may I have a little assistance?

[aliceelizabeth]

in lay man's terms, can anyone explain the way you find out your longitude at sea.

I understand it myself and have spent today reading 'longitude' but am having trouble explaining it in simple terms for a lesson I am planning for the children at school.

[Astro_Baby]

I'll have a go

In simple terms its the left/right position in time. ie if your at 1hrs 5 mins longitude you are 1 hrs and 5 mins from Greenwich with the sun as a reference point.

Put simply if you were floating above the earth and stationary and fixed a crosshair on Greenwich and then waited 1 hrs and 5 mins the cross hair would now be at the location thats 1 hr 5mins longitude. ( wherever that may be ).

You can calculate longitude either by having a precise time fix OR by having an astronomical table that tells you the precise location of stars/planets at a given time. ie if you know where two stars are at an exact time you can calculate your position through some (relatively) simple trigonometry.

By the way - as an aside- if you have read longitude I would seriously recommned getting along to the Greenwich observatory to look at Harrisons sea clocks which are amazing and very beautiful and they all work as well apart from H5 which runs but isn't wound up because its mechanism needs lots of regular maintenance if its run.

[aliceelizabeth]

thank you yes I live in South London and was over there the other day doing a bit of research. I am quite frequently in fact as I like to lounge around the maritime museum in the summer and used to work at mudchute. I only wish I had recorded the guide's speech, it'd be so blasted useful!

[barkis]

I know modern technology makes it easy these days with GPS, but the fascinating story of John Harrison, which was screened on TV some time ago, told the story of the invention of a very accurate clock, which was essential way back then, to find Longitude at sea. A great story, and of course, very true.

Read the book, simply called Longitude. you will enjoy it.

Ron.

[beamish]

I believe one of J Harrison's original wooden clocks is still working in Lincolnshire. ie wooden mechanism :shock: and is even by modern std.s still accurate.

[aliceelizabeth]

I believe one of J Harrison's original wooden clocks is still working in Lincolnshire. ie wooden mechanism :shock: and is even by modern std.s still accurate.

do you think it's the one del boy found in his garage?

[timetraveller]

hello Alice Elizabeth

This is how i would explain how to find longitude at sea .

First I would tell the children that the earth is the shape of a circle and that a circle is divided up into 360 degrees or segments and that if you divide 360 by the number of hours in a day (24) you get each hour equalling 15 degrees because 360/ 24 =15.

once they understand that i would then briefly introduce the lines of longitude and latitude using a globe or a ball ,explaining that the longitude lines run around the the earth from the north pole down to the south pole and back around the other side from the south pole to the north pole.

And that the zero line or 12 o'clock actually runs through the capital city of England ( London ) at greenwhich. and that all the rest of the lines of longitude or great circles ran in the same direction around the globe spaced out at exactly 15 degree or hourly intervals.

And that these lines run anti clockwise, from west to east so that 12 o'clock was in London and then you count 1 hour intervals as you travel west or -1 hour intervals as you travel east. .

I would then simply explain that latitude lines run around the globe in line with the equator and above and below so that the lines cross to form a big grid. also spaced out at intervals of 15 degrees.

once they understood that i would go onto explain that because these lines are spaced out equally then the distance travelled is equal to time .

I would then explain that as the earth rotates over the course of the day the sun slowly changes its position compared to were you are stood slowly moving across the sky from east to west in a arc and that the sun reaches its highest point in relation to your position at midday then as you move west the time that the sun reaches its highest point will be different from each place so if midday is measured as 12 in London in Bristol it wont be midday for another 15 to 20 minutes because you are further round the globe on a different line of longitude and because of this time difference if you can measure the exact time on the ship at midday and compare it with another clock set for midday as it would be at Greenwich then the difference in time multiplied by your speed will give you your distance travelled in longitude and to measure your latitude you just measure the angle of the sun at set time intervals like midday from your position and the number of degrees and minutes will once again give you your distance travelled

this is the simplest i can make it i hope it helps and I wish you good luck in your lesson

best wishes Pete

[aliceelizabeth]

thank you pete, they're 12 / 13.

it seems simple enough once you have it fixed in your own mind but try and explain it and arg?!

[timetraveller]

Good morning Alice

yes I think that's the big trouble with a lot of scientific ideas they are relatively straight forward to understand right up to the point of actually having to explain it in simple terms to somebody else then minefield.

One imagines the trouble people like Leonardo Galileo Issac Newton Einstein and Steven Hawkins would have had in conveying their ideas and findings to their piers.

I mean how do you explain something like moving clocks run slow to someone struggling to come to terms with the complexity of Gravity.

I think this is what sets apart the truly great men and women of science and all other subjects their ability to communicate ideas in a way which others can grasp and understand . Good teachers are truly special people and i envy them their ability .

And just as an aside and having had more time to mull over your particular task an even more simple explanation for longitude would be that the measurement of longitude is simply the difference in time of a set event relative to a known point and your position equals distance i don't think it is possible to break it down any more simply than that , but i may well be proved wrong

[beamish]

I believe one of J Harrison's original wooden clocks is still working in Lincolnshire. ie wooden mechanism :shock: and is even by modern std.s still accurate.

do you think it's the one del boy found in his garage?

No, I think it was H4 he found. The clock in Lincolnshire is a tower clock !

[Kaptain Klevtsov]

A simple way might be to tell them about time zones. The sun rises at different times according to the longitude, so in New York they don't get sunrise until we've had breakfast and gone to school in the UK.

If you know what time the sun rises in Greenwich and set your clock to that, by seeing what time the sun comes up where you are, you can work out your longitude. For each hour delay, you are 15 degrees west of Greenwich, and if the sun rises early, you are east of Greenwich.

The same works for the stars, but the sums are harder and its less obvious to kids that the stars track across the night sky.

Kaptain Klevtspv

Kaptain Klevtsov

[proflight2000]

Wow the Mudchute, I remember that place as a kid when I lived in Poplar. Used to have the inner city farm there as well, do not live down there anymore but the name is never forgotten.

[mike.brightmore]

A link to info about John Harrison's stable clock at Brocklesby Hall in Lincolnshire;

http://www.bhi.co.uk/hj/April05AOM.pdf

I think they are having a spot of bother with it at the moment. We have just set up a steering group to establish the 'Longitude Harrison Foundation' in order to better promote Harrison's life and work.

Mike

[beamish]

cheers Mike

[mike.brightmore]

Basically, after 40 years of dedication and 4 models, John Harrison built a chronometer that kept accurate time at sea, i.e. could cope with the ship's tossing, turning and rolling, changes in temperature and pressure etc. This involved innovative ideas like the bi-metallic strip to compensate for temperature changes and a system of caged roller bearings etc. The clock (or oversized pocket watch as it ended up) was set at Greenwich mean time at the beginning of the voyage and left at that. Whilst at sea the navigators would establish when local noon was (when the Sun was at its highest point in the sky). That was at 12 o'clock noon. They then read the time given by the Harrison clock (Greenwich mean time) and the difference gave them their longitude in hours and minutes. As the Earth turns 360 degrees every 24 hours so (as explained in previous posts) each hour difference equals 15 degrees.

So a ship sailing from England to America and nearing its destination would find that when it was noon (the Sun at it highest point) near the east coast of the USA, the Harrison clock would show GMT to be 5.00 p.m. This would give their longitude as + 5 hours or 75 degrees west.

Incidentally, shortly after the Apollo 11 mission, Neil Armstrong was invited to a dinner at 10 Downing Strret. He proposed a toast to John Harrison, saying, "His invention enabled man to explore the Earth with precision and, when most of the Earth had been explored, to dare to build navigation systems for voyages to the Moon."