"Longitude"

[Barry W]

I was wondering if others of you out there have read the book (by Dava Sobel) or seen the docu-drama "Longitude". It's the story of John Harrison's trials and ultimate solution to the problem of time keeping at sea. After the 1707 shipwrecks of the Royal Navy's fleet, he spent the remainder of his life perfecting a time piece for use at sea.

I ask because we're having a good discussion in the help and advice section on RA. I'm still trying to decide RA's importance to the amateur backyard astronomer (in terms of precision measurements). It's relationship to longitude, however, is extremely important and so is the accurate calculation in relation to time. In history, this is what led a carpenter to challenge the highly educated Royal Astronomers to gain recognition for his work and acceptance for safe navigation practices. Pretty much everything we can do now with EQ mounts and computerized tracking from the safety of our generally fixed positions stem from Harrison's work.

I think it's an amazing story of the practical applications of understanding astronomy.

Cheers,

Barry

[beamish]

It is a great book and the docu-drama wasn't too bad either !

[Talitha]

Yes the book is fantastic. On my recent trip to the UK, we went to the Royal Observatory where H1-H4 are kept... they're beautiful timepieces, and it was a thrill to be able to see them in person.

[Jarndyce]

Yes, fascinating book and film.

IMHO Harrison's clocks are masterpieces - not just in engineering, but also in innovation and creative thinking. All without any CNC.

They look pretty cool too...

[roundycat]

read the book, seen the film. I do not believe I ever came across anyone with such perseverance as Harrison, brilliantly portrayed by Michael Gambon. Harrison should be declared a national treasure.

Dennis

[Astro_Baby]

Read the book and found it fascinating. So much so I went up to Greenwich to see the clocks for myself and as Talitha says they are wonderful pieces to see. H4 especialy as H1, 2 and 3 are quite large then you come to H4 which is about the size of a saucer. Apart from H4 they are all running. H4 can run but they dont wind it to preserve it apparently. Amazing also to think that Harrisons marvels of design were left to rot in an attic until they were restored in the 1920s.

[barkis]

I thoroughly enjoyed the dramatised documentary of Longitude.

I'm sure some artistic licence was applied to it, but the fundamental story of Harrison and his quest to solve the problem of Longitude at sea was revealed. I could watch it again, and I have already seen it twice. Apart from his technical expertise, the man had a tenacious will to succeed. As far as the use of Right Ascension in modern day astronomical observing, I don't think it has a practical use in amateur circles. (No pun intended). Modern Observatory telescopes are fitted with encoders which give readouts to several decimal places I would imagine. Image acquisition is never a problem for them.

The amateur relies either on his Goto scope, or an ability to star hop to locate his/her target.

As a few experienced members have already stated, the circles fitted to instruments amateurs mainly buy, are much too small to be

capable of great accuracy. They may allow an adept user to get to an approximate position, but a spiral search usually ensues from then on.

The amateur telescope maker of yesteryear, who not only made his own mirror, but also the OTA, and the mount, including a drive system. Mine was initially synchronous motors, the drive rate adjusted by changing the frequency. Then came steppers, which offered much better control. I never did get into servos.

The mount consisted of an A frame, with self aligning bearings holding the polar shaft, on top of which, a housing for the declination shaft and bearings was attached. I managed to acquire a 12" diameter wheel, with a matching worm drive. It came from a film developing unit, used for aerial film development.

It wasn't very well machined, and the reason my astrophotography on the whole was a miserable failure. DSO's at least.

The wheel was clutched to the polar shaft, so I could slew the scope manually, without damage to the worm drive.

Onto the wheel, I mounted a 10" diameter RA Circle, which would rotate East to West with the scope when driven, but could be slipped independently.

With a pointer fixed over the circle, representing my North South line, or local meridian, and a pointer attached to the polar shaft extending to the east side and across to the west side, and depending on which side of the meridian my scope was operating, that side pointer was the one I would use for indexing.

Setting the declination circle was easy of course. Once star drifting had established the polar axis was spot on, the circle could be adjusted via a star of known declination. Once locked, it rarely had to be adjusted. Once the drive was switched on, and running at sidereal rate, it was a matter of locating a star, setting the RA circle to its coordinates, by slipping the circle to that hour, minutes.

The North South pointer now indicated my local sidereal time, the pointer to the west or east, pointed to the star I had chosen as the reference at the start, be it west or east, one or the other.

Now all I needed to do to locate another object, was to look at my Nortons, or other Chart for the coordinates of my next target. Turn the scope to its declination, and manually slew the scope to the RA of the chosen object. East or West of the meridian. As long as the drive was left to run, direct indexing of any object could be done.

It was never spot on all the time, but even if slight panning was required, I would find the target.

It was not the most sophisticated build you would ever see, but it did work, and finding fuzzies was never a problem for me.

Most of the bits for this construction, came from scrap dealers yards. Amazing what can be found. RS Components was I spent the most money.

Hope I haven't bored the life out of anyone with this.

Just part of my earlier years pursuing this great life of astronomy.

Ron.

[adamsp123]

The development of time pieces that could work at sea was an Historic moment in travel by sea by allowing relative accurate measure of longitude, of course north of the equator latitude was easy, just measure the angle of the polaris = latitude - pretty accurately.

But remember the sea going clock was only one piece of the puzzle, without accurate measurement of stars RA with respect to the first point of Aries by use of the transit telescopes at Greenwich Observatory, sea chronometers would have be useless.

As a matter of fact the ships chronometer was never altered to read correct time, a log was kept of the chronometers error with respect to GMT, it was also wound up by a fixed number of turns and done at the same time in "ships time" ie say a 1200 by the chronometer.

When taking star/sun/lunar sights the time was recorded using a "deck watch" as the ships chronometer was never moved and any error between deck watch and chronometer was recorded and adjusted when calculating the ships position.

A nerdy piece of info? sorry!

pete

[Barry W]

Good Stuff!

Pete's absolutely right. Time was only one piece of the puzzle, but it was the final piece to fit! By that time there was already a significant amount of information gathered on the positions of the stars and movements of solar system objects. The addition of accurate timekeeping made the navigation that much more precise. I remember the ship's chronometer (we held two in case one broke) and our "deck watches" were our own watches checked for error every day against the ships clock in GMT . We had to account for every know error and difference in the calculations.

Unfortunately nowadays, everything is GPS based with UTC being the standard. It's still the same thing though - your position is plotted by knowing the precise time and distance from multiple satellites. (maybe I'm even more a a nerd about this, but it's sad that astro nav is becoming more of a lost art!)

Cheers,

Barry

[Paladin]

I don’t want to drag this intellectual thread down too far, but wasn’t it Dell Trotter in Only fools & Horses who found the missing Harrison, to become a millionaire?

I’ll get my coat!

Paul

[Sam]

Longitude is a great book and what a fascinating subject. Hard to believe that something we take for granted now was so key to navigation and gave the nation that dreamed it up first unprecedented advantage in both trade and naval power. Like Carol said the H clocks in Greenwich are amazing to see and well worth the visit if you're in the area - and it's free!

Sam

[Rosanella]

Yes, I read this little book in a day and thoroughly enjoyed it. Then took my daughter and a friend to Greenwhich to see Harrison's clocks a few days later. I had to see them!

Quite a feat for Harrison that showed that the lunar distance method of longitude supported by the 5th astronomer royal Maskelyne, could be surpassed by a mechanical device. And quite a defeat for Maskelyne who opposed Harrison without pose. What a battle!

Brilliant little book :-) and Deva Sobel's style of writing is always very enjoyable.