Maskelyne v Harrison – the longitude show in London

Harrison's third clock
Harrison’s third clock


We finally managed to take in the excellent Ships, Clocks and Stars exhibition at the National Maritime Museum, Greenwich, which runs only until 4 January. Among other things, it puts the record somewhat straighter on the eminent 18th century Astronomer Royal, Nevil Maskelyne, chief villain of Dava Sobel’s best selling nautical history of the race to measure longitude at sea. Maskelyne was the man who conspired against John Harrison, the genius who built the first true chronometer, or so the narrative goes in Sobel’s book, Longitude.

Not so, it is clear from the detailed story set out in the exhibition, which of course includes the famous clocks, which are extraordinary creations. It seems the rival method using the moon to measure time (and thus longitude) worked perfectly well, though not as accurately. Far from being biased against Harrison in favour of the lunar method, Maskelyne seems to have been rather even handed, encouraging rigorous trials of both.

In the trials, the lunar method turned out to be good to a degree, or 60 miles, which was a lot better than most navigators had been able to achieve before on long passages, but chronometers could reduce the error to as low as 10 miles. By producing an accurate timepiece that could withstand violent movements and large temperature changes over many months, Harrison revolutionised navigation, made it possible for sailors to know reliably where they were, and thus saved countless lives and cargoes from being wrecked.

Both the chronometer method and the lunar method required measurements by sextant followed by calculations using detailed astronomical tables. One of Maskelyne’s great achievements was to set up a system for producing the annual nautical almanac, which is still published today. The calculations were done by computers, a term which in those days referred to people. In fact it was an attempt to automate the calculations in the early 19th century that led Charles Babbage to design the first mechanical computer, a small part of which is in the exhibition. The attempt failed because the costs of building the machine escalated out of control.

Harrison also seems to have been a bit of a curmudgeon, falling out with Maskelyne when pressure was put on him to demonstrate that his watch design could be reproduced by others for more general use at sea. Some of his rewards were held back until he published and publicly explained the details of the clock’s construction and made two copies. In the end, he won the argument by appealing to Parliament, which awarded him more than had been promised originally.

The fourth clock, dramatically smaller.

There was much more to discover in the exhibition. I was particularly interested in the method of finding the time and thus the longitude by observing the moons of Jupiter, which proved impossible to do on a heaving ship, but was accurate on land. This giant solar system clock was used to redraw the maps of Europe. When the French Atlantic coastline proved to be much further east than earlier maps had shown – shrinking the country’s area significantly – the King of France remarked that the cartographers had stolen more land from him than all his enemies combined.

 The new book, published this year to coincide with the exhibition, is Maskelyne, Astronomer Royal, edited by Rebekah Higgitt (one of the curators).

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