16 August 2005

Book Review: Longitude (2005), by Dava Sobel, *****

 (Testo italiano di seguito)

 Synopsis

Sobel has done the impossible and made horology sexy – no mean feat

New Scientist

Anyone alive in the 18th century would have known that ‘the longitude problem’ was the thorniest scientific dilemma of the day – and had been for centuries.

Lacking the ability to measure their longitude, sailors throughout the great ages of exploration had been literally lost at sea as soon as they lost sight of land. Thousands of lives, and the increasing fortunes of nations, hung on a resolution.


Review

This is a very well written account of the efforts of a man to solve the most difficult navigation problem of all times. Before the era of GPS, mariners had to find latitude and longitude with on board instruments. Latitude was easy, one just had to measure the angle of the sun or stars over the horizon. But that measurement said nothing about longitude, the other indispensable parameter to fix a position at sea. The only way to measure that was to match latitude and time.

Knowing what time it was both at a place of known longitude (the port of departure for example) and at the ship's location would allow a mariner to calculate the difference and fix a position. At any given latitude, one hour means exactly a precise distance, which varies from 15 miles at the equator to zero at the poles.

But onboard clocks were tricked by temperature, pressure, magnetic fields, the ship's rolling and pitching, changes on the earth's gravity. It was John Harrison who first build a watch to measure time precisely at sea. This book is the fascinating story of how he did it.

You can read more about the author here.

Buy your book here:


Buy your DVD on the film based on this book here




Ottimo e avvincente libro della storia di John Harrison e dei suoi 4 orologi. Prima dell'era dei GPS, i marinai dovevano calcolare la latitudine e longitudine con gli strumenti di bordo. La latitudine era facile, bastava misurare l'altezza del sole sull'orizzonte. Ma quell'altezza non diceva nulla sulla longitudine se non accoppiata all'ora esatta. Infatti ad ogni ora di fuso orario corrispondono 15 miglia all'equatore e via via sempre meno andando verso i poli. Facendo la differenza con l'ora in una citta di longitudine conosciuta, e avendo in mano il dato della latitudine, si poteva calcolare la longitudine.

Senza sapere l'ora, il marinaio non può sapere su quale meridiano si trovasse la nave. Gli orologi del tempo però erano imprecisi e in mare soffrivano di cambiamenti di temperatura, gravità terrestre, pressione barometrica, umidità e del rollìo e beccheggio della nave. John Harrison costruì i primi orologi che, accuratissimi in mare in tutte le condizioni di navigazione, permisero di fare il "punto nave".

Il libro è scritto in modo avvincente e fa appassionare a quello che potrebbe essere visto come un noioso problema scientifico!

Compra la traduzione italiana qui:


In the US buy your copy here

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