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The telescope, measurement of time and longitude

6.3 - Astronomers vs. mechanics

Determining the distance of the Moon from a star (Petrus Apianus, Introductio Geographica, 1553) Anonymous, Portrait of Isaac Newton, 18th century (Galleria degli Uffizi, Florence, Jovian Collection, no. 298)

Possessing a method for determining longitude at sea would bring significant economic advantages, such as shortening ocean routes and avoiding shipwrecks. Not by chance, many governments offered rewards for devising a method that would be precise to the degree (approx. 100 km at the equator).

The method Galileo proposed to Spain and The Netherlands was a hybrid one, both astronomical and mechanical. Other scientists preferred purely astronomical methods. Instead of recurring to Jupiter's satellites, Newton tried to perfect the "lunar distances method". This consisted of preparing tables predicting the positions of the Moon in relation to the stars. Having measured the angular distance of the Moon from a star, a navigator could determine the time of the reference place in the tables. Unfortunately, the nearness of the Moon to the Earth introduced errors of perspective in the observations due expressly to the observer's longitude.

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