Edmond Halley set out to sea to use magnetism as a possible solution to the problem of determining longitude.

It is over 300 years since Edmond Halley, the second Astronomer Royal, completed two voyages on the ship Paramour to investigate the variation between true and magnetic north and the possibility of using the difference to find longitude at sea.

Lodestone

From the 12th century in Europe, lodestone, or magnetite (Fe304), a naturally occurring magnetic ore, was used to magnetise mariners' compass needles. On longer voyages it was necessary to carry a piece of lodestone on board to remagnetise the needles. Until the late 18th century, compass needles were made of soft iron and so would lose magnetism relatively quickly.

The 15th and 16th centuries saw an increase in exploration and international trade and as a result the compass became more important as mariners travelled further afield. It was especially important for oceanic voyages in cloudy weather.

Edmond Halley

Almost 100 years after Gilbert published his famous book on magnetism, Edmond Halley (1656–1742), already an established astronomer and natural philosopher, set sail in command of the ship Paramour to investigate a possible solution to the longitude problem.

By this stage in his career, Halley, now aged 42, had already mapped the stars of the southern hemisphere from the island of St Helena, worked for Flamsteed at the Royal Observatory and been instrumental in the publication of Isaac Newton's Prinicipia.

In 1698, in the hope that Halley might be able to solve the longitude problem, the Royal Navy made him a captain and he set sail on the Paramour to chart the variation of the Earth's magnetic field in the western and eastern hemispheres. As it turns out, the variations fluctuate with time and cannot be used to find longitude at sea.

The compass

Though not useful in determining longitude, the compass continued to be important in navigation. In design it changed little between around 1550 and 1750. The azimuth compass was a step up from the standard mariners compass. These compasses incorporated a means of aligning the compass with a celestial body such as the Sun or the Pole star. The reading from this alignment would then give another reading for north which could be compared with that given by the compass needle thus allowing the variation to be easily read.

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