Astronomy Bio...Nevil Maskelyne

Jay Bitterman

Nevil Maskelyne was born in London on October 6, 1732. He was an influential British astronomer, physicist and, like so many others of his time, a priest. He was the founder of the Nautical Almanac and he became Astronomer Royal at age 32.

In 1754 he received his bachelor's degree in divinity from Westminster School, Trinity College, Cambridge. Although ordained a year later he decided to become an assistant to James Bradley at the Greenwich Observatory. It was probably his early interest in solar eclipses that led Maskelyne into his career as an astronomer. In 1756 he was made a Fellow of the Royal Society. In 1757 he was awarded his master's degree from Trinity College and elected a Fellow of the College. In 1760 he became the fifth Director of the Greenwich Observatory and Astronomer Royal and continued to carry out his clerical duties in such parishes as Shrawardine, Shropshire, and North Runcton, Norfolk.

Under the auspices of the Royal Society, he and R. Waddington undertook his first major observational project by going to the island of St Helena to observe the 1761 transit of Venus. They wanted to determine accurately the distance of the Earth from the Sun. Unfortunately, at the appropriate moment the weather turned bad and, in any case, he lost confidence in his instruments. He missed the 1769 transit again because it was not until 1772 that Maskelyne perfected his technique for observing transits. Nevertheless his sea journey to and from St Helena stimulated an interest in marine navigation by astronomical methods and he spent a considerable amount of effort trying to devise a better means of determining longitude at sea.

In 1767 Maskelyne founded the Nautical Almanac. This contained a compendium of astronomical tables and navigational aids that included many of the results of Maskelyne's studies of the Sun, the Moon, the planets and the stars. His observations of the proper motions of several of the brighter stars were used by William Herschel to demonstrate the movement of the Sun, which until 1783 had been presumed stationary.

Maskelyne's experiment on plumb line deflection in 1774 aroused great interest among fellow geodeticists. It was one of many attempts to determine the gravitational constant and to solve Newton's gravitational equation, and thus deduce the density of the Earth. To measure the gravitational effect that an isolated mountain might exert, he traveled to Schiehallion, a mountain in Perthshire. He determined the latitude both north and south of the mountain both by using a plumb line and by direct survey. He found that the mountain's mass between the two points of measurement caused the plumb line to be deflected so that the separation of the points was 27% greater than was found by direct geographical measurement. He made certain assumptions about the mass and volume of the mountain. Maskelyne deduced from the magnitude of the plumb line's deflection a gravitational constant and came to the conclusion that the Earth had a density of between 4.56 and 4.87 times that of water. This was reasonably close to the value now accepted (approximately 5.52 times that of water).

Maskelyne's work in astronomy contributed to a number of fields of study, but perhaps his most enduring contribution was the establishment of the Nautical Almanac. In 1775 he was awarded the Royal Society's Copley Medal. In 1777 he received a doctorate in divinity from Cambridge University. In 1802 he was elected to the French Academy of Sciences. He died in Greenwich on February 9, 1811.

Published in the October 2000 issue of the NightTimes