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The lunar distance method

The lunar distance method

A black and white portrait of a man dressed in clothing from the eighteenth century. He sits facing to the right of the image, with his head turned toward the viewer. He wears a dark coat, dark vest and a light coloured undershirt. His hair appears to be a wig in the popular style of the time, with rolls above each ear. His expression is neutral and detached. Behind him is a curtain that hangs obliquely, from right to left. Beyond the curtain is part of what appears a painting studio backdrop, that shows open land and what may be a building on a distant hill. The portrait is in an oval format. Under it is written 'Dr Maskelyne IRS, Astronomer Royal', in ornate script.

In the 1750s, German astronomer and map-maker Tobias Mayer devised a lunar distance method for finding longitude at sea. Sailors used a sextant to measure the angle between the moon and a star to establish the time in Greenwich, England, and then compared it with the local time on board ship. This required very precise observations of the sky, only possible following the invention of the achromatic telescope.

English Astronomer Royal Nevil Maskelyne was a keen advocate of the lunar method over its rival claimant to the Board of Longitude's prize, John Harrison's marine chronometer. Mayer's widow received £3000 for her husband's work.

Left: Portrait of Nevil Maskelyne, 1804. Courtesy: The Print Collector.

 

When James Cook sailed for the Pacific in 1768 he was carrying a nautical almanac containing tables showing the positions of celestial bodies for the following year. The tables had been feverishly prepared by Maskelyne's team of scientists to help navigators complete the time-consuming longitude calculations.

Diagram showing the angles and distances measured in the lunar method.

This diagram shows the angles and distances measured in the lunar method.
Courtesy: Based on an image by Michael Daly.

A sextant sitting upon a raised, clear plastic display base. The sextant has three 'legs', two short ones at each end and a longer one at the front toward the viewer. The long leg protrudes down through the display base. The sextant body is made up of a horizontal flat section upon which sits another flat section. On that part are vertical cylindrical and square sections. Above the long leg at the front rests a lense with an aperture on the end closest to the viewer. Behind the lense is a group of flat coloured glass lenses that apppear to be mounted so each one can be moved into position in front of the viewing lense. Much of the sextant is made from brass. The display base and the sextant sit in front of a featurless white backdrop. Shadows under the sextant suggest a light source off to the right-hand side of the image, beyond the frame.

Captain James Cook used this sextant, made by Dollond, London, about 1770.
Courtesy: State Library of New South Wales