Longitude

To define the precise position of a point on the earth's surface, we express it by means of latitude, longitude and altitude. The first two are angular measures, and the third is a linear measure (i.e. the height above sea level).

Determining longitude of two points on the earth's surface requires that the time be accurately known at both points. All that is then required is a comparison of the two times at any moment.

Today, the difference in time between any two places can easily be discovered. But until the development of accurate marine chronometers in the late eighteenth century, mariners sought to establish their longitude by determining the angular distance between the moon and certain stars, or the sun.

From 1767 this task was made easier by the publication of the first edition of the Nautical Almanac, authorised by Nevil Maskelyne (1732-1811), the fifth Astronomer Royal. This book contained tables of predictions by the Royal Greenwich Observatory of the moon's position relative to nine conspicuous stars and the sun. The tables showed the moving moon's angular distance from these targets at intervals of three hours, Greenwich time, throughout the year.

After using a sextant to measure the moon-star angle as seen in his own sky, Cook could then match the angle against the appropriate table to determine Greenwich time. He could then compare Greenwich time with his own local time by using a sextant to measure equal altitudes of the morning and afternoon sun.

• O'Conner and E.F. Robertson, Longitude and the Acad'mie Royale, 1997, http://www-gap.dcs.st-and.ac.uk/~history/HistTopics/Longitude1.html. [ Details ]