The influence of the Royal Observatory at Greenwich upon the design of 17th and 18th century angle-measuring instruments at sea

Abstract

When Charles II founded his Royal Observatory in Greenwich Park in 1675, position finding at sea was not much advanced on the techniques of 200 years earlier when the Portuguese began the art of ocean navigation. It was possible to find a ship's latitude reasonably easily to within 20 or 30 minutes but the business of finding longitude still seemed an insoluble problem. The answer was thought to lie in the lunar-distance method put forward by Werner in 1514, but for this method to be successful, not only was it necessary for the Moon's position to be predicted to within 1 minute of arc several years in advance, but also the fixed stars upon which the method depended had to be observed to a like accuracy. Greenwich Observatory was established with the object of finding a solution to the longitude problem, but of necessity Flamsteed concentrated his early efforts on the accurate observation and plotting of star positions. The alternative means of solving the longitude question was the meckanick's answer, a clock that would keep time under seagoing conditions and temperatures. In this way a Standard Time could be carried about with which to compare the ship's local time — the difference in time being the longitude. It is not generally realized that neither of these methods could be of much use to a seaman unless he were equipped with a sufficiently accurate instrument with which to observe the exacting measurements required. The Lunar-distance method required distances between the Moon and Sun or a star to be observed, as well as the altitudes of the bodies concerned, before the computation could begin. The chronometer method needed an accurate instrument for an altitude sight to determine local time to be taken when the Sun was moving at its most rapid rate near the Prime Vertical. No instrument of this capacity existed when the Greenwich Observatory was founded in 1675, for British seamen were still using the backstaff, invented by Captain John Davis about 1594, and the cross staff, first used at sea about 1510. Neither of these instruments was capable of consistently measuring a celestial angle to an accuracy of ± 10 minutes of arc due to inherent defects in their design, though their scales were graduated to 90° with a theoretical accuracy of 1 minute of arc.