Did Joseph Jackson and John Morgan make Sextants before John Bird?

摘要

John Hadley presented his design of the octant to the Royal Society in 1731, it was a double-reflecting instrument to measure the altitude of a celestial body above the horizon at sea. The first two examples were made by Hadley himself and by Johnathan Sisson (c.1690-1747); they were tried out in 1732, and proved to be a success. The octant was compact, easy to handle on the unsteady deck of a ship, and more reliable than the commonly used cross-staff and backstaff. During the sea trials Hadley also showed that it could be used to accurately measure the distance in degrees and minutes of arc between two celestial bodies, which meant it could be used for the method of finding longitude by lunar distances. The theory of this method had long been known, but the lack of an accurate instrument for the observation (for which neither cross-staffs nor backstaffs were suitable), and lunar tables for the calculation, had prevented it from coming into use. The invention of the octant was the first step towards the practical use of this method, the lunar tables submitted to the Admiralty by Tobias Mayer in the mid-1750s were the second.