Galilean moons century when other inner moons were discovered, and Marius' names became widely used. Galileo was able to develop a method of determining longitude based on the timing of the orbits of the Galilean moons. The times of the eclipses of the moons could be precisely calculated in advance, and compared with local observations on land or on ship to determine the local time and hence longitude. The main problem with the technique was that it was difficult to observe the Galilean moons through a telescope on a moving ship; a problem that Galileo tried to solve with the invention of
Reflecting instrument presented his reflecting quadrant in 1731. Halley noted that Hadley's design was quite similar to the earlier Newtonian instrument. As a result of this inadvertent secrecy, Newton's invention played little role in the development of reflecting instruments. What is remarkable about the octant is the number of persons who independently invented the device in a short period of time. John Hadley and Thomas Godfrey both get credit for inventing the octant. They independently developed the same instrument around 1731. They were not the only ones, however. In Hadley's case, two instruments were designed. The first was an instrument very similar