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
Newtonian gauge Newtonian gauge In general relativity, Newtonian gauge is a perturbed form of the Friedmann–Lemaître–Robertson–Walker line element. The gauge freedom of general relativity is used to eliminate two scalar degrees of freedom of the metric, so that it can be written as where the Latin indices "a" and "b" are summed over the "spatial" directions and formula_2 is the Kronecker delta. Conformal Newtonian gauge is the closely related gauge in which which is related by the simple transformation formula_4. These metrics are perturbed forms of the Friedmann–Lemaître–Robertson–Walker metric. They are called Newtonian gauge because formula_5 is the Newtonian gravitational potential of