D Vautier
6/2019
When Galileo first discovered the moons of Jupiter in 1609 it occurred to him that this could be a good way to tell time. After all Io, the closest moon was screaming around the planet at an incredible rate of a little less than two days (actually 1.769137786 days). In fact Io does make a very good timekeeper if there is a need to determine rates between sightings but the planet’s distance from earth varies so much that over the course of many years it is hard to use these movements to tell Greenwich time.
The orbit of Jupiter changes from 460 to 508 million miles from the sun. The orbit of earth varies also. Jupiter can be anywhere from 365 to 601 million miles from earth. So the distance to Jupiter can very by as much as 236 million miles. It takes light about 44 minutes to travel that far so to use the moons of Jupiter to keep Greenwich time was not very practical.
Nevertheless in 1675 Ole Roemer did calculate the speed of light based on the moons of Jupiter. He did this by estimating that the orbit times did not change but the time it took light to get to earth when it was far from Jupiter compared to when earth was close to Jupiter could measure the speed of light. Roemer was not off too much.
But there continued to be efforts to calculate Greenwich time based on the positions of the moons of Jupiter. Some people did try to calculate the position of the earth in its orbit and the distance to Jupiter, but not successfully.