Geoscience Reference
In-Depth Information
Fig. 4
An eclipse of one of Jupiter's satellites is viewed from Paris and from somewhere else
on the Earth, X. At the moment when the eclipse happens, Jupiter is seen in different locations
in the sky, and the eclipse occurs at different times of the day at the two places. The difference
between the local times corresponds to the longitude difference between Paris and X
calculated and observed by the Italian astronomer Gian Domenico Cassini, who had
become the director of the Paris Observatory. The natural clock of the motion of the
satellites can be used to regulate a more convenient but less accurate artificial clock
that can be used to observe the positions of stars and to determine longitude (
Fig 4
).
These techniques were used not only to lay out the north-south meridian but also to
determine by what longitude a place was offset from the meridian.
