In recognition of Anthony’s outstanding animation of Europa and Ganymede, can I suggest that we present him with the inaugural
RAPMO trophy; aka the “2009 Römer Award for Planetary Moon Occultations".
In 1676,
Ole Römer, a Danish astronomer working at the Paris Observatory, made the first real measurement of the speed of light. He made a systematic study of Io, one of the moons of Jupiter, which was eclipsed by Jupiter at regular intervals, as Io went around Jupiter in a circular orbit at a steady rate.
Römer found that for several months the Io eclipses lagged more and more behind the expected time, but then they began to pick up again. In September 1676, he correctly predicted that an Io eclipse on November 9 would be 10 minutes behind schedule. He explained that as the Earth and Jupiter moved in their orbits, the distance between them varied. The light from Io took time to reach the earth, and took the longest time when the earth was furthest away. When the Earth was furthest from Jupiter, there was an extra distance for light to travel equal to the diameter of the Earth’s orbit compared with the point of closest approach.
Cheers
Dennis