The velocity of light
 
In 1675, the Danish astronomer Roemer had measured the velocity of light by noting variations in the times of eclipses of Jupiter’s satellites. Galileo himself had observed on many occasions that the satellites he had discovered disappear into the shadow cast by Jupiter and had suggested that, by constructing tables of the eclipse times, they could be used as astronomical clocks for finding ships’ longitudes at sea. The disappearance or reappearance of a satellite could be looked upon as a light signal. If the velocity of light was infinitely large, these light signals would occur at regular intervals since the orbits of the satellites about Jupiter were very nearly circular.
Roemer found that the eclipses did not take place at absolutely regular time intervals. All four satellites’ eclipses were sometimes early and sometimes late. He also noticed that the amounts by which they were early or late depended upon the positions of Jupiter and the Earth: eclipses were earliest when Jupiter was nearest the Earth and latest when Jupiter and the Earth were on opposite sides of the Sun. Roemer concluded correctly that the discrepancies were due to the velocity of light being finite, so that the light signal indicating the beginning or end of an eclipse took time to cross space from the vicinity of Jupiter to the Earth. This time varied because of the changing Earth–Jupiter distance. The maximum discrepancy between the earliest and latest eclipse times was then the time it

Figure 1. The effect of aberration.
took light to cross the diameter of the Earth’s orbit. Roemer, from a rough knowledge of this distance and the time interval, calculated the velocity of light, obtaining a value not far removed from the modern value of 299 792·5 km s⁻¹.