being fully 15 minutes behind its calculated time. Its appearance,
moreover, had been growing gradually later, as the earth retreated
towards the part of its orbit most distant from Jupiter. Roemer
reasoned thus: 'Had I been able to remain at the other side of the
earth's orbit, the moon might have appeared always at the proper
instant; an observer placed there would probably have seen the moon 15
minutes ago, the retardation in my case being due to the fact that the
light requires 15 minutes to travel from the place where my first
observation was made to my present position.'

This flash of genius was immediately succeeded by another. 'If this
surmise be correct,' Roemer reasoned, 'then as I approach Jupiter
along the other side of the earth's orbit, the retardation ought to
become gradually less, and when I reach the place of my first
observation, there ought to be no retardation at all.' He found this
to be the case, and thus not only proved that light required time to
pass through space, but also determined its rate of propagation.

The velocity of light, as determined by Roemer, is 192,500 miles in a
second.

For a time, however, the observations and reasonings of Roemer failed
to produce conviction. They were doubted by Cassini, Fontenelle, and
Hooke. Subsequently came the unexpected corroboration of Roemer by the
English astronomer, Bradley, who noticed that the fixed stars did not
really appear to be fixed, but that they describe little orbits in the
heavens every year. The result perplexed him, but Bradley had a mind
open to suggestion, and capable of seeing, in the smallest fact, a
picture of the largest. He was one day upon the Thames in a boat, and
noticed that as long as his course remained unchanged, the vane upon