They admirably illustrate the total ignorance of Galileo concerning a problem upon which he experimented with utter failure, as also the slow pace of scientific progress, and the necessity of the co-operative efforts of many men and many sciences to perfect it.
It required the genius and research of Roemer, Bradley, Molyneux, Arago, Fizeau, Foucault, and Struve, joined to the patient experiment and mechanical skill of Bréguet, Bessel, and Graham—the labor of all these men extending through a period of one hundred and ninety years (1672 to 1862)—to complete its demonstration.
And first, as to the velocity of light. In 1672, Roemer, a Danish astronomer residing in France, began observations on the satellites of Jupiter and their eclipses, which resulted in the discovery of progressive transmission of light and the determination of the value of its velocity. Up to his day, it had almost become a fixed principle that the passage of light through space was absolutely instantaneous.
From the time of Galileo, an immense mass of exact calculations of the eclipses of the first satellite of Jupiter had been accumulating, and Roemer found that at certain times the satellite came out of the shadow later, and at other times sooner, than it should have done, and this variation could not be accounted for on any known principles. Remarking that it always came too late from the shadow when the earth in its annual movement was at more than its mean distance from Jupiter, and too soon when it was at less, he formed the conjecture that light requires an appreciable time to traverse space.
Becoming satisfied of the truth of his theory, he, in September, 1676, announced to the French Academy of Sciences that an emersion of the first satellite, to take place, on the 16th of November following, would occur ten minutes later than it should according to ordinary calculation.
The event verified his prediction. Nevertheless, doubters and cavillers abounded, and Roemer's theory was not accepted without dispute. It was claimed that the delays and accelerations in the immersions and emersions, instead of being attributed to change of position of the observer, and to the progressive transmission of light, might be regarded as indicating a real perturbation in the movement of the satellite, due to a cause not yet discovered.
These doubts were removed fifty years later by the English astronomer Bradley, who discovered the phenomenon of aberration, which consists in an apparent displacement which all the stars and planets experience on account of the combination of the velocity of the earth with the velocity of light.
Bradley's discovery was accidental. A superior instrument, constructed by Graham, and destined to observe with the greatest precision the passage of the stars near the zenith, had been placed at the observatory of Kew for the purpose by Molyneux.
Bradley used this instrument to arrive at some precise data of the annual parallax of the stars. His first observations led him to the discovery of aberration, the details of which, of the highest possible interest, may be found in the Philosophical Transactions, Royal Society, No. 406, December, 1728.