ultimately made by an astronomer. It was observed in 1675 by Olof Roemer[1] (b. 1644, d. 1710) that the eclipses of the first satellites of Jupiter were apparently affected by an unknown disturbing cause; the time of the occurrence of the phenomenon was retarded when the earth and Jupiter, in the course of their orbital motions, happened to be most remote from each other, and accelerated in the contrary case. Roemer explained this by supposing that light requires a finite time for its propagation from the satellite to the earth; and by observations of eclipses, he calculated the interval required for its passage from the sun to the earth (the light-equation, as it is called) to be 11 minutes.[2]
Shortly after Roemer's discovery, the wave-theory of light was greatly improved and extended by Christiaan Huygens (b. 1629, d. 1695). Huygens, who at the time was living in Paris, communicated his results in 1678 to Cassini, Roemer, De la Hire, and the other physicists of the French Academy, and prepared a manuscript of considerable length on the subject. This he proposed to translate into Latin, and to publish in that language together with a treatise on the Optics of Telescopes; but the work of translation making little progress, after a delay of twelve years, he decided to print the work on wave-theory in its original form. In 1690 it appeared at Leyden,[3] under the title Traité de la lumière où sont expliquées les causes de ce qui luy arrice dans la réflexion et dans la réfraction. Et parti-
- ↑ Mént, de l'Acad. 1. (1666-1699), p. 575.
- ↑ It was soon recognized that Roemer's value was too large; and the astronomers of the succeeding half-century reduced it to 7 minutes. Delambre, by an investigation whose details appear to have been completely destroyed, published in 1817 the value 498·2s, from a discussion of eclipses of Jupiter's satellites during the previous 150 years. Glasenapp, in an inaugural dissertation published in 1875, discussed the eclipses of the first satellite between 1848 and 1870, and derived, by different assumptions, values between 496s and 501s, the most probable value being 600·8s. Sampson, in 1909, derived 498·64s from his. own readings of the Harvard Observations, and 498·79s from the Harvard readings, with probable errors of about ±0·02s. The inequalities of Jupiter's surface give rise to some difficulty in exact determinations.
- ↑ Huygens Lad by this time returned to Holland.
