this angle is to the sine of the visible inclination of the object to the line in which the eye is moving, as the velocity of the eye is to the velocity of light. Observations such as Bradley's will therefore enable us to deduce the ratio of the mean orbital velocity of the earth to the velocity of light, or, as it is called, the constant of aberration; from its value Bradley calculated that light is propagated from the sun to the earth in 8 minutes 12 seconds, which, as he remarked, "is as it were a Mean betwixt what had at different times been determined from the eclipses of Jupiter's satellites."[1]
With the exception of Bradley's discovery, which was. primarily astronomical rather than optical, the eighteenth century was decidedly barren, as regards both the experimental and the theoretical investigation of light; in curious contrast. to the brilliance of its record in respect of electrical researches. But some attention must be given to a suggestive study[2] of the aether, for which the younger John Bernoulli (b. 1710, d. 1790). was in 1736 awarded the prize of the French Academy, His: ideas seem to have been originally suggested by an attempt[3]
- ↑ Struye in 1845 found for the constant of aberration the value 20″·445, which he afterwards corrected to 20″·463. This was superseded in 1883 by the value 20″·492, determined by M. Nyrén. The observations of both Struve and Nyrén were made with the transit in the prime vertical. The method now generally used depends on the measurement of differences of meridian zenith distances (Talcott's method, as applied by F. Küstner, Beobachtungs-Ergebnisse der kön. Sternwarte zu Berlin, Heft 3, 1888): the value at present favoured for the constant of aberration is 20″·523. Cf. Chandler, Ast. Journal, xxiii, pp. 1, 12 (1903).
The collective translatory motion of the solar system gives rise to aberrational terms in the apparent places of the fixed stars; but the principal term of this character does not vary with the time, and consequently is equivalent to a permanent constant displacement. The second-order terms (i.e. those which involve the ordinary constant of aberration multiplied by the sun's velocity) might be measurable quantities in the case of stars near the Pole; and the same is true of the variations in the first-order terms (i.e. those which involve the sun's velocity not multiplied by the constant of aberration) due to the circumstance that the star's apparent R. A. and Declination, which occur in these terms, are not constant, but are affected by Precession, Nutation, and Aberration. Cf. Seeliger, Ast. Nach., cix., p. 273 (1834).
- ↑ Printed in 1752, in the Recueil des pièces qui ont remporter les prix de l'Acad., tome iii.
- ↑ Aeta eruditorum, mdcci, p. 19.
