548

HISTORY OF PHYSICAL ASTRONOMY.

afforded a more favourable chance of detecting the parallax of a fixedstar, than that founded on observations of the absolute position of thestar in the celestial sphere. It does not appear that Galileo attemptedon any occasion to reduce it to practice.

Hooke first employed the telescope in observations for the purpose ofdetecting the annual parallax of the fixed stars. In order to avoid theeffects of refraction, he confined his observations to the star y Draconis,which passes near the zenith of Gresham College, London , where hisobservatory was established. From four observed zenith distances of thestars, as determined by him in the months of July, August, and Octoberof the year 1669, with a zenith sector 36 feet long, he concluded thatthe star had an annual parallax, amounting to somewhere between 27"and 30"*. It is to be remarked, however, that Picard, a few years after-wards, observed the altitude of a Lyroe at both solstices, but was unableto detect the slightest trace of parallax f.

Previous to the discovery of the Aberration of Light, the displacementdue to that principle exhibited itself in the observations of severalastronomers, who, in some instances, were induced erroneously to at-tribute it to the effects of parallax. In all probability the variations inthe apparent place of y Draconis, observed by Hooke, proceeded fromthis cause. This was certainly the case with respect to Flamsteed, who,from a series of observations of the zenith distance of the polar star,made by him at Greenwich with the mural arc, between the years 1689and 1697, concluded that the star possessed an annual parallax equalto at least 40" J. J. Cassini shortly afterwards remarked, with justness,that the displacement indicated by the observations of Flamsteed, wasincompatible with the supposition of its being produced by parallax §.It has been already mentioned, that the variations in the position ofthe pole star, detected by Flamsteed on this occasion, were, in reality,the results of aberration, the maximum value of which is deduciblewith an astonishing degree of accuracy from the observations of thatastronomeij|. It may be mentioned, that, about the same time, Wallissuggested observations of the greatest azimuth, east or west of a circum-polar star, as favourable for the detection of parallax, since the effect ofrefraction would thereby be completely avoided If. It does not appear thatany astronomer attempted to reduce this method to practice.

Horrebow states that, in the years 1692-3, Roemer remarked a seriesof irregularities in the declinations of the stars, which could not beaccounted for either by parallax or refraction, but which he suspectedto arise from a variation in the position of the terrestrial axis, the theoryof which he hoped to assign on some future occasion**. It cannotadmit of any doubt that the anomalies observed by that distinguishedastronomer were attributable to the effects of aberration which hithertocontinued to be unaccounted for. Dismissing the declinations as not suffi-ciently trustworthy in so delicate an enquiry, Roemer attempted to deducethe parallax of the fixed stars from observations of their right ascensions.By a series of observations of Sirius and a. Lyrce, made at differentseasons of the year, he found that the aggregate of the parallaxes of thetwo stars produced an annual variation in the difference of their right

* “ Attempts to Prove the Motion of the Earth, ” p. 7.

f Lemonnier, Hist. Celeste, p. 252. | Wallis’s “ Opera Mathematica,”tom. iii.,p. 705.

§ Mem. Acad, des Sciences, 1699, p. 177. || See chapter xiv.

^ Phil. Trans., 1093, p. 844. * * Basis Astronomise, p. 66.