Parallax, the apparent displacement of a heavenly body arising from a change of the observer's position. The angle subtended at the body by the line joining the two stations is the measure of the parallax. As the positions of the heavenly bodies have reference in practical astronomy to the earth's centre, a correction for parallax is necessary in every observation, except when the body is in the zenith, where the parallax vanishes. It is greatest in the horizon, and is there termed horizontal parallax. It is manifestly equal to the angle subtended by the earth's radius as supposed to be seen from the body, as the earth's radius varies with the latitude, and the equatorial radius is commonly selected as the measure of parallax. By the mean horizontal equatorial parallax of the moon, for instance, is understood the angle subtended by the earth's equatorial semi-diameter at the moon's mean distance. The same is the case with the sun. And even if the word. equatorial be omitted, it is to be understood that equatorial parallax is signified unless the contrary be implied. The parallax and the sine of the parallax are appreciably equal for all objects except the moon, and either is used indifferently.
In the case of the moon there is a difference, and unfortunately two usages are employed. "Where the mean equatorial horizontal lunar parallax is spoken of, the word parallax is used in its usual sense; but what is called the lunar constant of parallax is in reality the angle which has for its circular measure the sine of the true parallax. - Annual parallax is the variation of a star's place by being observed from opposite points of the earth's orbit. This is extremely minute, notwithstanding the great length of the base line, and is so difficult of determination that it long defied the endeavors of astronomers to detect it. (See Astronomt.) The apparent absence of stellar parallax was considered by Tycho Brahe fatal to the Copernican doctrine of the earth's orbital motion. Galileo suggested a mode of investigating the problem by observations on two stars of different magnitudes situated close together. This mode has been successfully applied by modern observers. Hooke was the first to use the telescope in this investigation, but he failed. The aberration of light had not then been discovered, and the result he announced as parallax was probably due to this cause.
The same is to be said of Flam-steed. The attempts of astronomers to determine parallax led to two signal discoveries, the aberration of light by Bradley (1725), and the systems of double stars by the elder Herschel (1803.) The earliest approximately successful researches on this problem were made by the elder Struve, begun in 1835 on the star a Lyrse, though his conclusions were not received with entire confidence by astronomers. The first unequivocal success was reached shortly afterward by Bessel at Konigsberg on the star 61 Oygni, and by Henderson at the Cape of Good Hope on the star a Oentauri.