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The problem of the distances of the stars is of peculiar interest in connection with the Copernican system. The greatest objection to this system, which must have been more clearly seen by astronomers themselves than by any others, was found in the absence of any apparent parallax of the stars.
Under the assumed name of Parallax he visited most of the chief towns of England, propounding what he calls his system of zetetic astronomy. Why he should call himself Parallax it would be hard to say; unless it be that the verb from which the word is derived signifies primarily to shift about or dodge, and secondarily to alter a little, especially for the worse.
The nearest star of all, Alpha Centauri, visible only in latitudes south of our middle ones, is perhaps half as far as Bessel's star, while Sirius and one or two others are nearly at the same distance. About 100 stars, all told, have had their parallax measured with a greater or less degree of probability.
Astronomical Observations relative to the Lunar Mountains. Phil. Trans., vol. lxxi. Astronomical Observations on the Rotation of the Planets on their Axes, made with a View to decide whether the Daily Rotation of the Earth be always the same. On the Comet of 1781, afterwards called the Georgium Sidus. Phil. Trans., vol. lxxii. On the Parallax of the Fixed Stars. Catalogue of Double Stars.
Numbers of naval officers, however, who were thoroughly well assured of the fact that the earth is a globe, were not able to demolish the crafty arguments of Parallax publicly, during the discussions which he challenged at the close of each lecture.
The fixed stars were alleged to be stationary, and it was necessary to suppose that they are almost infinitely distant, inasmuch as they showed to the observers of that time no parallax; that is to say, they preserved the same apparent position when viewed from the opposite points of the earth's orbit.
But in Kepler's case the obviously implied physical theory of the planetary motions, even before Newton explained the simplicity of conception involved, made astronomers quite ready to waive the claim for a rigid proof of the earth's motion by measurement of an annual parallax of stars, which they had insisted on in respect of Copernicus's revival of the idea of the earth's orbital motion.
For a complete knowledge of a star's motion the proper motion and parallax must also be known. When Huggins first applied the Doppler principle to measure velocities in the line of sight, the faintness of star spectra diminished the accuracy; but Vogel, in 1888, overcame this to a great extent by long exposures of photographic plates.
Indeed, to be strictly logical, the followers of Parallax ought on this account to adopt the faith that the earth is not flat, but basin-shaped, which hitherto they have not been ready to do.
While Brooks was reciting, it was easy for Brown, sitting next, to open his book, and calculating narrowly the parallax, to hold it concealed below the rail, while he diligently conned the page following. In his turn he rose well-primed, and spouted glibly, and so on down the class. Rumour went that our childlike professor declared he had never known anything like it.
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