Page:Popular Astronomy - Airy - 1881.djvu/228

214 parallax" to denote the extreme difference of apparent positions of a star, as seen from the sun on the one hand, and from the earth on the other hand), which amounts to this: the distance of the star is 200,000 times greater than the distance of the sun from the earth. That is a thing, however, which requires many observations for its verification.

I then mentioned another way in which the distances of the stars may be ascertained; a method which is free from all those defects of which I have spoken. This method is by the observation of two stars, of which one is believed to be very much nearer to the sun than the other. For then we may assume that the distant star will have no sensible change of place from parallax, depending on the position of the earth in its orbit. And then, in observing the stars from the various parts of the earth's orbit, we can compare the apparent place of that star which we believe to be the nearer with the place of the other. Practically this is of importance. The refraction, precession, nutation, and aberration, are sensibly the same; and there is no uncertainty whatever from the computation of the various quantities which cast so much uncertainty on the results derived from other observations. This is the method pursued by Bessel in determining the distance of the star 61 Cygni. He measured the angular distance of this star from two small stars near it, by means of an instrument called the Heliometer, well known on the Continent, but of which there was at that time no specimen in England. With this he determined the parallax of the star 61 Cygni to be one-third of a second; that amounts to the same as saying that the distance is 600,000 times greater than the distance of the earth from the sun. It is deserving of attention