Page:Popular Astronomy - Airy - 1881.djvu/229

Rh that 61 Cygni is a double star; but we know from long observation that the two stars partake of the same motions, and probably are a connected system like the earth, and moon, and therefore we speak of them and of their distance as if they were only one star.

I have here spoken repeatedly of our supposition that some stars are nearer to us than others. The grounds of this supposition are generally the amount of what is called the proper motion of the stars. Upon comparing the places of the stars, as we observe them in different years, and applying the corrections for precession, nutation, and aberration, so as to reduce every observation of every star to what it ought to exhibit on the first day in the year, agreeably to the common practice of astronomers, we find that a great number of the stars have what is called proper motion. We are obliged to give up the idea of fixity entirely. The term "fixed stars" is a good term for young astronomers to use; but the vast majority of the stars which have been well observed, seem to have a motion of their own, and that is known by the term proper motion. In all good catalogues of stars there is a reserved column, distinguishing the proper motions of the stars, showing the direction in which the stars appear to be moving through other stars, and the amount of their motion in a year. This has only been discovered after many years' observation; it is in every case a small quantity; but still in most instances the quantity has been correctly ascertained. Those of Sirius and Arcturus are pretty large; but the largest known are those of two small stars, 61 Cygni (whose motion is nearly three seconds in a year), and a star known by the name of Groombridge, 1830,