Page:Popular Astronomy - Airy - 1881.djvu/230

216 (whose motion is nearly four seconds in a year.) The attention of astronomers has therefore been directed to both those stars, and it appears certain that the former has sensible parallax, and probable that the latter has parallax of a somewhat smaller amount. In closing this account of the method of measuring the distance of the stars, I will only remind you that I have redeemed my pledge of showing how the distance of the stars is measured by means of a yard measure, and I will very briefly recapitulate the principal steps. By means of a yard measure a base-line in a survey was measured; from this, by the triangulations and computations of a survey, an arc of meridian on the earth was measured; from this, with proper observations with the Zenith Sector, the surveys being also repeated on different parts of the earth, the earth's form and dimensions were ascertained; from these, and a previous independent knowledge of the proportions of the distances of the earth and other planets from the sun, with observations of the transit of Venus, the sun's distance is determined; and from this, with observations leading to the parallax of the stars, the distance of the stars is determined. And every step of the process can be distinctly referred to its basis, that is, the yard measure.

Before dispatching the subject of observation of stars, I will make one remark. The proper motions of many are very irregular in direction and magnitude; but with regard to some others there is a rude regularity which may be conceived in this way. I speak of it in connection with what is supposed to be the motion of the solar system in space. Suppose that I am walking through a crowd of people, or through a forest, if I keep my attention on those