Page:Popular Astronomy - Airy - 1881.djvu/208

194 and this proves, both, that the earth's axis has no motion except those of precession and nutation, and that the stars are at an inconceivable distance.

But there may be other stars as S, whose distance we have some reason for conjecturing to be not so enormously great. Now, the only way in which we can measure its distance is one strictly analogous to that used for measuring the distance of the moon; with this difference, that we cannot observe from two places at once. On account of the immense distance of the stars, it would be necessary to observe the place of the star from two positions, as far distant as the breadth of the earth's orbit; but we cannot do that. We can, however, observe the position of the star from the earth when the earth is in two positions, as E' and E"', on opposite sides of the earth's orbit; that is, at times half a year apart.

I have used, as an elucidation of parallax, the effect of the two eyes in the head. If you have your head in any fixed position, and you shut one eye, you cannot determine accurately the distance of an object; but if you open both eyes the distance is seen immediately. But with one eye a person can judge of distance very well if he moves his head. In like manner, one observer on the earth can observe the distance of a star, provided he takes advantage of the change of places at different times; that is, provided he allows his eye to be moved round for him by the revolution of the earth round the sun; it is, however, necessary for us to be fully possessed of every element for correction of the star's place, so as to clear it of every source of change, except the difference of apparent place depending on the star's distance and the earth's place in its orbit. This is the reason why I have deferred the mention of this measure until I had