Page:Popular Astronomy - Airy - 1881.djvu/224

210 surface. A second or two either way becomes of great importance. But when we come to treat of the distances of the stars, we find that the parallax which can be exhibited, even in the difference of the position of the stars as seen when the earth is at different parts of its orbit, (which is frequently called the annual parallax,) is not certainly in any case two seconds; and in every case but one is certainly less than a single second. An error of a fraction of a second is here of very great importance; it deranges the whole of the results. It is therefore of the utmost importance to take into account the quantity of precession, which amounts to 50 seconds in the year; nutation, which amounts to 9 seconds one way or another; and aberration, which amounts to 20 seconds in one direction or another. All this we must know perfectly well before we enter into the question as to the parallax of the stars.

Now, with regard to the observations of the distances of the stars, I remarked that those observations were, in character, not exactly similar to the observations which are made for ascertaining the distances of the sun and the moon and Mars, which we have spoken of before. In all other cases we were able to plant Observatories at different parts of the earth, and from these different parts of the earth to make observations at the same instant on the subject in question, whether it was the distance of the sun, or of the moon, or of Mars, that was to be measured. But we cannot do so with respect to the stars, and for this reason: the mere observation of the stars from two points of the earth does not present any sensible difference whatever. We have no reason to believe that the apparent places of a star, as seen from one part or another of the earth, are different