Page:Popular Astronomy - Airy - 1881.djvu/225

Rh by the ten-thousandth part of a second. We can, however, do this: we can make observations on the place of a star, when the earth is in two positions widely different. If S is a star, in Figure 54, whose distance we wish to estimate, we can do it if we can observe it when the earth is in the position E', and also in the position E'". There is half a year between these observations; but still, if we can take into account all the changes in the star's apparent place that happen in the course of half a year, we shall be able to get some notion of the real change in the position of that star which arises solely from the different position of the earth in its orbit. Now let us see what we have to do. The position of the star at each of these times is affected by the three causes I have mentioned: precession, nutation, and aberration. The way in which we must reduce the observation of the star is this: we must make such a correction to the place in which we do really see the star, as will reduce it to the place in which we should have seen the star at a certain time, provided that the variable parts of the corrections were done away with. For instance, I observe the star on the 1st of February; precession has been going on for many centuries; I do not, however, reduce the place of the star very far back, I only apply the correction which is due to the change between January 1 and February 1, and thus refer the star's place to the beginning of the year. I then remark that nutation has sensibly disturbed the star's place. I do not, however, apply the correction (as regards nutation) to show where the star would have been on the 1st of January; but I apply a correction to show where the star would have been seen if there were no such thing as nutation at all. I take