Page:Popular Astronomy - Airy - 1881.djvu/206

192 clock. This is a prodigious velocity, but the measure of it is very certain.

These three quantities, (precession, nutation, and aberration,) are the corrections to a star's apparent place, which it is necessary for us to take into account in every observation of a star, at whatever part of the earth it is observed; and besides these, it is necessary at every place to apply the proper correction for refraction, which may be different at every different place of observation. Having obtained these elements of calculation, we can proceed at once with the measure of the distance of the fixed stars.

In Figure 54, let E',E",E'",E"", be four positions

of the earth in its orbit (seen in perspective), P a place of observation, S a star, (the earth being in such a