Page:Popular Astronomy - Airy - 1881.djvu/226

212 the same steps as regards aberration; that is to say, I correct the place of the star so as to shew where it would have been seen if there were no such thing as aberration. I have then got my observation of the star corrected for these various causes in such a manner, that its place is totally freed from the disturbances of a periodical nature, and from the change of precession which it has undergone from the 1st January. About the 1st August I repeat the observation. I then apply to this observation, corrections of the same kind; that is to say, I correct it for the change which, by precession, the place of the star has undergone since the 1st January. I apply the correction for nutation on August 1, so as to show what its place would have been if no nutation existed; and I then apply the correction for aberration on August 1, so as to show what its place would have been if no aberration existed; and then from the observation made on August 1, I have the position of the star as it would have been seen at this part of the earth's orbit; all the periodical causes of disturbance being removed, and the precession existing in just the same state as it did on the 1st January. There then remains only one disturbing cause, and it is that which depends on the position of the earth in the different parts of its orbit. In order to find whether the place of the star is really affected by that cause, the step most convenient is to observe the North Polar distance of the star. If the magnitude of the earth's orbit be not anything sensible, as viewed from the star, then the North Polar distance of the star, corrected as I have mentioned, will be the same in these two positions of the earth. But if the North Polar distance of the star, duly corrected, be not the