Page:Popular Astronomy - Airy - 1881.djvu/223

Rh The question for us now is this: does this determination of the velocity of light agree with the deduction made from the aberrations of the stars? We found that the light travels ten thousand times faster than the earth moves in its orbit: if the light occupy 8m. 18s. in coming from the sun to the earth, does that imply a speed ten thousand times as great as the speed of the earth? The fact is, that the two calculations, though perfectly independent, support each other with the greatest nicety; and there is no doubt of the correctness of the measure of the velocity of light.

The subject on which I then proceeded at the Lecture yesterday was the measure of the distances of some of the fixed stars; and I observed in the first place, that it was necessary for me to premise these various things, namely, the explanation of precession, nutation, and aberration, and for this reason: that the apparent places of the stars are disturbed by them to a very sensible degree, both in right-ascension and North Polar distance; and that the very utmost accuracy is necessary in everything relating to the observations upon which the measure of the distances of the stars are to be founded. I assume that we now know the meaning of the term "parallax." Between the apparent places of the moon, as seen at one point of the earth and as seen at another point of the earth, there may be a difference of a degree and half, or more. Now when we have a degree and half of difference, an error of a second is of no particular consequence. The parallax of the sun, as found in the way described in a former lecture, is a very much smaller quantity, between 8 and 9 seconds; that is, there is a difference of 8 or 9 seconds in the sun's places, as seen at the centre of the earth and on the