Page:Popular Astronomy - Airy - 1881.djvu/189

Rh only from the dimensions of the earth, which, if the measure were conducted in any ordinary way, would be too small a base for so great a measure.

I said, that the distance of the moon from the earth is 240,000 miles, and that if a railway carriage were to travel at the rate of 1000 miles a day, it would be eight months in reaching the moon. But that is nothing compared with the length of time it would occupy a locomotive to reach the sun from the earth; if travelling at the rate of 1000 miles a day, it would require 260 years to reach it.

I will now proceed with explanations of some of the higher branches of Astronomy, which, though difficult, will be found most valuable: first, as instances of very important applications of the principle of gravitation; secondly, as showing the nature of some of the corrections to observations, which it is necessary to understand, in order to see clearly the different steps that must be made before we can arrive at a measure of the distance of the fixed stars.

I shall speak first of the Precession of the Equinoxes. This is a thing which was known as a fact of observation to the ancients. The person who discovered it was the Greek Astronomer Hipparchus, a hundred and fifty years before the Christian era. It was first explained by Sir Isaac Newton, by whom the principles of gravitation were made known. I will endeavour to convey the explanation to you; but it is a thing not to be done without much difficulty. For this purpose I must, in the first place, recall to your minds the laws of gravitation. The fundamental law of gravitation is this: that every particle of every body attracts every particle of every other body: one body does not attract another body as a mass, but every body attracts every other body as a collection