Page:Popular Astronomy - Airy - 1881.djvu/180

166 is a proof that the assumed value of horizontal parallax is wrong, and a new trial must be made. I mentioned to you that this method is interesting, not only as being in use at the present time, but also because it is the method first used by Greek Astronomers.

Both in the last and in the present century, the distance of the moon has been found with great accuracy by these methods. Its mean value may be roughly stated to be 240,000 miles, or about thirty times the breadth of the earth. It seems a long way, and it is a long distance to measure, considering that it is ascertained by the use of a yard measure. You will observe that it is really and truly measured so. A yard measure was used to measure the base in the trigonometrical survey; by means of this, and a series of triangles, a long line was measured on the earth; by knowing its length, and by making observations with the Zenith Sector, the form and dimensions of the earth were found; and by knowing the size of the earth, and by observing the angles which relate to parallax, the moon's distance is found. You see we have thus proceeded step by step from the yard measure to the moon's distance. We may thus picture to ourselves the distance of the moon. If a railway carriage travelled at the rate of 1000 miles a day, it would at that rate be eight months reaching the moon.

I then proceeded to point out to you by what means the measure of the sun's distance had been attempted; and first I pointed out to you some methods which have failed. In the first place I remarked that the distance of the sun from the earth might apparently be measured, just in the same way as the distance of the moon from the earth, by observations at two distant observatories. This method