Page:Popular Astronomy - Airy - 1881.djvu/182

168 I then pointed out a third method, which has been tolerably successful, (although it is not the one regarded as being the most accurate), namely, by observing the parallax of Mars, which moves round the sun in an orbit between those of the earth and Jupiter. It is founded upon these considerations. First, that from seeing how much the planets go right or left of the sun, or how much their motion deviates from motion in a circle round the earth, the proportion of the distance of Mars from the sun to the distance of the earth from the sun is known quite independently of any knowledge of the absolute distances; and that in fact this proportion was known with considerable accuracy many centuries ago. In the year 1700 it was nearly as well known as it is now. Secondly, the proportion of the distances from the sun being known, it followed that the proportion of the distance of Mars from the earth to the distance of the earth from the sun was known. Suppose then, at a certain time, (I am obliged to say at a certain time because the orbit of Mars is very eccentric,) when the sun, the earth, and Mars, are in a straight line, suppose at that time we know the distance of Mars from the sun is four-thirds of the distance of the earth from the sun, it follows from that, that the distance of Mars from the earth at that time, is one-third of the distance of the sun from the earth. If we can by any method find the distance of Mars from the earth at that time, and if we multiply it by three, we shall get the distance of the sun from the earth. Thirdly, the distance of Mars from the earth can be obtained by the simple method of parallax, as in the first method for the moon; and with considerable accuracy. At two observatories, as in Europe and at the Cape of Good Hope, the position of Mars when on the