The distances of the heavenly bodies are ascertained by what is known as "parallax." Suppose the ellipse (Fig. 54), marked Jan., Apr., July, Oct., represents the course of the Earth round the Sun, and that A B are two stars. If in January we look at the star A, we see it projected against the front of the sky marked 1. Three months later it would appear to be at 2, and thus as we move round our orbit the star itself appears to move in the ellipse 1, 2, 3, 4. The more distant star B also appears to move in a similar, but smaller, ellipse; the difference arising from the greater distance. The size of the ellipse is inversely proportional to the distance, and hence as we know the magnitude of the earth's orbit we can calculate the distance of the star. The difficulty is that the apparent ellipses are so minute that it is in very few cases possible to measure them. [Illustration: Fig. 54.--The Parallactic Ellipse.] The distances of the Fixed Stars thus tested are found to be enormous, and indeed generally incalculable; so great that in most cases, whether we look at them from one end of our orbit or the other--though the difference of our position, corresponding to the points marked January and July in Fig. 54, is 185,000,000 miles--no apparent change of position can be observed. In some, however, the parallax, though very minute, is yet approximately measurable. The first star to which this test was applied with success was that known as 61 Cygni, which is thus shown to be no less than 40 billions of miles away from us--many thousand times as far as we are from the Sun. The nearest of the Stars, so far as we yet know, is [Greek: alpha] Centauri, the distance of which is about 25 billions of miles. The Pleiades are considered to be at a distance of nearly 1500 billions of miles.