Astronomy Author:Robert Stawell Ball Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: nearly 2,000 years ago, and the differences between the drawing and the present appearance of the Great Bear are so slight that they may perhaps be entirely due ... more »to the errors which Ptolemy made. We have spoken of the motion of the stars as apparent, and we have now to explain how we know that the motion is only apparent, and to show to what the apparent motion is really due. To do this we must first consider the figure of the earth. We shall now introduce a convention which is very useful. The stars are no doubt at very varied distances from the earth, but, nevertheless, we have seen that the appearance of the heavens can be adequately represented on a globe where all the stars are at the same distance from the centre. Let us now suppose a colossal globe to be described with the earth at its centre and an enormously great radius. Then, if the stars were all bright points stuck on the interior of this globe, the appearance of the heavens would not be altered. This imaginary globe we call the celestial sphere. CHAPTER III. THE FIGURE OF THE EARTH. § 12. The Earth a Sphere.—To an observer situated upon the surface of the earth the contrast is very wide indeed between the appearance of the earth and the appearances presented by the sun and moon. Theearth appears to be a flat plain, more or less diversified ; the sun and moon appear to be globular; the earth appears to be at rest, while the sun and moon are apparently in constant motion; and, lastly, the earth appears to have a bulk incomparably greater than that of either the sun or the moon. If, however, we could change our point of view to a suitable position in space, we should form a more just conception of the relation of the earth to the sun and moon. We would then see that each of the three bodies was really s...« less