It will thus be seen how enormously the solar system was enlarged in extent by the discovery of the outermost planets. The finding of Uranus plainly doubled its breadth; the finding of Neptune made it more than half as broad again. Nothing indeed can better show the import of these great discoveries than to take a pair of compasses and roughly set out the above relative paths in a series of concentric circles upon a large sheet of paper, and then to consider that the path of Saturn was the supposed boundary of our solar system prior to the year 1781. We have seen that the usual shape of celestial bodies themselves is spherical. Of what form then are their paths, or _orbits_, as these are called? One might be inclined at a venture to answer "circular," but this is not the case. The orbits of the planets cannot be regarded as true circles. They are ovals, or, to speak in technical language, "ellipses." Their ovalness or "ellipticity" is, however, in each case not by any means of the same degree. Some orbits--for instance, that of the earth--differ only slightly from circles; while others--those of Mars or Mercury, for example--are markedly elliptic. The orbit of the tiny planet Eros is, however, far and away the most elliptic of all, as we shall see when we come to deal with that little planet in detail. It has been stated that the sun and planets are always rotating. The various rates at which they do so will, however, be best appreciated by a comparison with the rate at which the earth itself rotates. But first of all, let us see what ground we have, if any, for asserting that the earth rotates at all?