Page:The story of the comets.djvu/79

IV. of amateurs these elements are almost unintelligible; and even to advanced students they often convey only a vague idea of the true form and position of the orbit. But all questions as to orbits will be dealt with in a separate chapter.

To the early astronomers the motions of comets caused great embarrassment. Tycho Brahe thought that they moved in circular orbits; Kepler suggested that comets moved in right lines. Though he was wrong as to this he was more correct in concluding that they were further off than the Moon. He formed this opinion by noting that the Comet of 1577 seemed to occupy the same position amongst the stars whether viewed from Uraniburg or from Prague, 400 miles distant. Hevelius seems to have been the first to remark that cometary orbits were much curved near perihelion, the concavity being towards the Sun. He also threw out an idea as to the parabola being the ordinary form of a comet's path, though it does not seem to have occurred to him to assume that the Sun was likely to be the focus of such a path. Borelli suggested the ellipse or the parabola as likely curves to be pursued by a comet. Sir William Löwer was probably the first to hint that comets sometimes moved in very eccentric ellipses; this he did in a letter to his "especiall good friende Mr. Thomas Harryot ", dated Feb. 6, 1610. Dörfel, a native of Upper Saxony, was the first practical man, for he came to the conclusion that the Comet of 1680 moved in a parabolic orbit. Sir I. Newton also gave his attention to the subject. Confirming Dörfel, Sir Isaac showed further that the motion of that comet was in accordance with the general theory of Gravitation.