Already in my first chapter have I called attention to the fact that comets have at all times been a source of great interest to the world in general, but it will be worth while to pursue the matter and rake up some of the historical notices of comets from the early times down to the Middle Ages and later; with a glance now and again at what some of the great poets have said on the subject.

Going back to the times of the earliest known students of Astronomy—the Chaldæans, it is to be noted that they seem to have considered comets as analogous in their nature to planets; that is to say permanent bodies revolving round the Sun in orbits, so much more extensive, however, that they were only visible when they came near the Earth. This opinion, which is the oldest hint we have of the existence of periodical comets, was also held by philosophers of the Pythagorean School.