SECTION I. COMETS PARTICIPATE IN THE DIURNAL MOTION. COMETS participate in the diurnal motion of the heavens. During the time of their apparition they rise and set like the sun, the moon, the stars, and the planets. In this respect, therefore, they do not differ from other celestial bodies. Let the observer, when a comet is in sight, note the point in the heavens which it occupies when his attention is first directed to it. This is easily done by referring the nucleus, the brilliant point from which the tail proceeds, to two adja cent stars. Let a certain time elapse-an hour, for example; at the end of that time the three luminous points, the two stars and the comet, will be found to have changed their position with respect to the horizon, each having described an arc of a circle in the heavens. The common centre of these arcs is the celestial pole, a point situated within a very small distance of the pole-star; the lengths of these arcs depend upon the interval of time between the observations, and the angular distance of each body from the pole. The direction is that of the general movement of the heavens and the stars; that is to say, from east to west. We have here, then, a fact which clearly teaches us that a comet moves in regions beyond the atmosphere of the earth; for the diurnal motion is an apparent motion, foreign to the comet, and belongs in reality to the observer, or, as we may say, to the observatory. It is caused by the rotation of the earth upon its axis. The entire atmosphere of the earth participates in this movement, and a body immersed in it-although it might, of course, have a separate motion of its own--would not participate in the diurnal motion. This is so elementary a fact that there is no need to insist upon it further. The ancients, and even those amongst the moderns who have regarded comets as meteors of atmospheric origin, have been compelled either to consider the earth as immovable or to admit that comets, after being formed within the atmosphere, withdraw from our globe, and, becoming independent, move in the heavens-a theory, as we have already seen, adopted by Hevelius. SECTION II. MOTIONS OF COMETS. Distinction between comets, nebula, and temporary stars-Comets, in their motions, are subject to stationary periods and retrogressions-The apparent complications arise, as in the case of the planets, from the simultaneous movement of these bodies and the earth. THERE is nothing in the foregoing section to distinguish comets from the multitude of brilliant stars which nightly illuminate the azure vault of heaven. Comets, it is true, appear in regions where before they had not been visible, and after a time they disappear; but in this respect they resemble those remarkable stars which have been seen to shine out suddenly in the midst of a constellation, to increase in brilliancy for a time, and afterwards to become faint and disappear; such as the famous temporary stars of 1572 (the Pilgrim), 1604, 1670, and 1866, which appeared and became extinct in the constellations of Cassiopeia, Serpens, Vulpecula, and Corona Borealis respectively. These stars, however, have, without exception, been distinguished by this peculiarity, that from the first to the last day of their apparition they continued immovable in the spot where they first appeared; or, more correctly, that their only motion was that due to the diurnal revolution of the heavens. Situated, like the fixed stars, at immense distances from our system, they had no appreciable movement of their own during the whole time of their visibility-in some |