MARS, in astronomy, the planet that revolves next beyond the earth in our system, is of a red fiery colour, and al ways gives a much duller light than Ve nus, though sometimes he equals her in size. He is not subject to the same limi tation in his motions as Mercury or "Ve nus, but appears sometimes very near the sun, and at others at a great distance from him ; sometimes rising when the sun sets, or setting when he rises. Of this planet it is remarkable, that when he ap proaches any of the fixed stars, which all the planets frequently do, these stars change their colour, grow dim, and often become totally invisible, though at some little distance from the body of the planet ; but Dr. Herschel thinks this has been ex aggerated by former astronomers. Mars appears to move from west to oast round the earth. The mean duration of his side real revolution is 686.979579 days. Ills motion is very unequal. When we first perceive this planet in the morning, when he begins to separate from the sun, his motion is direct, and the most rapid pos sible. This rapidity diminishes gradually, and the motion ceases altogether, when the planet is about 137° distant from the sun; then his motion becomes retrogade, and increases in rapidity till he comes into opposition with the sun. It then gradually diminishes again, and becomes nothing, when Mars approaches within 137° of the sun. Then the motion becomes direct, after having been retrograde for seventy three days, during which interval the planet described an arc of about 16°. Continuing to approach the sun, the planet at last is lost in the evening rays of that luminary. All these different phenomena are renewed after every opposition of Mars, but there are considerable differ ences both in the extent and duration of his retrogradations.
Mars does not move in the plane of the ecliptic, but deviates from it several de grees. His apparent diameter varies ex ceedingly. His mean apparent diameter is 27", and it increases so much, that when the planet is in opposition, the appa rent diameter is 81". Then the parallax
of Mars becomes sensible, and about double that of the sun. The disk of Mars changes its form relatively to its position with regard to the sun, and becomes oval. Its phases show that it derives its light from that luminary. The spots observed on its surface have informed astronomers, that it moves round its axis from West to East in 1.02733 days, and its axis is in clined to the ecliptic at an angle of about 59.70.
They were first observed in 1666 by Cassini at Bologna, with a telescope about 16i feet long ; and continuing to observe them for a month, he found they came into the same situation in twenty four hours and forty minutes. The pla net was observed by some astronomers at Rome, with longer telescopes, but they assigned to it a rotation in thirteen hours only. This, however, was afterwards shewn by M. Cassini to have been a mis take, and to have arisen from their not distinguishing the opposite sides of the planet, which, it seems, have spots pretty much alike. Ile made further observa tions on the spots of this planet in 1670, from whence he drew an additional con firmation of the time the planet took to revolve. The spots were again observed in subsequent oppositions, particularly for several days in 1704, by Maraldi, who took notice that they were not always well defined, and that they not only changed their shape frequently in the space be tween two oppositions, but even in the space of a month. Some of them, how ever, continued of the same form long enough to ascertain the time of the pla net's revolution. Among these there ap peared this year an oblong spot, resem bling one of the belts of Jupiter when broken. It did not reach quite round the body of the planet, but had, not far from the middle of it, a small protube rance towards the North, so well defined, that he was thereby enabled to settle the period of its revolution at twenty-four hours thirty-nine minutes, only one minute less than what Cassini had determined it to be.