MARS, in Astronomy, one of the eleven planets, situated without the earth's orbit, and remarkable for the extent of its atmosphere and the redness of its light. See ASTRONOMY Index.
The red colour of this planet, according to Mr Brewster*, is owing to the same cause as the redness of the morning and evening clouds. When a beam of Fargood's white light passes through any medium, its colour inclines Vol. ii.
clines to red, in proportion to the space through which it has travelled, and the density of the medium. The momentum of the red or least refrangible rays being greater than that of the violet or most refrangible rays, the former will make their way through the resisting medium, while the latter are either reflected or absorbed. The colour of the beam, therefore, when it reaches the eye, must partake of the colour of the least refrangible ray; and the redness of this colour must increase with the number of the violet rays that have been obstructed. Hence we see, that the sun, moon, and stars appear red when in the horizon; and that every luminous object seen through a mist is of a ruddy hue. Now, the planet Mars is allowed to have an atmosphere of great density and extent, as is manifest from the dim appearance of the fixed stars that are placed at a considerable distance from his disk. The sun's light, therefore, by which this planet is illuminated, having to pass twice through the atmosphere of Mars before it reaches the earth, must be deprived of a great proportion of the violet rays; and consequently the colour of the resulting light by which Mars is visible, must be red.—As there is a considerable difference of colour among the other planets, and likewise among the fixed stars, are we not entitled to conclude, that those in which the red colour predominates, have the greatest or the densest atmospheres? According to this principle, Saturn must have the next greatest atmosphere to that of Mars.