Recreations in Astronomy / With Directions for Practical Experiments and Telescopic Work

by Henry White Warren

MARS.

The god or war; Its sign

, spear and shield.

Mean distance from the sun, 141,000,000 miles. Diameter, 4211 miles. Revolution, axial, 24h. 37m. 22.7s.; orbital, 686.98 days. Velocity per minute, 899 miles. Satellites, two.

At intervals, on an average of two years one month and nineteen days, we find rising, as the sun goes down, the reddest star in the heavens. Its brightness is exceedingly variable; sometimes it scintillates, and sometimes it shines with a steady light. Its marked peculiarities demand a close study. We find it to be Mars, the fiery god of war. Its orbit is far from circular. At perihelion it is 128,000,000 miles from the sun, and at aphelion 154,000,000; hence its mean distance is about 141,000,000. So great a change in its distance from the sun easily accounts for the change in its brilliancy. Now, if Mars and the earth revolved in circular orbits, the one 141,000,000 miles from the sun, and the other 92,000,000, they would approach at conjunction within 49,000,000 miles of each other, and at opposition be 233,000,000 miles apart. But Mars at perihelion may be only 128,000,000 miles from the sun, and earth at aphelion may be 94,000,000 miles from the sun. They are, then, but 34,000,000 miles apart. This favorable opportunity occurs about once in seventy-nine years. At its last occurrence, in 1877, Mars introduced to us his two satellites, that had never before been seen by man. In consequence of this greatly varying distance, the apparent size of Mars differs very much (Fig. 62). Take a favorable

Fig. 62.—Apparent Size of Mars at Mean and Extreme Distances. time when the planet is near, also as near overhead as it ever comes, so as to have as little atmosphere as possible to penetrate, and study the planet. The first thing that strikes the observer is a dazzling spot of white near the pole which happens to be toward him, or at both poles when the planet is so situated that they can be seen. When the north pole is turned toward the sun the size of the spot sensibly diminishes, and the spot at the south pole enlarges, and vice versa. Clearly they are ice-fields. Hence Mars has water, and air to carry it, and heat to melt ice. It is winter at the south pole when Mars is farthest from the sun; therefore the ice-fields are larger than at the north pole. It is summer at the south pole when Mars is nearest the sun. Hence its ice-fields grow smaller than those of the north pole in its summer. This carrying of water from pole to pole, and melting of ice over such large areas, might give rise to uncomfortable currents in ocean and air; but very likely an inhabitant of earth might be transported to the surface of Mars, and be no more surprised at what he observed there than if he went to some point of the earth to him unknown. Day and night would be nearly of the same length; winter would linger longer in the lap of spring; summer would be one hundred and eighty-one days long; but as the seas are more intermingled with the land, and the divisions of land have less of continental magnitude, it may be conjectured that Mars might be a comfortable place of residence to beings like men. Perhaps the greatest surprise to the earthly visitor would be to find himself weighing only four-tenths as much as usual, able to leap twice as high, and lift considerable bowlders.

Satellites of Mars.

The night of August 11th, 1877, is famous in modern astronomy. Mars has been a special object of study in all ages; but on that evening Professor Hall, of Washington, discovered a satellite of Mars. On the 16th it was seen again, and its orbital motion followed. On the following night it was hidden behind the body of the planet when the observation began, but at the calculated time—at four o'clock in the morning—it emerged, and established its character as a true moon, and not a fixed star or asteroid. Blessings, however, never come singly, for another object soon emerged which proved to be an inner satellite. This is extraordinarily near the planet—only four thousand miles from the surface—and its revolution is exceedingly rapid. The shortest period hitherto known is that of the inner satellite of Saturn, 22h. 37m. The inner satellite of Mars makes its revolution in 7h. 39m.—a rapidity so much surpassing the axial revolution of the planet itself, that it rises in the west and sets in the east, showing all phases of our moon in one night. The outer satellite is 12,579 miles from Mars, and makes its revolution in 30h. 18m. Its diameter is six and a quarter miles; that of the inner one is seven and a half miles. This can be estimated only by the amount of light given.