The orbit of Mars has the very small inclination of 1° 51' to the plane of the ecliptic. The planet revolves around the Sun in a period of 687 days, which constitutes its sidereal year, the year of Mars being only 43 days less than two of our years.
Mars travels along its orbit with a mean velocity of 15 miles per second, being about ⁸⁄₁₀ of the velocity of our globe in its orbit. The synodical period of Mars is 2 years and 48 days, during which the planet passes through all its degrees of brightness.
Mars is a smaller planet than the Earth, its diameter being only 4,200 miles, and its circumference 13,200 miles. It seems well established that it is a little flattened at its poles, but the actual amount of this flattening is difficult to obtain. According to Prof. Young, the polar compression is ¹⁄₂₁₉.
The surface of this planet is a little over ²⁸⁄₁₀₀ of the surface of our globe, and its volume is 6½ times less than that of the Earth. Its mass is only about ⅒ while its density is about ¾ that of the Earth. The force of gravitation at its surface is nearly ¾ of what it is at the surface of our globe.
The planet Mars rotates on an axis inclined 61° 18' to the plane of its orbit, so that its equator makes an angle of 28° 42' with the same plane. The period of rotation of this planet, which constitutes its sidereal day, is 24 h. 37 m. 23 s.
The year of Mars, which is composed of 669⅔ of these Martial days, equals 687 of our days, this planet rotating 669⅔ times upon its axis during this period. But owing to the movement of Mars around the Sun, the number of solar days in the Martial year is only 668⅔, while, owing to the same cause, the solar day of Mars is a little longer than its sidereal day, and equals 24 h. 39 m. 35 s.
The days and nights on Mars are accordingly nearly of the same length as our days and nights, the difference being a little less than three-quarters of an hour. But while the days and nights of Mars are essentially the same as ours, its seasons are almost twice as long as those of the Earth. Their duration for the northern hemisphere, expressed in Martial days, is as follows: Spring, 191; Summer, 181; Autumn, 149; Winter, 147. While the Spring and Summer of the northern hemisphere together last 372 days, the Autumn and Winter of the same hemisphere last only 296 days, or 76 days less. Since the summer seasons of the northern hemisphere correspond to the winter seasons of the southern hemisphere, and vice versa, the northern hemisphere, owing to its longer summer, must accumulate a larger quantity of heat than the last. But on Mars, as on the Earth, there is a certain law of compensation resulting from the eccentricity of the planet's orbit, and from the fact that the middle of the summer of the southern hemisphere of this planet, coincides with its perihelion. From the greater proximity of Mars to the Sun at that time, the southern hemisphere then receives more heat in a given time than does the northern hemisphere in its summer season. When everything is taken into account, however, it is found that the southern hemisphere must have warmer summers and colder winters than the northern hemisphere.
Seen with the naked eye, Mars appears as a fiery red star, whose intensity of color is surpassed by no other star in the heavens. Seen through the telescope, it retains the same red tint, which, however, appears less intense, and gradually fades away toward the limb, where it is replaced by a white luminous ring.
