MARS, in astronomy, the outermost of the four earth-like planets revolving about the sun. Its orbit lies completely outside that of the earth, its mean distance from the sun being 141,500,000 miles, and with the single exception of the orbit of Mercury, it has the most eccen tric orbit of the solar system, the greatest dis tance of the planet from the sun exceeding its least distance by 26,400,000 miles. Mars occu pies 687 days in completing the circuit of its path, so that this is the length of the year on this planet. As the orbits of Mars and of the earth both approximate to circles, having the sun near their centres, it follows that the dis tance apart of these planets varies enormously. When Mars, the earth and the sun are in one straight line, Mars and the earth being on the same side of the sun, this distance may be so little as 35,500,000 miles, while if the former planet is beyond the sun in the most remote part of its orbit, the distance may be so great as 248,600,000 miles. In the former case Mars is more favorably situated for observation from the earth than any other planet in the solar system; the planet Venus when nearest us is, indeed, much less far away, but at this time it is the dark, or night, side of Venus that is turned toward us, while when we are nearest to Mars it is the fully illuminated, day side of the planet upon which we look. On account of the eccentricity of the orbit of Mars, its dis tances from the earth differ greatly at different oppositions; if an opposition occurs while Mars occupies that part of its orbit which is nearest the sun, the distance may be so small as 35,500, 000 miles, while if at this time Mars is at the point of its orbit most remote from the sun, even at the instant of nearest approach the distance may exceed 61,600,000 miles. The in terval between successive close approaches of Mars and the earth is 780 days, much the long est synodical period in the planetary system, — while the usually favorable close approaches occur in groups separated by 15 or 17 years. These favorable oppositions always occur dur ing the months of August or September; the date of the last one was 1909, while the three succeeding ones will be seen in the years 1924, 1939 and 1941. It is at these times that very unusually favorable opportunities are afforded for studying the surface of the planet. The day on Mars is 24 hours, 37 minutes, 22.67 seconds in length, being thus but little longer than our own; the axis of the planet is inclined at an angle of 24° 0' to the axis of its orbit, so that spring, summer, autumn and winter succeed one another there in almost precisely the same way as with us, but the Martian seasons arc each of nearly twice the duration of ours since the year is nearly two of our years in length. (The inclination of the earth's
axis is 23° 27').
In 1877, Asaph Hall, of Washington, dis covered that Mars is attended by two minute satellites to which he gave the names Phobos and Deimos. Both of these bodies are very minute objects, and as may be seen from the following table their motions are in many re spects unique among the satellites of the solar system.
The distance away of each of the moons from the planet is remarkably small; thus, even Deimos is less than one-seventeenth as far away from Mars as our own moon is from the earth, while Phobos is separated from the planet's surface by less than one diameter of the planet. The latter moon is, in fact, so close to Mars that it would remain forever invisible to any observers on the planet in higher latitudes than 69 degrees, being hidden by the curvature of the planet's surface. But still more striking is the apparent motion of these bodies as it would be witnessed from the surface of Mars itself. As Phobos completes a revolution in 7 hours 39 minutes. which is far less than the time of the rotation of the planet, it follows that this moon would be seen to rise in the west, run rapidly eastward among the stars and finally set in the east_ Eleven hours later it would again appear in the west, to repeat the same retrograde motion. Deimos, though rising in the east in the usual way, would mount the sky very slowly: the various constellations would be seen to drift past it and the moon would not finally attain the western horizon until about 66 hours after rising. Both moons would be seen to go through their phases in the course of their peculiar motions. With such minute objects it is impossible for us to discern any measur able discs, but from observations of the amount of light which their surfaces reflect, their diam eters can be, at least approximately, determined. The figures of the above table are those found in this way at the Lowell Observatory. These indicate that Phobos, as seen from the nearest point on Mars, would appear somewhat larger than our moon does to us, though its surface would be but one-half as bright, while the ap parent diameter of Deimos would be but three minutes of arc. This satellite would therefore only be visible as a disc with difficulty to the naked eye; it would have merely the appear ance of a very brilliant star.