The inclination of Mars’s axis to the ecliptic has not been ascertained with all desirable certainty, but if Sir W. Herschel’s estimate that the obliquity on Mars is 28¾° (the Earth’s obliquity being 23½°) is correct, it is evident that there must be a very close similarity between the seasons of the Earth and the seasons of Mars, thereby furnishing another link of proof to support the statement made at the commencement of this chapter that, taken all in all, Mars is the planet which bears most resemblance to the Earth. The apparent absence of satellites in the case of Mars was long a matter of regret to astronomers; they seemed to think that such a planet ought to have at least one companion. At last, in 1887, two were found by Hall at Washington, U. S., using a very fine refractor of 26 inches aperture. These satellites, which have been named Phobos and Deimos, are, however, very small, for Phobos at its best only resembles a star of mag. 11½, whilst Deimos is no brighter than a star of mag. 13½; from this it will be understood that only very large telescopes will show either of them. Phobos revolves round Mars in 7½ hours at a distance of about 6000 miles, whilst Deimos revolves in 30 hours at a distance of about 15,000 miles. It has been thought that neither of them can be more than about 6 or 7 miles in diameter, and therefore that they can not afford much light to their primary. Mars revolves round the Sun in 686d. 23h. 30m., at a mean distance of 141 million of miles, which the eccentricity of its orbit may increase to 154 millions or diminish to 128 millions. The planet’s apparent diameter varies between 4″ in conjunction and 30″ in opposition. Owing to the great eccentricity of the orbit the planet’s apparent diameter as seen from the Earth varies very much at different oppositions. The real diameter is rather more than 4000 miles. CHAPTER VIII. THE MINOR PLANETS. In 1772 a German astronomer named Bode, of Berlin, drew attention to certain curious numerical relations subsisting between the distances of the various planets. This “law,” as it has been sometimes called, usually bears Bode’s name, though it was not he but J. D. Titius of Wittemberg who really first discovered it. Take the numbers— 0, 3, 6, 12, 24, 48, 96, 192, 384; each of which (the second excepted) is double the preceding; adding to each of these numbers 4 we obtain— 4, 7, 10, 16, 28, 52, 100, 196, 388; which numbers approximately represent the distances of the planets from the sun expressed in radii of the Earth’s orbit. A little table will make the matter more clear.