The physical constitution of the Moon was a subject which also engaged the attention of our indefatigable enthusiast. As early as 1780 he attempted the measurement of the lunar mountains, and came to the conclusion that few of them exceeded 2600 feet in height. Later research, however, has proved these figures to be inadequate. Next he addressed himself to a study of the lunar volcanoes, three of which he declared to be in a state of ignition; two of them apparently on the decline, the third still active. He was so convinced of the reality of the phenomenon, that on the 20th of April 1787 he wrote:—"The volcano burns with greater violence than it did last night." The real diameter of the volcanic light he estimated at 16,400 feet. Its intensity he described as superior to that of the nucleus of a comet then flashing across our system. The objects situated near the crater were fully illuminated by the glare of its burning matter.
It may seem strange that, after observations so exact and minute, few astronomers now admit the existence of active volcanoes in the Moon. The reasons for their incredulity are thus stated:—
The various parts of the Moon do not all reflect with the same intensity. Here, that intensity may be dependent on the form; elsewhere, on the nature of the materials. Those persons who have examined the lunar orb with telescopes, know how very considerable the difference arising from these two causes may be,—with how much keener and stronger a radiance one point of the Moon will sometimes shine than those around it. Well, it would seem to be obvious that the ratio of intensity between the brilliant parts and the faint parts must always be the same, whatever the origin of the illuminating light. In that portion of the lunar sphere which receives the glow and glory of the sun, we know that some points exist, the brightness of which is extraordinary compared with the feeble flickering gleam of those around them. And these same points, when seen in the dim reflection of the Earth, will still predominate in intensity over the neighbouring regions. In this way Arago and others explain the observations of Herschel, without admitting the existence of active volcanoes in the Moon. That volcanoes there are, is a familiar fact; but they would seem to have exhausted their activity in long-past ages. The lunar surface is now a dreary waste of rugged lava and ashes, covered with the matter ejected from craters once in a state of furious eruption. The Moon, in fact, is a world which has burned itself out. How strange the thought that in a far-back period the inhabitants of Earth, had Earth then been inhabited, might have seen the glare of countless volcanoes diffused, lurid and threatening, over the face of their satellite! How strange the thought that the once active fires should all have died away, and the Moon have thus been prepared for the better reception and reflection of the solar radiance in order to illuminate the nights of Earth!
The planets, needless to say, were the objects of Herschel's assiduous attention. Mercury was the one which least interested him; but he ascertained the perfect circularity of its disc. With respect to Venus, he endeavoured to determine the time of its rotation from 1777. We owe to him the discovery of the true shape of the "red planet Mars,"—that, like the Earth, it is an oblate spheroid, or flattened at the poles. After Piazzi, Olbers, and Harding had discovered the small planets, Ceres, Pallas, Juno, and Vesta, he applied himself to the measurement of their angular diameters. His researches led him to the conclusion that these four new bodies could not properly be ranked with the planets, and he proposed to call them Asteroids—a name now generally adopted. Since Herschel's time, the number of these minor planets known to astronomers has increased to upwards of one hundred.
With respect to Jupiter, our astronomer arrived at some important facts in connection with the duration of its rotation. He also made numerous observations on the intensities and comparative magnitudes of its satellites.
We come next in order to Saturn, the gloomy planet which the ancient astrologers regarded with so much dislike. Here, too, we find traces of Herschel's labours. Not only has he enlarged our knowledge of its equatorial compression, of its physical constitution, and of the rotation of its luminous belt or ring, but he added two to the number of its satellites. Five only of these were known at the close of the seventeenth century; of which Cussiric discovered four, and Huygens one. It was universally believed that the subject was exhausted.
But, on the 28th of August 1780, Herschel's colossal tube revealed to his delighted gaze a satellite nearer to the Saturnian ring than those previously observed. And a few days later, on the 17th of September, a seventh and last satellite crossed his field of vision. It was situated between the former and the ring; that is, it is the nearest to it of the seven.
But the most remarkable of Herschel's achievements was the discovery of the planet Uranus, and the detection of its satellites.
On the 13th of March 1781, between ten and eleven o'clock at night, the great astronomer was engaged in examining the small stars near H in the constellation Gemini, with a seven-foot telescope, bearing a magnifying power of two hundred and twenty-seven times. It appeared to him that one of these stars was of an unusual diameter; and he came to the conclusion, therefore, that it was a comet. It was under this denomination that it was discussed at the meeting of the Royal Society. But the researches of Herschel at a later period showed that the orbit of the new body was circular, and accordingly it was elevated to the rank of a planet. As already stated, Herschel named it, in compliment to George III., the Georgium Sidus; in this copying the example of Galileo with his "Medicaean stars." Afterwards, astronomers christened it Herschel, and subsequently Uranus, in conformity with the mythological nomenclature of the other planets.