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.