In former times the philosophers who studied the solar system ranged themselves in several “schools of thought,” to use a modern hackneyed phrase. Some upheld the Ptolemaic system, which took its name from a great Egyptian astronomer, Claudius Ptolemy, though it does not appear that he was actually the first to suggest it. The Ptolemaic system regarded the Earth as the centre, with the following bodies, all called planets, revolving round it in the order stated:—the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. It will be observed that there are seven bodies here named, and as seven was regarded as the “number of perfection,” it was in later times considered that only these seven bodies (neither more nor less) could really be the Earth’s celestial attendants. Though Ptolemy was in one sense an Egyptian, there yet prevailed amongst the Egyptians at large another theory slightly different from Ptolemy’s. According to the “Egyptian theory,” Mercury and Venus were regarded as satellites of the Sun, and not as primary planets appurtenant to the Earth. After Ptolemy’s era many centuries elapsed, during which the whole subject of the solar system lay practically dormant, and it continued so until the revival of learning brought new theorists upon the scene. The most important of these was Copernicus, who, in the sixteenth century, propounded a theory which eventually superseded all others, and, with slight modifications, is the one now accepted. Copernicus placed the Sun in the centre of the system, and treated it as the point around which all the primary planets revolved. So far, so good; but Copernicus went astray on the question of the orbits of the planets. He failed to realise the true character of the curves which they follow and treated these curves as “epicycles,” which word may be described as representing a complicated combination of little circles which taken together form a big one. It was left to Kepler and Newton to settle all such details on a true and firm basis. But before this stage was reached a man of the highest astronomical attainments and practical experience, Tycho Brahe, made shipwreck of his reputation as an astronomer by solemnly reviving the idea of the Earth being the immovable centre of everything. He treated the Moon as revolving round the Earth at no great distance and the Sun as doing the same thing a little farther off; the five planets revolving round the sun as solar satellites. The “Tychonic system,” as it is called, has something in common with the Ptolemaic system without being by any means as logical as the latter. That such far-fetched ideas as Tycho’s should have been palmed off on the world of science so recently as 300 years ago is passing strange; but the explanation appears to be that his action arose out of a misconception of certain passages of Holy Scripture, which seemed irreconcilable with the Copernican theory. It must not be forgotten that Copernicus’s famous