TUTOR. The system I have been describing to you was known and taught by Pythagoras, a Greek philosopher, who flourished about 500 years before Christ, as he found it impossible, in any other way, to give a consistent account of the heavenly motions. This system, however, was so extremely opposite to all the prejudices of sense and opinion, that it never made any great progress, nor was ever widely spread in the ancient world. Ptolemy, an Egyptian philosopher, who flourished 130 years after Christ, supposed that the earth was fixed in the center, and that the sun and the rest of the heavenly bodies moved round it in twenty-four hours, or one natural day, as this seemed to correspond with the sensible appearances of the cœlestial motions. This system was maintained from the time of Ptolemy to the revival of learning in the sixteenth century. At length, Copernicus, a native of Poland, a bold and original genius, adopted the Pythagorean system, and published it to the world in the year 1530. This doctrine had been so long in obscurity, that the restorer of it was considered as the inventor. Europe, however, was still immersed in ignorance; and the general ideas of the world were not able to keep pace with those of a refined philosophy. This occasioned Copernicus to have few abettors, but many opponents. Tycho Brahe, in particular, a noble Dane, sensible of the defects of the Ptolemaic system, but unwilling to acknowledge the motion of the earth, endeavoured, about 1586, to establish a new system of his own; but, as this proved to be still more absurd than that of Ptolemy, it was soon exploded, and gave way to the [8]Copernican or true Solar System. [Footnote 8: See Plate I. fig. 2.] PUPIL. I confess, I should have thought with Ptolemy, that the earth was in the center, and that the sun moved round it. TUTOR. You must at present content yourself with knowing that it is not so; and it shall be my business to prove it. PUPIL. May I beg the favour of the information you intended respecting the planets? TUTOR. I will grant it with pleasure. The planets are spherical bodies, which appear like stars, but are not luminous; that is, they have no light in themselves; though they give us light; for they shine by reflecting the light of the sun. PUPIL. You say, Sir, that they appear like stars; if so, how am I to know them from stars? TUTOR. Very easily: for the stars, or as they are more properly called fixed stars, always keep the same situation with respect to each other; whereas the planets, as they move round the sun, must be continually changing their places among the fixed stars, and with one another. PUPIL. Is there any other method of distinguishing them besides what you have mentioned? TUTOR. Yes. The planets never twinkle like the fixed stars, and are seen earliest in the evening and latest in the morning. PUPIL. How is the twinkling of the stars in a clear night accounted for?