Such was the system of Astronomy which prevailed during the Middle Ages, and was only discredited at an epoch nearly simultaneous with that of the discovery of the New World by Columbus. The true arrangement of the solar system was then expounded by Copernicus in the great work to which he devoted his life. The first principle established by these labours showed the diurnal movement of the heavens to be due to the rotation of the earth on its axis. Copernicus pointed out the fundamental difference between real motions and apparent motions; he proved that the appearances presented in the daily rising and setting of the sun and the stars could be accounted for by the supposition that the earth rotated, just as satisfactorily as by the more cumbrous supposition of Ptolemy. He showed, moreover, that the latter supposition must attribute an almost infinite velocity to the stars, so that the rotation of the entire universe around the earth was clearly a preposterous supposition. The second great principle, which has conferred immortal glory on Copernicus, assigned to the earth its true position in the universe. Copernicus transferred the centre, about which all the planets revolve, from the earth to the sun; and he established the somewhat humiliating truth, that our earth is merely a planet pursuing a track between the paths of Venus and of Mars, and subordinated like all the other planets to the supreme sway of the Sun. This great revolution swept from astronomy those distorted views of the earth's importance which arose, perhaps not unnaturally, from the fact that we happen to be domiciled on that particular planet. The achievements of Copernicus were soon to be followed by the invention of the telescope, that wonderful instrument by which the modern science of astronomy has been created. To the consideration of this important subject we shall devote the first chapter of our book. [Illustration: PLATE II. A TYPICAL SUN-SPOT. (AFTER LANGLEY.)] CHAPTER I. THE ASTRONOMICAL OBSERVATORY. Early Astronomical Observations--The Observatory of Tycho Brahe--The Pupil of the Eye--Vision of Faint Objects--The Telescope--The Object-Glass--Advantages of Large Telescopes--The Equatorial--The Observatory--The Power of a Telescope--Reflecting Telescopes--Lord Rosse's Great Reflector at Parsonstown--How the mighty Telescope is used--Instruments of Precision--The Meridian Circle--The Spider Lines--Delicacy of pointing a Telescope--Precautions necessary in making Observations--The Ideal Instrument and the Practical One--The Elimination of Error--Greenwich Observatory--The ordinary Opera-Glass as an Astronomical Instrument--The Great Bear--Counting the Stars in the Constellation--How to become an Observer.