INTRODUCTION.

V

superior planets as tliey approached towards opposition, when contemplatedin connexion with this doctrine, naturally led him to conceive that theyalso might probably revolve round the sun as the centre of their motions.This conclusion was strengthened by the opinion of Pythagoras , who hadplaced the sun immoveable in the centre of the universe, and assigned tothe earth an annual motion in the ecliptic. Finally it occurred to him, thatNicetas, of Syracuse , and some other ancient philosophers, had supposedthe heavens to be at rest, and sought to explain their diurnal changes byascribing to the earth a motion round a fixed axis. Having reflectedprofoundly upon these various principles, he found that, by combiningthem together, the resulting system accounted with the most scrupulousfidelity for all the phenomena of the celestial motions, while it wasdistinguished by a union of harmony and simplicity which admirablyaccorded with the general economy of nature. The alternate vicissitudesof night and day, the varied circle of the seasons, the stations and retro-gradations of the plauets, and their variable appearance at different timesof the year, all offered themselves as immediate consequences of thisbeautiful system.

According to Copernicus , then, the sun is placed immoveable in thecentre of the universe, and all the planets, including the earth, revolveround him in the order of the signs in concentric orbits, Mercury andVenus revolving within the earth's orbit, and all the other planets withoutit. While the earth is traversing her annual orbit, she is also constantlyrevolving from west to east round a fixed axis passing through the celestialpoles, accomplishing a complete revolution every twenty-four hours.Copernicus explained the motion of the moon by supposing her to revolvein a monthly orbit round the earth, while at the same time she accompaniedher in her annual motion round the sun. He also very ingeniously accountedfor the precession of the equinoxes, by attributing to the earth’s axis a slowconical motion in a direction opposite to the apparent motion of the stars.This great man has given to the world a full exposition of his principlesin his famous work, “ De Revolutionibus Orbium Celestium.” It is said,that he received the first copy of this work, upon the contents of whichhe had meditated thirty-six years, only a few hours before his death.

Although Copernicus greatly simplified the system of the world, hestill retained the machinery of epicycles to represent the motions of theplanets, and therefore left an ample field of research to his successors.But before the investigation of the actual form of the planetary orbitscould be prosecuted with any hopes of success, it was necessary that agreat improvement should be effected in practical astronomy. The art ofobservation still continued in the same condition in which it existedamong the Greeks and Arabians. Copernicus was less conspicuous forthe qualities of an observer than for his sagacity in unfoldiug the prin-ciples of nature. The various tables of astronomy had all fallen con-siderably into error, and the necessity of reconstructing them upon a moreaccurate basis appeared indispensable to the future progress of the science.It is clear, then, that the present crisis required less a theorist of the firstorder, than an astronomer who might possess sufficient genius and practicalskill to perfect the methods of observation, to imagine new T instruments,and by these means to establish a number of accurate facts relative to themotions of the planets. These qualities were eminently fulfilled in Tycho Brahe , whose labours introduce a new era in the art of observation.This illustrious astronomer was born in the year 1516 at Knudsthorp, a