hich includes the moon with the earth for its centre； makes the round of that great circle between the planets； in yearly motion about the sun； and revolves about the centre of the universe； in which the sun rests motionless； and that all which looks like motion in the sun is explained by the motion of the earth。 The extent of the universe； however； is so great that； whereas the distance of the earth from the sun is considerable in comparison with the size of the other planetary orbits； it disappears when compared with the sphere of the fixed stars。 I hold this to be more easily comprehensible than when the mind is confused by an almost endless number of circles； which is necessarily the case with those who keep the earth in the middle of the universe。 Although this may appear incomprehensible and contrary to the opinion of many； I shall； if God wills； make it clearer than the sun； at least to those who are not ignorant of mathematics。 〃The order of the spheres is as follows： The first and lightest of all the spheres is that of the fixed stars； which includes itself and all others； and hence is motionless as the place in the universe to which the motion and position of all other stars is referred。 〃Then follows the outermost planet； Saturn； which completes its revolution around the sun in thirty years； next comes Jupiter with a twelve years' revolution； then Mars； which completes its course in two years。 The fourth one in order is the yearly revolution which includes the earth with the moon's orbit as an epicycle。 In the fifth place is Venus with a revolution of nine months。 The sixth place is taken by Mercury； which completes its course in eighty days。 In the middle of all stands the sun； and who could wish to place the lamp of this most beautiful temple in another or better place。 Thus； in fact； the sun； seated upon the royal throne； controls the family of the stars which circle around him。 We find in their order a harmonious connection which cannot be found elsewhere。 Here the attentive observer can see why the waxing and waning of Jupiter seems greater than with Saturn and smaller than with Mars； and again greater with Venus than with Mercury。 Also； why Saturn； Jupiter； and Mars are nearer to the earth when they rise in the evening than when they disappear in the rays of the sun。 More prominently； however； is it seen in the case of Mars； which when it appears in the heavens at night； seems to equal Jupiter in size； but soon afterwards is found among the stars of second magnitude。 All of this results from the same causenamely； from the earth's motion。 The fact that nothing of this is to be seen in the case of the fixed stars is a proof of their immeasurable distance； which makes even the orbit of yearly motion or its counterpart invisible to us。〃＇1＇
The fact that the stars show no parallax had been regarded as an important argument against the motion of the earth； and it was still so considered by the opponents of the system of Copernicus。 It had； indeed； been necessary for Aristarchus to explain the fact as due to the extreme distance of the stars； a perfectly correct explanation； but one that implies distances that are altogether inconceivable。 It remained for nineteenth…century astronomers to show； with the aid of instruments of greater precision； that certain of the stars have a parallax。 But long before this demonstration had been brought forward； the system of Copernicus had been accepted as a part of common knowledge。 While Copernicus postulated a cosmical scheme that was correct as to its main features； he did not altogether break away from certain defects of the Ptolemaic hypothesis。 Indeed； he seems to have retained as much of this as practicable； in deference to the prejudice of his time。 Thus he records the planetary orbits as circular； and explains their eccentricities by resorting to the theory of epicycles； quite after the Ptolemaic method。 But now； of course； a much more simple mechanism sufficed to explain the planetary motions； since the orbits were correctly referred to the central sun and not to the earth。 Needless to say； the revolutionary conception of Copernicus did not meet with immediate acceptance。 A number of prominent astronomers； however； took it up almost at once； among these being Rhaeticus； who wrote a commentary on the evolutions； Erasmus Reinhold； the author of the Prutenic tables； Rothmann； astronomer to the Landgrave of Hesse； and Maestlin； the instructor of Kepler。 The Prutenic tables； just referred to； so called because of their Prussian origin； were considered an improvement on the tables of Copernicus； and were highly esteemed by the astronomers of the time。 The commentary of Rhaeticus gives us the interesting information that it was the observation of the orbit of Mars and of the very great difference between his apparent diameters at different times which first led Copernicus to conceive the heliocentric idea。 Of Reinhold it is recorded that he considered the orbit of Mercury elliptical； and that he advocated a theory of the moon； according to which her epicycle revolved on an elliptical orbit； thus in a measure anticipating one of the great discoveries of Kepler to which we shall refer presently。 The Landgrave of Hesse was a practical astronomer； who produced a catalogue of fixed stars which has been compared with that of Tycho Brahe。 He was assisted by Rothmann and by Justus Byrgius。 Maestlin； the preceptor of Kepler； is reputed to have been the first modern observer to give a correct explanation of the light seen on portions of the moon not directly illumined by the sun。 He explained this as not due to any proper light of the moon itself； but as light reflected from the earth。 Certain of the Greek philosophers； however； are said to have given the same explanation； and it is alleged also that Leonardo da Vinci anticipated Maestlin in this regard。＇2＇ While； various astronomers of some eminence thus gave support to the Copernican system； almost from the beginning； it unfortunately chanced that by far the most famous of the immediate successors of Copernicus declined to accept the theory of the earth's motion。 This was Tycho Brahe； one of the greatest observing astronomers of any age。 Tycho Brahe was a Dane； born at Knudstrup in the year 1546。 He died in 1601 at Prague； in Bohemia。 During a considerable portion of his life he found a patron in Frederick； King of Denmark； who assisted him to build a splendid observatory on the Island of Huene。 On the death of his patron Tycho moved to Germany； where； as good luck would have it； he came in contact with the youthful Kepler； and thus； no doubt； was instrumental in stimulating the ambitions of one who in later years was to be known as a far greater theorist than himself。 As has been said； Tycho rejected the Copernican theory of the earth's motion。 It should be added； however； that he accepted that part of the Copernican theory which makes the sun the centre of all the planetary motions； the earth being excepted。 He thus developed a system of his own； which was in some sort a compromise between the Ptolemaic and the Copernican systems。 As Tycho conceived it； the sun revolves about the earth； carrying with it the planets…Mercury； Venus； Mars； Jupiter； and Saturn； which planets have the sun and not the earth as the centre of their orbits。 This cosmical scheme； it should be added； may be made to explain the observed motions of the heavenly bodies； but it involves a much more complex mechanism than is postulated by the Copernican theory。 Various explanations have been offered of the conservatism which held the great Danish astronomer back from full acceptance of the relatively simple and； as we now know； correct Copernican doctrine。 From our latter…day point of view； it seems so much more natural to accept than to reject the Copernican system； that we find it difficult to put ourselves in the place of a sixteenth…century observer。 Yet if we recall that the traditional view； having warrant of acceptance by nearly all thinkers of every age； recorded the earth as a fixed； immovable body； we shall see that our surprise should be excited rather by the thinker who can break away from this view than by the one who still tends to cling to it。 Moreover； it is useless to attempt to disguise the fact that something more than a mere vague tradition was supposed to support the idea of the earth's overshadowing importance in the cosmical scheme。 The sixteenth…century mind was overmastered by the tenets of ecclesiasticism； and it was a dangerous heresy to doubt that the Hebrew writings； upon which ecclesiasticism based its claim； contained the last word regarding matters of science。 But the writers of the Hebrew text had been under the influence of that Babylonian conception of the universe which accepted the earth as unqualifiedly centralwhich； indeed； had never so much as conceived a contradictory hypothesis； and so the Western world； which had come to accept these writings as actually supernatural in origin； lay under the spell of Oriental ideas of a pre…scientific era。 In our own day； no one speaking with authority thinks of these Hebrew writings as having any scientific weight whatever。 Their interest in this regard is purely antiquarian； hence from our changed