A Consideration of the Calculations of Astronomers on the Foregoing

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CHAPTER TEN

A CONSIDERATION OF THE CALCULATIONS OF ASTRONOMERS ON THE FOREGOING

Whatever this observation may have contributed to correcting the motion of Venus has now been explained. Let us next examine how it agrees with the calculations of astronomers. I doubt not how it will be more fully evi- dent what indispensable support it brings to one working in astronomy when it becomes known how much even the most distinguished astronomers have difffered not only among themselves, but have also difffered from the truth. It is not fĳitting in this controversy, surely, to tax Ptolemy, that ancient father of astronomy, and Alfonso,1 who royally expended so much on his celestial tables. Those errors of their calculations are now easily excused by the passage of time. Let the exercises engage those younger men, who, in a small group, illuminate our age with their love of astronomy. Strive then, you most reputable astronomers of this century! Behold here for you the noble palm of victory—Venus promises Urania,2 more beautiful than any Helen, to him who properly captures her. There are four important astronomers from whose tables ephemerides are computed at this time. All do not yet agree upon the worth of these ephemerides. It is therefore well to examine them one by one.

1. Copernicus embraced the new (or rather renewed) hypotheses, and laws3 of the motions of the planets in six books of his Revolutions, from which Erasmus Reinhold afterward constructed the Prutenic Tables.4

1 Alfonso X of Castile (1221–1284), who commissioned an improved set of tables over existing ones, and variants of which were dominant during the next three centuries. 2 The muse of astronomy. This is but one of the many examples of the union of celestial bodies and events with Greek mythological fĳigures provided by Horrocks, traditional in the astronomy of his day and in the classical period. 3 The use of the term “laws” to refer to astronomical phenomena was not unusual before the seventeenth century. Kepler used the term “laws of motion” in connection with elliptical planetary orbits. Epitome astronomiae Copernicana, GW VII, 291, l. 31. The link between “law” and quantitative relationships, however, became more common in the later seventeenth century. 4 Erasmus Reinhold (1511–1553), Prutenicae tabulae coelestium motuum (Tübingen, 1551). 40 chapter ten

From those Origanus, Magini, and others5 calculated ephemerides which are still extant, and are chiefly used by our prognosticators, though now the Prutenic Tables are held to be of less value by the learned. 2. C. Longomontanus, the disciple of Tycho Brahe, and as it were, the heir of his discoveries, who, in his Danish Astronomy,6 treading faith- fully enough in the footsteps of his master, brings to a conclusion those things which Tycho, prevented by death, was unable to fĳinish. 3. The most ingenious J. Kepler, formerly an assistant to Tycho and aid- ing in his calculations, was afterwards astronomer to three emperors. He most fortunately completed his new and most truthful astronomy,7 eruditely preceding the publication of other earlier learned writings, and at length the Rudolphine Tables. 4. Finally, last of all, P. Lansberge, who looks down upon the endeavors of his predecessors with the greatest impudence. He strives to sub- stitute in place of other tables his own Perpetual Tables of the Celes- tial Motions, loaded to satiety with the praises of it by himself and others.

It seems useful to provide here the calculations of these four, so that it may it may be seen who would have discerned the hidden intricacies of Venus most clearly, and in whom we can most safely have confĳidence. Indeed, this observation is most suitable for this purpose. For to whatever extent a calculation is able to reasonably serve for the greatest elongations from the Sun, though it be somewhat in error elsewhere, it is more sharply tested in conjunction with the Sun, especially the inferior conjunction; and unless the calculation sticks to the truth very closely, it easily pro- duces gaping fĳissures, covering the tiniest errors beneath the Sun, but with difffĳiculty. It also happens (I do not know by what chance) that whatever is mistaken anywhere in the hypotheses of the astronomers, shows itself very greatly in this instance, with the errors all concentrated here and not (as sometimes occurs) canceling one another. I shall be content, however, to set down only the calculation from their tables, and will not weary myself and my readers with a geometrical

5 David Origanus (1558–1628); (Frankfurt an der Oder, 1599) and Novae motuum coelestium ephemerides. . . . (Frankfurt an der Oder, 1609). Giovanni Antonio Magini (1555–1617) (Venice, 1582). 6 Christian Severin Longomontanus (1562–1657), Astronomia Danica (Amsterdam, 1622). 7 Astronomia nova. . . . (Prague, 1609).