Kepler loved to give reins to his imagination he was equally impressed with the necessity of scrupulously comparing speculative results with observed facts, and of surrendering without demur the most beloved of his fancies if it was unable to stand this test.  If Kepler had burnt three-quarters of what he printed, we should in all probability have formed a higher opinion of his intellectual grasp and sobriety of judgment, but we should have lost to a great extent the impression of extraordinary enthusiasm and industry, and of almost unequalled intellectual honesty which we now get from a study of his works.”

Professor Forbes is more enthusiastic.  In his “History of Astronomy,” he refers to Kepler as “the man whose place, as is generally agreed, would have been the most difficult to fill among all those who have contributed to the advance of astronomical knowledge,” and again a propos of Kepler’s great book, “it must be obvious that he had at that time some inkling of the meaning of his laws—­universal gravitation.  From that moment the idea of universal gravitation was in the air, and hints and guesses were thrown out by many; and in time the law of gravitation would doubtless have been discovered, though probably not by the work of one man, even if Newton had not lived.  But, if Kepler had not lived, who else could have discovered his Laws?”

APPENDIX I.

List of Dates.

Johann Kepler, born 1571; school at Maulbronn, 1586; University of Tuebingen, 1589; M.A. of Tuebingen, 1591; Professor at Gratz, 1594; “Prodromus,” with “Mysterium Cosmographicum,” published 1596; first marriage, 1597; joins Tycho Brahe at Prague, 1600; death of Tycho, 1601; Kepler’s optics, 1603; Nova, 1604; on Comets, 1607; Commentary on Mars, including First and Second Laws, 1609; Professor at Linz, 1612; second marriage, 1613; Third Law discovered, 1618; Epitome of Copernican Astronomy, 1618-1621; Rudolphine Tables published, 1627; died, 1630.

APPENDIX II.

Bibliography.

For a full account of the various systems of Kepler and his predecessors the reader cannot do better than consult the “History of the Planetary Systems, from Thales to Kepler,” by Dr. J.L.E.  Dreyer (Cambridge Univ.  Press, 1906).  The same author’s “Tycho Brahe” gives a wealth of detail about that “Phoenix of Astronomers,” as Kepler styles him.  A great proportion of the literature relating to Kepler is German, but he has his place in the histories of astronomy, from Delambre and the more modern R. Wolfs “Geschichte” to those of A. Berry, “History of Astronomy” (University Extension Manuals, Murray, 1898), and Professor G. Forbes, “History of Astronomy” (History of Science Series, Watts, 1909).

GLOSSARY.

Apogee:  The point in the orbit of a celestial body when it is furthest
    from the earth.