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.

Apse: An extremity of the major axis of the orbit of a body; a body is
at its greatest and least distances from the body about which it
revolves, when at one or other apse.

Conjunction: When a plane containing the earth's axis and passing
through the centre of the sun also passes through that of the moon
or a planet, at the same side of the earth, the moon or planet is in
conjunction, or if on opposite sides of the earth, the moon or
planet is in opposition. Mercury and Venus cannot be in opposition,
but are in inferior or superior conjunction according as they are
nearer or further than the sun.

Deferent: In the epicyclic theory, uneven motion is represented by
mot