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They compass the whole sweep of Celestial Geometry, and stamp their seer as unapproachably the greatest of astronomers, as well as one of the chief benefactors of mankind. The announcement of Kepler's first two laws was made in his New Astronomy, "Astronomia Nova, seu Physica Caelestis, tradita Commentariis de Motibus Stellae Martis: Ex Observationibus G.V. Tychonis Brahe." Folio. Prague: 1609.
His first law states that the planets describe ellipses with the sun at a focus of each ellipse. These two laws were published in his great work, Astronomia Nova, sen. Physica Coelestis tradita commentariis de Motibus Stelloe; Martis, Prague, 1609.
The whole book, Astronomia Nova, is a pleasure to read; the mass of observations that are used, and the ingenuity of the propositions, contrast strongly with the loose and imperfectly supported explanations of all his predecessors; and the indulgent reader will excuse the devotion of a few lines to an example of the ingenuity and beauty of his methods.
In his Astronomia Reformata, , he published a rough and incorrect map of the Moon, compiled from observations made by Grimaldi of Ferrara; but in designating the mountains, he named them after eminent astronomers, and this idea of his has been carefully carried out by map makers of later times.
And, indeed, the "Astronomia Nova" presents an unequalled illustration of observation vivified by theory, and theory tested and fructified by observation. To appreciate the significance of the discovery of the elliptical orbit of the planets, it is necessary to understand the complicated confusion that prevailed in the conception of planetary motions.
I do ingenuously acknowledge, I used those prayers according to the form and direction prescribed for some weeks, using the word astrologia for astronomia; but of this no more: that Ars Notoria, inserted in the latter end of Cornelius Agrippa signifieth nothing; many of the prayers being not the same, nor is the direction to these prayers any thing considerable.
He saw the difficulty of gravitation acting through the void space. He compared universal gravitation to magnetism, and speaks of the work of Gilbert of Colchester. A few of Kepler's views on gravitation, extracted from the Introduction to his Astronomia Nova, may now be mentioned: Every body at rest remains at rest if outside the attractive power of other bodies.
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