History of Astronomy

by George Forbes

A few of Kepler’s views on gravitation, extracted from the Introduction to his Astronomia Nova, may now be mentioned:—

1. Every body at rest remains at rest if outside the attractive power of other bodies.

2. Gravity is a property of masses mutually attracting in such manner that the earth attracts a stone much more than a stone attracts the earth.

3. Bodies are attracted to the earth’s centre, not because it is the centre of the universe, but because it is the centre of the attracting particles of the earth.

4. If the earth be not round (but spheroidal?), then bodies at different latitudes will not be attracted to its centre, but to different points in the neighbourhood of that centre.

5. If the earth and moon were not retained in their orbits by vital force (aut alia aligua aequipollenti), the earth and moon would come together.

6. If the earth were to cease to attract its waters, the oceans would all rise and flow to the moon.

7. He attributes the tides to lunar attraction. Kepler had been appointed Imperial Astronomer with a handsome salary (on paper), a fraction of which was doled out to him very irregularly. He was led to miserable makeshifts to earn enough to keep his family from starvation; and proceeded to Ratisbon in 1630 to represent his claims to the Diet. He arrived worn out and debilitated; he failed in his appeal, and died from fever, contracted under, and fed upon, disappointment and exhaustion. Those were not the days when men could adopt as a profession the “research of endowment.”

Before taking leave of Kepler, who was by no means a man of one idea, it ought to be here recorded that he was the first to suggest that a telescope made with both lenses convex (not a Galilean telescope) can have cross wires in the focus, for use as a pointer to fix accurately the positions of stars. An Englishman, Gascoigne, was the first to use this in practice.

From the all too brief epitome here given of Kepler’s greatest 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?