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49. Keppler
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286 49 Keppler Winter 1892 Houghton Library Johann Keppler it was who discovered the form of the planets’paths in coursing round the sun and the law of their varying speed. This achievement, by far the most triumphant unravelment of facts ever performed ,—cunninger than any deciphering of hieroglyphics or of cuneiform inscriptions—occupied its author’s whole time from October 1600 to October 1604, and the greater part of four years more. That fairylike town Prague was the scene of these studies and there in April 1609 was published the immortal Commentaries on the Motions of the Star Mars. To gain any idea of a scientific research, one must look with one’s own eyes and brain at the things with which it deals. Now the year 1892 happens to be a good one for watching Mars, and if the reader will from his own naked-eye observations set down upon a star-map (say upon the figures in the Century Dictionary) the course of the planet from the third week in March to the end of the year, as it traverses the constellations Sagittarius, Capricornus, and Aquarius, the true greatness of Keppler will begin to dawn upon him. For the telescope was only invented in the very year in which Keppler’s book was published; so that he had before him only naked-eye observations, and saw only what anybody may see. During the year 1892 Mars will describe a loop among the stars, moving first eastwardly then gradually bending to the south, then to the west, then to the north, and last to the east again, so that on October 6 he will cross his previous path at the point where he was on June 10. This motion in a loop is characteristic of all the planets; and to account for it, the ancients very naturally supposed each to move round a circle itself carried round another circle, within which, though not at its centre, the earth was immovably fixed. They could not make the centre of the first circle move at a uniform rate round the circumference of the second, but took within the latter, at the same distance from its centre that the earth was, but on the opposite side, another fixed point, round which the centre of the first circle described equal angles in equal times. They 49. Keppler, 1892 287 found themselves further obliged to suppose that the first circle had a perpetual tilting, or reciprocating, motion around an axis tangent to the second. Copernicus, however, had shown that it was better to suppose earth and planets to move round a common centre very near the sun, while still continuing to make them move on circles that were carried round on other circles and balanced back and forth. Keppler was the scientific executor of the astronomer Tycho Brahe, who had measured as well as he could with the rude instruments of those days the celestial latitude and longitude of Mars in ten alternate years. From the study of these observations together with a few of his own of inferior value (for he was both near-sighted and awkward) and three by the ancient observer Ptolemy, Keppler found out and proved conclusively, that there are no such tiltings, or librations, as had been supposed, but that all the motions of Mars take place in a plane having an excessively slow motion if any, and furthermore, what Copernicus had failed to discover, that the sun lies in this plane, and also that Mars does not move in one circle carried by another, but simply in an ellipse having the sun at one of its foci 1 , and also that this ellipse itself turns round at a very slow rate, and also that the line from the sun to Mars describes in its motion equal elliptical sectors in equal times. Was it not wonderful to make out all this, and with perfect certainty too, from mere naked-eye observations which anybody could nowadays improve upon with the commonest instruments? The Keppler family had once been noble; but Johann’s near ancestors were artisans of Nüremberg, coarse people, hard and shrewd, but not long-lived. His grandfather, a bookbinder, had removed to Weil der Stadt near Stuttgart, where, owing to his reputation for sagacity, he had risen to be burgomaster, and where Johann was born. His father was a soldier and inn-keeper; his mother, a yellow blonde, little and spare, with a terrible...