described. He also benefited from another set of careful observations by the Danish astronomer Tycho Brahe, as analyzed by his student Johannes Kepler—a contemporary of Galileo.
Both Brahe and Kepler were remarkable characters. Brahe, from a privileged background, became the most eminent astronomer in Europe after his observations of the supernova of 1572. He was given an entire island by the Danish monarch King Frederick II to use as an observatory site, only to be forced to move some years later by Frederick’s successor. Unhindered by, or perhaps because of, his arrogance (and a false nose made of
metal), Brahe managed to improve in one decade the precision in astronomical measurement by a factor of 10 over that which it had maintained for the previous thousand years—and all of this without a telescope! In Prague, where he had gone in exile from Denmark, Brahe hired Kepler a year before his own death to perform the intricate calculational analysis required to turn his detailed observations of planetary motions into a consistent cosmology.
Kepler came from another world. A child of a family of modest means, his life was always at the edge, both financially and emotionally. Besides his scientific pursuits, Kepler found time to defend his mother successfully from prosecution as a witch and to write what was probably the first science fiction novel, about a trip to the moon. In spite of these diversions, Kepler approached the task of analyzing the data in Brahe’s notebooks, which he inherited upon Brahe’s death, with an uncommon zeal. Without so much as a Macintosh, much less a supercomputer, he performed a miracle of complicated data analysis that would occupy the better part of his career. From the endless tables of planetary positions, he arrived at the three celebrated laws of planetary motion that still bear his name, and that provided the key clues Newton would use to unravel the mystery of gravity.
I mentioned one of Kepler’s Laws earlier—namely, that the orbits of the planets sweep out equal areas in equal times—and how Newton was able to use this to infer that there was a force pulling the planets toward the sun. We are so comfortable with this idea nowadays that it is worth pointing out how counterintuitive it really is. For centuries before Newton, it was assumed that the force needed to keep the planets moving around the sun must emanate from something pushing them around. Newton quite simply relied on Galileo’s Law of Uniform Motion to see that this
was unnecessary. Indeed, he argued that Galileo’s result that the motion of objects thrown in the air would trace out a parabola, and that their horizontal velocity would remain constant, would imply that an object thrown sufficiently fast could orbit the Earth. Due to the curvature of the Earth an object could continue to “fall” toward the earth, but if it were moving fast enough initially, its constant horizontal motion could carry it far enough that in “falling” it would continue to remain a constant distance from the Earth’s surface. This is demonstrated in the following diagram, copied from Newton’s Principia:
Having recognized that a force that clearly pulled downward at the Earth could result in a body continually falling toward it for eternity—what we call an orbit—it did not require too large a leap of imagination to suppose that objects orbiting the sun, such as the planets, were being continually pulled toward the sun, and not pushed around it. (Incidentally, it is the fact that objects in orbit are continually “falling” that is responsible for the weightlessness experienced by astronauts. It has nothing to do with an absence of
gravity, which is almost as strong out at the distances normally traversed by satellites and shuttles as it is here on Earth.)
In any case, another of Kepler’s laws of planetary motion provided the icing on the cake. This law yielded a quantitative key that unlocked the nature of the
Both Brahe and Kepler were remarkable characters. Brahe, from a privileged background, became the most eminent astronomer in Europe after his observations of the supernova of 1572. He was given an entire island by the Danish monarch King Frederick II to use as an observatory site, only to be forced to move some years later by Frederick’s successor. Unhindered by, or perhaps because of, his arrogance (and a false nose made of
metal), Brahe managed to improve in one decade the precision in astronomical measurement by a factor of 10 over that which it had maintained for the previous thousand years—and all of this without a telescope! In Prague, where he had gone in exile from Denmark, Brahe hired Kepler a year before his own death to perform the intricate calculational analysis required to turn his detailed observations of planetary motions into a consistent cosmology.
Kepler came from another world. A child of a family of modest means, his life was always at the edge, both financially and emotionally. Besides his scientific pursuits, Kepler found time to defend his mother successfully from prosecution as a witch and to write what was probably the first science fiction novel, about a trip to the moon. In spite of these diversions, Kepler approached the task of analyzing the data in Brahe’s notebooks, which he inherited upon Brahe’s death, with an uncommon zeal. Without so much as a Macintosh, much less a supercomputer, he performed a miracle of complicated data analysis that would occupy the better part of his career. From the endless tables of planetary positions, he arrived at the three celebrated laws of planetary motion that still bear his name, and that provided the key clues Newton would use to unravel the mystery of gravity.
I mentioned one of Kepler’s Laws earlier—namely, that the orbits of the planets sweep out equal areas in equal times—and how Newton was able to use this to infer that there was a force pulling the planets toward the sun. We are so comfortable with this idea nowadays that it is worth pointing out how counterintuitive it really is. For centuries before Newton, it was assumed that the force needed to keep the planets moving around the sun must emanate from something pushing them around. Newton quite simply relied on Galileo’s Law of Uniform Motion to see that this
was unnecessary. Indeed, he argued that Galileo’s result that the motion of objects thrown in the air would trace out a parabola, and that their horizontal velocity would remain constant, would imply that an object thrown sufficiently fast could orbit the Earth. Due to the curvature of the Earth an object could continue to “fall” toward the earth, but if it were moving fast enough initially, its constant horizontal motion could carry it far enough that in “falling” it would continue to remain a constant distance from the Earth’s surface. This is demonstrated in the following diagram, copied from Newton’s Principia:
Having recognized that a force that clearly pulled downward at the Earth could result in a body continually falling toward it for eternity—what we call an orbit—it did not require too large a leap of imagination to suppose that objects orbiting the sun, such as the planets, were being continually pulled toward the sun, and not pushed around it. (Incidentally, it is the fact that objects in orbit are continually “falling” that is responsible for the weightlessness experienced by astronauts. It has nothing to do with an absence of
gravity, which is almost as strong out at the distances normally traversed by satellites and shuttles as it is here on Earth.)
In any case, another of Kepler’s laws of planetary motion provided the icing on the cake. This law yielded a quantitative key that unlocked the nature of the
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