Renaissance Astronomy
The following notes are excerpted from Journey to the Cosmic Frontier by John D. Fix, Archives of the Universe by Marcia Bartusiak, and Coming of Age in the Milky Way by Timothy Ferris.
Nicolaus Copernicus wrote Commentariolus (Little
Commentary) in 1514 and proposed a model of the heavens in which
he brashly asserted that the Sun is the center of the universe.
We revolve around the Sun just as any other planet,
he wrote. Copernicus was not concerned with a moving Earth, but
he objected to a celestial sphere rotating around a stationary
Earth. To him, a stationary heavens with the source of light and
heat resting at a stationary throne in the center was a more noble
configuration than a stationary Earth. By the time of Copernicus,
western philosophers — in particular, the theologian St. Thomas
Aquinas — had rediscovered the works of ancient Greece and
had come to revere Aristotle and his vision of perfect circular
motion. This belief would come back to haunt the work of Copernicus
as he discovered, just like Ptolemy thirteen centuries earlier,
that the heavens had no need to satisfy earthly aesthetics. Much
to his chagrin, he eventually had to add epicycles to his heliocentric
model and even reintroduce the idea of the equant for off-center motion.
In his completed treatise, De Revolutionibus Orbium Coelestium (On
the Revolutions of the Heavenly Spheres), a well-meaning theologian
wishing to protect Copernicus from controversy — and possibly heresy
charges — added an introduction to the book stating that a sun-centered
model was merely a mathematical convenience
and should not
be taken literally.
Copernicus' model provided a more elegant solution to the retrograde motion of planets (it is the motion of Earth that is the cause), but Ptolemy's model was still quite capable of providing accurate planetary positions. Copernicus' model was no more accurate than Ptolemy's and in some cases was much worse. Ironically, Copernicus' desire to restore an Aristotelian aesthetic to the motion of the heavens required a moving Earth, but resulted in simplified explanations of planetary motions.
While planetary motions were simplified, Copernicus actually complicated the lives of natural philosophers trying to unravel the mysteries of Earthly dynamics: namely, why we don't feel Earth's rotation and why objects thrown into the air are not left behind by a moving Earth.
The last of the great pre-telescopic observers, Tycho Brahe made observations that pave the way for unraveling the mystery of planetary motions, but also cast more doubt on the Aristotelian world view of an immutable heavens. After observing a nova stella (supernova) and comet, Tycho ascertained that both were located beyond the "region of Element", or beyond the Moon's orbit. That the comet moved across the sky, and presumably through the crystalline spheres, made Tycho question the reality of the spheres.
By making the most accurate naked-eye observation known, Tycho and his assistants were able to measure the length of the year to within 1 second, determine how the earth's atmosphere bent starlight, and refined the precession rate discovered by Hipparchus. As a transitional figure between medieval and Renaissance astronomy, Tycho could not accept Copernicus' bold, new model and stubbornly maintained that the heavens moved around Earth. His only concession was to propose that the earth was stationary and that the Sun — carrying all the planets with it — moved around Earth.
In 1600 Tycho hired Johannes Kepler, who knew of Tycho's work and was anxious to solve the riddle of planetary motion, to be his chief assistant. Unfortunately, Tycho's ego and awareness that Kepler was a far superior mathematican moved Tycho to assign various mundane tasks to his assistant instead of sharing the precious planetary observations needed by Kepler. The untimely death of Tycho changed all of that.
A confirmed Copernican, Johannes Kepler devoted much of his life after the death of Tycho Brahe to determining the motions of the heavens. His goal was to "demonstrate from observations" the motions of the planets. Kepler discovered a geometric rule that explained the size of a planet's orbit and its speed and published his findings in the Cosmographic Mystery in 1596. He based his fanciful ideas on the shapes of the five Platonic solids (a cube, tetrahedron, dodecahedron, icosahedron, and octahedron) nested inside each other. The mathematics impressed Tycho Brahe so much that he invited Kepler to work for him. Upon Tycho's death, Kepler set out to solve the riddle of the motion of Mars—a task assigned by Tycho prior to his death. Boasting he would solve the mystery in a week, it took nine years!
Because Tycho's observations were so accurate, Kepler was able to plot Mars' position at every degree along its orbit and realized that simply moving the Sun off-center of a circular orbit did not make observations match the predicted positions. Only be making the orbit elliptical did the measurement of equal areas swept out in equal time intervals work out. What we now consider to be Kepler's second law of planetary motion was actually solved first.
It was the very observations of Mars that convinced Kepler that Copernicus was correct and that Aristotle's Earth-centered world view could not stand. Kepler realized that Tycho's observations sometimes placed Mars nearer to Earth than the Sun. If crystalline spheres existed, Mars would actually smash through the Sun's sphere.
The laws of planetary motion formulated by Kepler are
- A planet moves along an elliptical orbit about the Sun with the Sun occupying one focus of the ellipse.
- During equal time intervals, a Sun-planet line sweeps out equal areas.
- The square of a planet's sidereal period is proportional to the cube of the orbit's semi-major axis.
Much of Kepler's work was inspired by William Gilbert's claim that the Earth was a giant magnet and that the Sun emitted magnetic rays that moved the planets. Though completely wrong, the theory moved the science of astronomy towards physics and away from geometric principles. Kepler came to a different conclusion and eventually concluded that there were two forces acting on planets: one spiraling outward from the Sun that pushed the planets along in their orbits and a second magnetic force of a attraction that pulled the planets towards the Sun. The combination of these two forces was thought to create the elliptical orbits of the planets. Though wrong, the idea that motion was the result of forces had been planted.
In 1627, Kepler produced the Rudolphine Tables, a far more accurate resource for calculating planetary positions based on his own laws of planetary motion. In his own time, this tome ensured Kepler's renown. By doing away with the need for fictitious circles and geometric figures, however, Kepler realized that his successors' task would be to solve the riddle of the underlying physical principles that govern the cosmos.
Galileo began his study of moving bodies in 1600. At the time, the most widely accepted theory of motion was the impetus theory—a concept that had been around for 200 years and shared its roots with the teachings of Aristotle. The central idea of the theory was that an object would remain in motion only as long as a force was at work on the object. Galileo, however, reasoned that mathematics and experimentation were the best tools for unraveling the mysteries of nature instead of relying on common sense.
Believing as others did, that heavy object fall faster than lighter ones and that the speeds of fall was proportional to the weights, Galileo set out to investigate the concept of free-fall using balls of various weights rolling down inclined ramps. The rolling balls moved much slower than ones dropped vertically and could thus be measured more accurately. He also used more accurate clocks than his predecessors. What Galileo found is that the distance a ball traveled was proportional to the square of the time it had been in motion. This was true no matter what the slope of the ramp or the weight of the ball. Reasoning that all objects would fall at the same rate in the absence of air, he concluded that heavy objects fall faster in air because the air resistance is negligible to them.
Galileo's experiments also lead him to conclude that motion was as natural a state as rest. Contrary to the impetus theory—in which an object remains in motion only until its impetus was used up, Galileo theorized that an object set in motion would continue in a state of motion forever or until something acted upon it. In other words, nothing was required to keep an object in motion once started; only stopping it required a force.
Galileo's observations with a telescope further convinced him of the validity of Copernicus' heliocentric model of the universe. The observations that cast doubt on the Aristotelian world-view of Ptolemy's geocentric cosmology were:
- Four starlets that followed Jupiter and continually orbited it.
- Earth-like features on the moon: mountains, valleys, and "seas"
- Spots on the surface of the Sun and evidence of the Sun's rotation
- Countless stars in the Milky Way
- Changing phases of Venus that indicate it orbits the Sun
Galileo's work on motion was continued by the French philosopher René Descartes, who believed that all motion resulted from collisions with tiny particles called corpuscles. If no collisions occur, an object remains at rest. An object in motion remains in motion at the same speed in the same direction. Descartes' conceptualization is a clear statement of inertia. Not only did he believe that an object would remain in motion unless something acted upon it, but he also believed the objects natural motion would continue in a straight line. The circular motion that formed the basis of both Ptolemy's and Copernicus' theories was not possible since they assumed no forces were present. According to Descartes, any deviation from linear motion required the application of a force.
For more information, visit The Galileo Project website.