| Home | Articles | Guide to Observing | Tour of the Constellations | Book Reviews |
People in the Middle Ages were far more sophisticated about the sky than most of us today. As a theatergoer left the Globe Theater after seeing the premiere of Julius Caesar, he could look upwards at the stars and know exactly his place in the Universe. He was at the center. Actually the exact center was in Jerusalem, but he was close enough to it. All those bright, twinkling points of light set in England's clear pre-industrial sky were revolving around the Earth.
Our theatergoer, if he had the time and inclination, could watch the constellation of Orion slowly rise in the east, circle around the southern sky and finally set over the western horizon. He probably realized that both the Sun and the Moon also moved as if they were attached to invisible spheres gradually rotating around the Earth.
The only thing that might have bothered our sixteenth-century skygazer would be the wandering "stars" such as Venus, Mars and Jupiter. They would be moving like any normal star from east to west and then inexplicably change direction without any rhyme or reason.
Most of us were taught that the great Polish astronomer Copernicus discovered that the Earth and its sister planets "really" moved around the Sun. But 1800 years before Copernicus, a Greek philosopher, Aristarchus had already written that the Earth revolves around the Sun and not vice versa. Nevertheless, the idea that held sway over the western world until Copernicus was that of Ptolemy, who lived in Egypt. Under the Ptolemaic system, the Earth was at the center. All the heavenly bodies were fixed on large invisible spheres, which rotated around the Earth at different rates of speed. All the fixed stars were on the same sphere because they all moved at the same rate.
Like Copernicus, we are going to concede the center to the Sun. We will let the Earth and its sister planets circle around the Sun.
But doesn't the Earth really revolve around the Sun? Not according to modern physics, which declares that Copernicus' system is no truer than Ptolemy's. So why choose it? We choose it for the same reason that Copernicus chose it; it's prettier. I have mentioned that on occasion the planets reverse their tracks and move from west to east. Medieval astronomers found it difficult to imagine a celestial sphere suddenly reversing direction, so they attached smaller spheres called epicycles to the larger ones to explain the periodic backward motion of the planets. As more accurate observations of the sky were made, more and more epicycles had to be added to explain the new observations. The epicycles did explain the observations just as well as Copernicus' system did. But his system was simpler and more elegant. Scientists are greatly concerned with aesthetics and always try to find a theory that will not only explain all the observable facts, but also explain them in the simplest way possible. If we have a choice between a solar system of a hodge podge of epicycles or a sun majestically circled around by its planets, we choose the latter.
Before going any further, let me clarify the difference between rotating and revolving. An object rotates when it turns around its center point or axis. A top or a roulette wheel rotates when it is spinning. All objects in the universe are rotating. That includes the planets, their satellites, our Sun and the other stars and even whole galaxies consisting of billions of stars. An object that is circling or orbiting around a point is revolving around that point. The orbit of a revolving object does not have to be an exact circle. The Earth and the other planets travel around the Sun in an ellipse. An ellipse is a circle that may or may not be "squashed down". When an object anywhere in the universe, revolves around another object, it does so in an ellipse.