Physics 111	Prof. P. Berman
Evolution of Scientific Thought

The Copernican System

De Revolutionibus is published in 1543, the year of Copernicus’ death. By 1630, one can say that the Copernican system was accepted by most astronomers. A number of events led to the assimilation of the Copernican system. Before listing some of these, it is important to note that, when Copernicus published his system and even by about 1630, the Copernican method of calculating did not yield results that were significantly better than those based on the Ptolemaic system. Nevertheless, the evidence or pressure was mounting to accept the heliocentric system as the “true” system of the solar system. What were some of the strengths of and weaknesses of the Copernican system?

The major strength was the elimination of the primary epicycle that gives rise to retrograde motion, varying brightness, and varying speeds of the planets in the Ptolemaic system. The retrograde motion is now explained as an apparent motion viewed from the Earth. This also explained why the apparent average periods of the inner planets was equal to one year.

To counteract this strength, there are numerous “weaknesses”:

· If the Earth is rotating on its axis, why don’t we feel the motion?

· If the Earth is rotating on its axis, why don’t we fly off?

· If the Earth is rotating about the Sun, why don’t we observe parallax of the stars?

· Motion of the Earth is against the scriptures.

The first of these questions had already been addressed by Buridan and his student Oresme at the University of Paris in the 14^th century. Oresme stated that the same relative motion would be observed if the Earth stood still and the stars rotated or if the Earth rotated on its axis and the stars stood still. (We will return to discuss the validity of this statement later in the course.) He gave an example of people on boats feeling at rest even though they are moving. He concludes that the Earth can be rotating, but we don’t feel the motion. Copernicus gives a similar argument and so does Galileo. This is not a “proof “ that the Earth is rotating, but an argument to indicate that it is possible for the Earth to be rotating and we don’t sense any motion. The second question was reversed by Copernicus – if we should fly off the Earth, why don’t the stars fly apart it they are rotating about the Earth. It will have to wait until Huygens and Newton to quantitatively determine the effects of rotation on an objects weight. To explain the lack of parallax, Copernicus had to assume the stars were more distant than had been previously thought. The violation of scriptures played an important role in the debate over the two world systems.

None of these “explanations” really is a proof for the Copernican system. Yet the Copernican system catches on for a variety of reasons, some of which are outlined below.

· De Revolutionibus was a printed book which was readily available to scholars.

· Rheticus’ publication in 1540 of the Narratio Prima serves as a simpler introduction to the Copernican system

· Prutenic Tables for planets and stars published about 1550 by Erasmus Reinhold were based on Copernican methods. These were the first detailed tables to be published in 300 years and were up to date, although not more intrinsically accurate than previous tables.

· Thomas Digges (1564-1595) speaks of an infinite Universe and Bruno (1548-1600) extends this idea to having no center for the Universe with the Earth and Sun at no special place.

· Tycho Brahe (1546-1601) makes exceptionally good measurements on planets (4’ of arc) and stars (1’ of arc). He was not a Copernican but proposed a system in which the Earth was stationary and the sun revolved around the Earth, but all the planets revolved around the sun. His system is mathematically equivalent to Copernicus’, but not physically equivalent (there are measurable consequences of the fact that the Earth rotates on its axis). The nova of 1572 burns for over a year and it is clear that there is no parallax – the celestial sphere is not immutable. Comets are also seen with no parallax, implying that they cannot be sublunar. The Aristotelian ideas are shaken deeply.

· Kepler (1571-1631) is a student of Maestlin, where he learns and becomes committed to the Copernican system. Kepler certainly seems to be driven by a desire to explain the solar system in terms of “harmonies” associated with the regular Platonic solids. Still he manages to overcome the mysticism to provide the calculations that establish the Copernican system. He had Brahe’s data to work with and concentrated on the orbit of Mars. In the Mysterium of 1597 Kepler proposes that an anima motrix from the sun, pushes the planets and the Earth around. This led him to consider the force exerted on the planets by the sun. He then gets to work on the orbit of Mars and discovers his second law – equal areas are swept in equal times. He made some errors in arriving at this correct law, but it was essential for him to view to solar system as Copernican for him to succeed. He had to calculate Mars’ orbit and then how we view Mars’ orbit from the moving Earth. In the figure the planet must move faster at A than B.

· In the Astronoma Vova of 1609, both the second law and Kepler’s First Law (planets move about the sun in elliptical orbits with one focus at the Sun) are included. With one fell swoop, the epicycles, equants, and eccentrics are gone for good! For good measure, Kepler’s Third Law, which we will discuss later is given in Hamonice Mundi published in 1619. This law states that the ratio of the period squared and the semimajor axis of the elliptical orbit cubed is a constant for all the planets. This determines the distance once the period is known. The Rudolphine Tables published in 1627 help acceptance of Kepler’s results.

· A contemporary of Kepler was Galileo (1564-1642). He was a devote Copernican, but seemed to hold to circular orbits, even after Kepler’s work. There was another nova in 1604 with no parallax, but the thing that made Galileo’s career was the use of a telescope to look at the heavens. With it he found that the stars were brighter, but not bigger, which implies that they are more distant than thought. Moreover he discovered many new stars. Next he observed that Jupiter has 4 planets – clearly everything can’t rotate about the sun. The most important discovery was the phases of Venus. This could occur for the Copernican or Brahe system, but not for the Ptolemaic system, where Venus would always appear as a thin crescent. His observations of rotating sun spots showed that the sun was not all that perfect. Galileo was, above all, an outspoken proponent of the Copernican system.

By Galileo’s death in 1642, the Copernican system was fairly well established. It remained for Newton to quantify the entire solar system dynamics. But that is another story.