Johannes Kepler published Book IV of the Epitome of Copernican Astronomy in 1617. In Book IV, Kepler formally presents his three laws of planetary motion that resolve the problems associated with the epicycles of Copernicus’ heliocentric model. Kepler’s First Law is called the Law of Ellipses. It states that “the orbits of the planets are ellipses, with the Sun at one focus.” Kepler’s Second Law is called the Law of Equal Areas in Equal Time. It states that “the line between a planet and the sun sweeps out equal areas in the plane of the planet’s orbit over equal times.” Kepler’s Third Law is called the Law of Harmony. It states that “the time required for a planet to orbit the sun, called its period, is proportional to half the long axis of the ellipse raised to the 3/2 power.” Although Kepler discovered these Laws, he did not know how they worked. Newton solved this problem with his Theory of Gravity.
Nicolaus Copernicus published the Revolutions of the Heavenly Spheres in 1543. In Book I, Copernicus presents his heliocentric model of the universe. He argues that the Sun, not the Earth, is the center of the universe. He also correctly determined the order of the planets. He wrote that the planets revolve around the Sun in the following spheres – Mercury, Venus, Earth, Mars, Jupiter, and Saturn. However, he stubbornly held onto the belief that the planets must revolve around the sun in perfect circles; for God is perfect. This caused him to mistakenly retain Ptolemy’s system of epicycles to explain the motions of the heavens. Continue reading COPERNICUS: Revolutions of the Heavenly Spheres [Introduction—Book I-Ch. 11]
In The Almagest, Ptolemy outlines antiquity’s geocentric model of the universe. The model is based on the assumptions that the universe is spherical, the Earth is a sphere, the Earth is at the center of the universe, and the Earth does not move. Although modern scientific advances have determined that many of these assumptions are false, Ptolemy correctly conjectures that the Earth is spherical (and yes, I know that the Earth is not a perfect sphere) which is an accomplishment considering that Ptolemy wrote The Almagest in 150 AD, a time when modern astronomical observation instruments were unavailable.