Riddle, Bob, Science Scope
One of the pieces of evidence that helped confirm the Sun's position (not the Earth's) as the centerpiece of our solar system is the retrograde motion of the planets. Retrograde motion refers to the apparent backward orbital motion of a planet toward the west. Retrograde is an apparent motion because all planets revolve around the Sun in the same direction, and from the surface of the Earth, this motion is toward the east. However, as the faster-moving Earth catches up with and passes an outer planet, there is a period when that outer planet appears to be moving backward, or toward the west. From our perspective, the planet seems to slow down, come briefly to a stop, and then start moving backward. After a period of time, the planet once again seems to pause in its orbit, but then it resumes its direct, or eastward, orbital motion.
Over time, the motions of the visible planets are easily seen with the naked eye, and it was probably those motions that led to these wandering stars being called planets. However, to the early astronomers, explaining the backward retrograde motion was somewhat of a problem complicated by two related factors (using circular orbits and a correct understanding of the arrangement of planets and the Sun in the solar system). It wasn't resolved until German mathematician Johannes Kepler (1571-1630) developed the laws describing planetary orbits, including the first law stating that planets do not follow circular orbits, but have elliptical-shaped orbits around the Sun.
Until Kepler's time, orbits of the planets were drawn as circles. We now know that the orbital shapes are actually elliptical, but they are close enough to being circular that calculating and predicting their future orbital positions could be made with a reasonable amount of accuracy. However, accuracy decreased as the span of time to be calculated increased. Basically, a planet never quite came back to exactly the same point in space at the same time each revolution, despite following a circular-shaped orbit. Each time around, the planet was farther east, and this shift during each revolution resulted in increasingly complex diagrams used for predicting planet positions.
Leading up to a correct explanation for retrograde motion were at least three models, two Earth centered and the third Sun centered. All three models have the planets and the Sun on circular orbits. However, unlike our modern model of the solar system where the planet moves along the orbital path around the Sun, in these models the planet is on a smaller circle called the epicycle. The planet spins around the center of the epicycle while the epicycle follows the larger orbital path, called the deferent. During part of the epicycle period, the planet moves westward, or in retrograde motion (see Figure 1a).
The earliest model was an Earth-centered one proposed by Claudius Ptolemy (90-168 AD). Ptolemy, an Egyptian astronomer and mathematician, was so influential that his model became the standard and was used for more than 15 centuries. In the Ptolemaic model, the Earth is in the center surrounded by the circular orbits of the Sun, and visible planets (see Figure 1a).
During the 15th century, Nicolaus Copernicus (1473-1543), a Polish astronomer and mathematician, proposed a Sun-centered model of the solar system that correctly had the Sun in the center, but relied on the use of circular orbits and epicycles to explain retrograde motion. Accuracy of planet position calculations was better; however, the Copernican model was more complicated, as it reduced the size of the deferent and epicycle and placed them both on a larger circular orbit around the Sun. Both the deferent and epicycle were in motion (see Figure 1b).
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Tycho Brahe (1546-1601), a Danish astronomer known for his accurate observations of planet positions, developed an Earth-centered model of the solar system that had the Sun following a circular orbit around the Earth while the planets orbiting the Sun also followed circular orbits (see Figure 1c). …