# How Did the Ancient Greeks Reconcilize the Observed Motions of the Planet With an Unmoving Earth?

The Greeks’ answer to the question How did the ancients reconcile the observed motions of the planet with an unmoving Earth is a fascinating one. They used observational astronomy which focuses on the observed motions of the planets and the heavenly bodies. However their answer was also incomplete. Observations also require periods and rotations.

## Observational astronomy

Greek astronomers reconciled observed motions of the planets with the unmoving Earth by introducing circles to the orbits of wandering stars. The Greeks believed that the heavens were a perfectly circular space so they created illustrations that appeared disorienting. In the fourth century BCE Copernicus’s model was accepted by Greek astronomers but it was not widely accepted.

Greek astronomy began with the Pythagorean school. This school of thought believed that the order of the cosmos was mathematical and the heavenly bodies were revolving around a central fire. They also constructed the first physical model of the solar system. The character of Greek astronomy derives from a comment from Plato who in the 4th century BCE instructed scientists to ‘save phenomena in a uniform circular motion.’

Hipparchus made important contributions to astronomy. His theories based on an impressive body of observational data. Hipparchus used the idea of an eccentric circle with the Earth at some point being eccentric to the geometric center of the universe. Hipparchus used this concept to explain the irregularities and inequalities of the Moon and Sun’s motion. Hipparchus also derived the mathematical equivalent of an epicycle-deferent system a method first developed by Apollonius of Perga.

A study of the front cover of a copy of the book reveals extensive information about the motions of the planets. In addition to listing the synodic periods of each planet the inscription also lists the days between each event and its duration in years. For example Venus takes 584 days to return to the same position relative to the sun. Saturn takes 442 years to return to its original position.

## Observed motions of the planets

The first known attempt to explain the apparent motions of planets is attributed to the ancient Greeks. The inferior planets Mercury and Venus were fixed to their orbits around the Sun but were never observed to move away from the Sun. During the time of Plato the question of why the planets appeared to move relative to the stars was posed by Plato. In the 2nd century A.D. Greek astronomer Ptolemy formulated the first solar system theory and his book became standard astronomy until the Renaissance.

The first major thinker to assert that the heavens must be perfect was Plato who left massive written records and intellectual clout. Plato reasoned that the most ideal orbit of a planet would be circular and constant in speed. This is because motion is constant but change is not. So if the heavens were perfect then planets would not change in speed.

Aristotle who worked in Alexandria in the Second Century AD had a more sophisticated approach to explaining how the planets moved. His model incorporated a sphere at the center of a larger sphere to explain the apparent motions of the planets. Nevertheless aristotle’s model required 56 spheres to account for the motions of the seven heavenly bodies.

Hipparchus’ theory of the Moon incorporated the idea of an epicycle or a small circular motion riding around a larger circular motion. The Ptolemaic reference at the Galileo project explains what an epicycle is and how it could be used to calculate the positions of all planets. Furthermore Hipparchus’ theories resulted in the development of trigonometry a mathematical tool used to calculate the positions of the planets.

## Observed positions of the heavenly bodies

The observable positions of the heavenly bodies in ancient Greece were known and recorded by a number of Greek astronomers. These astronomers were not all devoted astrologers however. In fact a number of these men were not astrologers at all and they did not use the astronomical observations as a method of teaching. However it is not impossible to determine whether they observed any of the heavenly bodies. Observed positions of the heavenly bodies in ancient Greece are known to date back to at least the fifth century BC.

The observations were accurate to within a few minutes. Greek astronomers tended to reject Aristarchus’s heliocentric model of the universe because the heliocentric model contradicted the widely held belief that the earth is the center of the universe. Nevertheless it is possible that these observations were made before the heliocentric universe was conceived. For this reason it is possible that the heavenly bodies were known to the Greeks around 300 BC.

The Greeks observed seven objects before they had the use of telescopes including the Sun the moon and five planets. Because they could observe the movement of the planets they understood that the Earth is not a sphere in space. They also realized that the planets were not on a celestial sphere and would transit the sun on their nightly course. In fact the ancients were the first to postulate the existence of planets and their motion relative to the sun.

Greeks were among the first astronomers to observe the heavenly bodies. The ancient Egyptians for example were fascinated with the night sky. They had a great interest in the stars and used them to guide their travels. Even Polynesians used stars and constellations to navigate between the islands of the Pacific Ocean. Ancient Greek astronomers were influenced by the Babylonians. Pythagoras a famous Greek astronomer observed the morning and evening star as one body. Similarly Aristotle of Stagira understood that the Earth was round and that the sun and moon moved at different speeds.

## Observed periods

Observed periods of the planets refer to the time between successive recurrences of a celestial body’s phases. This period also applies to artificial satellites of Earth. For example Jupiter’s synodic period is 398.8 days. Jupiter’s opposition to Earth occurs every 13 months. In addition the Moon and its artificial satellites also have synodic periods.

The observable periods of the planets sometimes referred to as synodic phases were first observed and documented by Babylonian astronomers. In addition to identifying planetary stationary points these astronomers recorded the time of the planets’ appearances and disappearances. Babylonian astronomers compiled lists of observations of planets during several tens of years.

The retrograde motions of outer planets can be explained by the ephemerides and the epicycle. The deferent is a large eccentric circle on which the planet is located. Most of the time the planet moves in one direction and retrograde when the epicycle turns. During the first part of its epicycle the planet is stationary; the second half of the cycle is in a reverse position.

Earlier than Tycho’s observations astronomers had to explain the motion of the planets in order to understand their position in the sky. In some cultures the planets were important for astrological reasons including the fact that they always appeared near the Sun. Mercury and Venus were even sometimes visible before the Sun rose and after it set. So astronomers had to explain how the planets were moving in relation to the Sun.

## Conciliation with an unmoving earth

The Greeks first recognized the planets as wanderers because of their erratic motions. Wanderers were the most significant problem in astronomy at that time. It was unclear what these ‘wanderers’ were but it was evident that they move in the same direction as the sun and sometimes reverse their motion to move in retrograde motion. In both cases the wanderer eventually finds a stationary point and resumes its prograde motion. This erratic motion was also the cause of the planetary rotations. The Greeks also believed that the Earth is a planet.

However the ancient Greeks recognized the Earth and Moon as spheres and rejected the notion that the Earth moves. They also measured the sizes of the Earth to a precise degree and Hipparchus made many observations to further their understanding of astronomy. He also discovered precession which he described as a shift in the celestial pole. In the Almagest Ptolemy summarized his work on classic astronomy. He used a geocentric model based on epicycles to explain the motions of the planets.

The ancient Greeks’ solution to this problem was a complex cosmological system involving multiple planets that orbit around the Earth. This system consists of several circles each of which has a center far away from the Earth. The planets’ center moves in the same direction as their epicycle center. Therefore planets at position x appear to move eastwards from Earth while those at position y move in the opposite direction.