Process of Science: Explaining the Motions of Planets. To ancient astronomers, the motions of the Sun, the Moon, and the stars seemed fairly simple. However, the motions of the planets, which were more complicated, were harder to account for. It was a challenge for astronomers to explain the observed motions of the planets and to relate those motions to variations in planetary brightness. Part A- Two competing models attempt to explain the motions and changing brightness of the planets: Ptolemy's geocentric model and Copernicus' heliocentric model. Sort the characteristics according to whether they are part of the geocentric model, the heliocentric model, or both solar system models.

Geocentric Model: - this model is Earth Centered

-Retrograde motion is explained by epicycles

Heliocentric Model: - This model is Sun Centered

-Retrograde motion is explained by the orbital speeds of planets

Both models:- Epicycles and deferents help explain planetary motion

-Planets move in circular orbits and with uniform motion

-The brightness of a planet increases when the planet is closest to Earth

Explanation:

Retrograde motion is an apparent change in the movement of the planet through the sky. Ptolemy's model of the solar systems was geocentric, where the Sun, Moon, planets and start all orbit the Earth in perfectly circular orbits. However this perfectly circular orbits around the Earth did not explain the occasional retrograde motion of the planets. In the Copernicus' heliocentric model, retrograde motion of planets is naturally explained. The explanation for retrograde motion in a heliocentric model is that retrograde occurs roughly when a faster moving planet catches up to and passes a slower moving planet.