The 7 Planets of the Ancients The term "planet" originally meant "wanderer“ There were thought to be 7 such wanderers or planets: - Mercury, Venus, Mars, Jupiter, Saturn, the Moon, and the Sun. 1.The Earth is missing, because it was not understood that the points of light wandering on the celestial sphere and the Earth on which we stood had anything in common. 2.Uranus, Neptune, and Pluto are missing because they would only be discovered when the telescope made them easily visible. Uranus is barely visible to the naked eye; it was discovered in Neptune and Pluto are too faint to see at all without a telescope; they were discovered in 1846 and 1930, respectively.

Aristotle’s Geocentric (Earth Centered) ideas were accepted from 350 BCE to 150 CE and beyond.

Retrograde Motion Apparent Backwards Motion

The Solution? Ptolemy 150 CE Epicycles and Deferents What does this updated model prove? - All motion is circular - All materials in the universe are perfect and can’t change their material - The Earth is the center of the Universe

The Development of Modern Astronomy In a book called On the Revolutions of the Heavenly Bodies (that was published as Copernicus lay on his deathbed), Copernicus proposed that the Sun, not the Earth, was the center of the Solar System. Such a model is called a heliocentric system. Aristotle and Ptolmey’s ideas were accepted until the 16th century.

Copernicus and the Need for Epicycles Copernicus did not question the assumption of uniform circular motion. Thus, in the Copernican model the Sun was at the center, but the planets still executed uniform circular motion about it. The difference was that the Copernican system required many fewer epicycles than the Ptolemaic system because it moved the Sun to the center

Ptolmey’s Model The Copernicus Model

The Copernican Revolution His ideas were published in 1543 in his book On the Revolutions of the Heavenly Bodies In 1616 the Catholic Church placed it on the list of forbidden books where it remained until 1835!