The 7 Planets of the Ancients

1
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.
• 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.
• 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 1781.
• Neptune and Pluto are too faint to see at all
  without a telescope they were discovered in 1846
  and 1930, respectively.

2
Aristotles Geocentric (Earth Centered) ideas
were accepted from 350 BCE to 150 CE and beyond.
3
Retrograde Motion

• Apparent Backwards Motion

4
The Solution?Ptolemy 150 CE
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The Development of Modern Astronomy
Aristotle and Ptolmeys ideas were accepted until
the 16th century.
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.
6
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
7
The Copernicus Model
Ptolmeys Model
8
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!