Geocentric model, Lecture notes of Astronomy

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Lecture 4

The Geocentric Model

of the Universe

January 3a, 2014

Size of the Earth – Eratosthenes

(200 BC)

• In Syene, Egypt, the Sun would shine straight down wells at noon on the summer solstice (June 21) - The Sun is at zenith at noon on the solstice along the Tropic of Cancer (23.5° north latitude)
• In Alexandria, the Sun was 7 ° 12’ south of zenith at that time.

The Geocentric Universe

• Ancient people (Aristotle) “knew” from observations that - The stars appear fixed on the sky relative to each other. - Planets, moon and sun change position with respect to the stars. - Mercury and Venus only seen near sun.
• These observations led them to theorize that the Earth is at the center and all objects move around it.

Basic Geocentric View

E

Sun

Moon

Mercury

Saturn Celestial Sphere

Jupiter

Mars

Venus

Retrograde Motion

• Planets usually move from west-to-east in the sky with respect to the stars, just like the Sun.
• Sometimes planets go east-to-west in sky, an event called retrograde motion.
• Retrograde motion was very hard to explain, representing the main challenge to ancient models of the solar system.

(^8) Retrograde Motion

• Direct Motion: Planets usually move from west-to-east in the sky with respect to the stars over long periods of time.
• Retrograde Motion: Planets sometimes change direction and go east-to-west with respect to the stars
• Mars retrograde motion (below and NASA site)

East West

• Predicted positions of planets with respect to stars with great accuracy.
• This theory was accepted for ~ centuries!

Ptolemy (140 AD)

• Size and rates of motion on epicycles was calculated using tabulated data - Hipparchus - Ptolemy

Java animation of Ptolemy's model

Geocentric Model Overview