Ptolemy and Retrograde Motion I

This animation shows how Ptolemy's geocentric model could reproduce planetary retrograde motion for a superior planet (i.e. a planet whose orbit lies outside of earth's orbit).

Category:

Education

Tags:

• physical
• science
• astronomy
• planet
• retrograde
• motion
• Ptolemy

[imarchello]

well, he failed.... that planet is orbiting around earth and around nothing at the same time - WTH?

Heliocentric model explains planet retrograde motion perfectly.

And heavy objects usually don't orbit around much lighter objects, like sun around earth.

the center of gravity would be deep inside the sun, therefore such orbit is impossible.

[mrg3]

A good intro astronomy course should have the historical development of the subject. Understanding why older models were accepted at the time can really help develop an understanding about how progress is made.

Even though this model has been superseded by later developments, the historic context is an important part of developing an understanding how modern science works.

[imarchello]

at 0:02 the planet would get pwned by earth's gravitation

[mrg3]

Ptolemy and his model predate Newton and his theory of gravity by quite a bit. Ptolemy was trying to create a simple mechanical explanation of the apparent motion of the stars, sun and planets.

I will also point out that this animation is designed to illustrate some aspects of Ptolemy's model and is not to scale.

[imarchello]

Just read about him. He tried to explain retrograde motion:

"This model consisted of a series of concentric spheres, with the Earth at the center (geocentric). The motions of the Sun, Moon, and stars was based on perfect circles. To account for the observed retrograde motion of the planets, it was necessary to resort to a system of epicycles, whereby the planets moved around small circular paths that in turn moved around larger circular orbits around the Earth."