Geocentrism facts for kids

Diagram illustrating geocentrism, showing all planets and stars circling the Earth

Geocentrism is the belief that the Earth is fixed at the centre of the Universe. Geocentrists accept that the earth is round. Before the 16th century most people believed in the theory of geocentrism. From Earth, it looks like the Sun and stars are moving across the sky. The Ancient Greek astronomer, Ptolemy wrote a book to explain in great detail how the spherical Earth is surrounded by things that move in the sky. From the time of Ptolemy around the 2nd and 3rd centuries AD through to the 16th Century AD educated people who knew the Earth is round almost always believed the Geocentric theory of Ptolemy.

From the 15th to the 17th century, astronomers, especially Copernicus, Galileo and Kepler, found evidence that the Earth is not fixed but moves round the Sun. That is called heliocentrism.

Geocentrism has been proven to be incorrect. However, there are still geocentrists today. In the United States, the National Science Foundation found that about 25% of Americans believe in geocentrism.

Ptolemaic system

The basic elements of Ptolemaic astronomy, showing a planet on an epicycle with an eccentric deferent and an equant point. The Green shaded area is the celestial sphere which the planet occupies.

Ptolemy argued that the Earth was a sphere in the center of the universe, from the simple observation that half the stars were above the horizon and half were below the horizon at any time (stars on rotating stellar sphere), and the assumption that the stars were all at some modest distance from the center of the universe. If the Earth were substantially displaced from the center, this division into visible and invisible stars would not be equal.

Pages from 1550 Annotazione on Sacrobosco's De sphaera mundi, showing the Ptolemaic system.

In the Ptolemaic system, each planet is moved by a system of two spheres: one called its deferent; the other, its epicycle. The deferent is a circle whose center point, called the eccentric and marked in the diagram with an X, is distant from the Earth. The original purpose of the eccentric was to account for the difference in length of the seasons (northern autumn was about five days shorter than spring during this time period) by placing the Earth away from the center of rotation of the rest of the universe. Another sphere, the epicycle, is embedded inside the deferent sphere and is represented by the smaller dotted line to the right. A given planet then moves around the epicycle at the same time the epicycle moves along the path marked by the deferent. These combined movements cause the given planet to move closer to and further away from the Earth at different points in its orbit, and explained the observation that planets slowed down, stopped, and moved backward in retrograde motion, and then again reversed to resume normal, or prograde, motion.

Ptolemy thought the solar system looked like this

The Ptolemaic order of spheres from Earth outward is:

1. Moon
2. Mercury
3. Venus
4. Sun
5. Mars
6. Jupiter
7. Saturn
8. Fixed Stars
9. Primum Mobile ("First Moved")

Related pages

• Flat Earth

Images for kids

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  Illustration of Anaximander's models of the universe. On the left, summer; on the right, winter.

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  This drawing from an Icelandic manuscript dated around 1750 illustrates the geocentric model.

See also

In Spanish: Teoría geocéntrica para niños