The sun-centered model vs. the earth-centered model 1

We know that the earth circles around the sun. However most people can¡Çt explain why the

earth-centered model is true let alone difference between two models in details. This article

is an astronomy reference for laymen.


¡¦ Arithtarchus was an ancient astronomer in the Greece. He discovered not only how to

measure the distance between the earth and the sun, but also how to prove that the earth

spins on its own axis. He preferred the sun-centered model because he concluded that the

sun didn¡Çt have to revolve around the earth if it span its axis.

Though his idea was very insightful, it was abandoned by Greeks. There are three reasons.

First, it contradicted their common sense. It was natural to think that the sun revolved

around the earth, if we saw its path everyday.

Second, nobody felt the movement of the earth. They didn¡Çt have any idea about inertia, so

they couldn¡Çt understand movement without feeling. ¢¨£± Moreover the idea of gravity was

unfavorable to the sun-centered model. They thought that gravity worked toward the

center, so the earth must have been the hub in the universe.

Third, they didn¡Çt observe any shift in the position of the stars. The distance of the

stars were so overwhelming for naked eyes that they had no device to detect such shifts.



Ptolemy and the sun-centered model

¡¦ It is often said that he is the protagonist of the sun-centered model, but it is not true.

He improved the old fashioned sun-centered model based on accumulated information

rather than invented it. The old sun-model outweighed the earth-centered model in terms of

above three points, however it had a crucial flaw: the disagreement with the observed orbit

of Mars. It was known as a vagabond by the sun-centered model supporters, because its

orbit was retrograded. If the Mars just revolved around the earth, the orbit would be a

pure circle. After all, he had to correct the old model to make it compatible with the

observation. His greatest invention was epicycle which was the second circle to make the

retrograded path. Thanks to this adjustment, his model has very close agreement

with the observation, but lost its simplicity.

However, the earth-centered model couldn¡Çt make any progress during Ptolemy¡Çs success, so

the sun-centered model had held central stage championed until the 16th century.