Читать книгу The Little Book of Mathematical Principles, Theories & Things - Robert Solomon - Страница 35

This is a system of the universe.

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In Ptolemy’s system the Earth is at the center and all other bodies revolve around it.

The earliest model for the motion of heavenly bodies about the Earth involved their moving in circles with the Earth at the center. More accurate observations showed that this was incorrect and modifications had to be made. These suggested that:

1. The Earth is not the center of the circle.

2. The bodies move in smaller circles (called epicycles) which were themselves moving in circles around the Earth.

3. The speed of the body was not constant as it moved around the circle.

In Ptolemy’s Mathematike Syntaxis, the mathematical compilation, better known by its Arabic name Almagest, all these modifications are used.

The diagram on the opposite page illustrates this model. A planet moves in a small circle. The center of this small circle moves in a big circle around a point C – this is not the Earth – and the angular speed is constant, not about the Earth and not about the center C of the big circle, but about a point called the equant E – on the other side of the Earth from C.

This system was very complicated. It was also reasonably accurate, however, and lasted as a guide for navigation, astronomy, calendar setting, and so on, for well over 1,000 years. Its accuracy was not challenged until the insistence on circular motion was abandoned and other curves were considered.

A model for planetary motion.

See: The Sun-Centered Universe, page 60; The Sun-Centered Universe Again, pages 64–65