Early people's search for an explanation for the motion of the ___________ is part of the ongoing desire to understand the world we live in. To understand the patterns we see, we come up with some basic ideas or ________________. We check these _____________ by using ____________. The Earth-centred or ________________ model was based on ideas of the ________________ philosopher _______________. The _______-centred or ___________________ model (accepted today) emerged during the ________________ century.

All __________________ bodies seem to move across the sky from ______________ to ___________________ during the day and night, revolving around ______________. Aristotle relied on the mathematics and geometry of ___________________ and ________________ to devise his Earth-centred model. He placed the stars on the surface of an outer _____________ which he termed a _________________ sphere. Inside the sphere, he arranged more concentric spheres on which were attached the __________, _____________ and the five known ___________________.

This was an unsatisfactory model due to many complexities. For example, additional levels of circular motion called _________________ had to be assigned to account for the observed motions of planets like ______________, _________________ and ___________________. However, if you built a model out of wheels, belts, and gears, it could be used to forecast astronomical events such as the phases of the _____________ and when ________________ would occur.

In the early 1500's, Nicholas _____________________ proposed a different model that was simpler. He proposed that, rather than _______________ being fixed and the ______________ travelling _________________ through the stars, the sun was _______________, and a rotating ____________________ travelled westward around it. The orbits were arranged on the same ________________ ________________ which is an imaginary, flat disk extending out from the Sun's _________________, along which the __________________ orbit the sun.

An Italian astronomer, ___________________, provided evidence for this ________centric model. Using an early __________________, he made several exciting discoveries. ______________ exhibited phases just as the ___________. He also spotted four ______________ orbiting Jupiter. Scientists have since found that Jupiter actually has 16 ___________.

List the order of the 6 planets from the Sun outwards in Fig. 13.8 (pg. 436):

Sun: ______________, _________________, ______________, _______________, _______________, _______________

A German mathematician named J. ______________ calculated that the planetary orbits are actually _________________ rather than circles. Then Newton provided his law of ___________________ saying that there is a _______________________ force between all objects pulling them together which gets ___________________ as the objects get closer. This force holds the solar system (____________, ______________, _____________) together.