IV

The Galileian System of Co-ordinates

A Galileian coordinate system is one where the Galieli-Newtan laws of mechanics are valid.

For example, Newton's first law, the law of inertia states that an object at rest remains at rest, and an object in motion continues in motion in a uniform straight line. Such objects remain in their conditions unless acted upon by an external force. From the frame of reference of the Earth's surface, the stars move in circles of immense radius. This violates the law of inertia. Therefore, to apply the law of inertia, we must exclude a system where stars move in circles. We must imagine that we are somewhere other than the Earth's surface where we can see the Earth rotating and the stars at their fixed points in the sky.