# The Galileian System Of Co–ordinates

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The law of inertia is fundamental to the mechanics of Galilei-Newton. It is stated as:

This law says something about:

- the motion of the bodies
- the reference-bodies or systems of co-ordinates in mechanics

The law of inertia applies to fixed stars.

If the Earth is a system of non-moving coordinates, then every star revolves in huge orbits around the earth in one Earth-day [geocentric]. This is opposed to the law of inertia.

This means that it is the stars that are in non-moving co-ordinates [heliocentric].

This system were the law of inertia applies is called a “Galileian system of coordinates.” The Classical mechanics of Galilei-Newton are valid only for a Galileian system of coordinates*.