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Einstein

Relativity: The Special and General Theory
Albert Einstein: Relativity
Part I: The Special Theory of Relativity
The Galileian System of Co−ordinates
As is well known, the fundamental law of the mechanics of Galilei−Newton, which is known as the
law of inertia, can be stated thus: A body removed sufficiently far from other bodies continues in a
state of rest or of uniform motion in a straight line. This law not only says something about the
motion of the bodies, but it also indicates the reference−bodies or systems of coordinates,
permissible in mechanics, which can be used in mechanical description. The visible fixed stars are
bodies for which the law of inertia certainly holds to a high degree of approximation. Now if we use
a system of co−ordinates which is rigidly attached to the earth, then, relative to this system, every
fixed star describes a circle of immense radius in the course of an astronomical day, a result which
is opposed to the statement of the law of inertia. So that if we adhere to this law we must refer
these motions only to systems of coordinates relative to which the fixed stars do not move in a
circle. A system of co−ordinates of which the state of motion is such that the law of inertia holds
relative to it is called a " Galileian system of co−ordinates." The laws of the mechanics of
Galflei−Newton can be regarded as valid only for a Galileian system of co−ordinates.
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