# Einstein:Book chapter 04 - The Galileian System of Co-Ordinates

Albert Einstein: **Relativity: The Special and the General Theory**

**
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17 - -
18
19
20
21
22
23
24
25
26
27
28
29 - -
30
31
32
**

## 4: The Galileian System of Co-Ordinates

**AS** is well known, the fundamental law of the mechanics of Galilei</person>-Newton</person>, which is known as the *law of inertia*, can be stated thus:</p>
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 co-ordinates, permissible in mechanics, which can be used in mechanical description. The visible fixed stars are bodies for which the law of inertia certainly holds for 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 co-ordinates 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 coordinates." The laws of the mechanics of Galilei</person>-Newton</person> can be regarded as valid only for a Galileian system of co-ordinates.
</div>