The observer is then in uniform circular motion.

Inertial systems are in rectilinear and uniform motion with respect to each other.

In relativity, any reference frame moving with uniform motion will observe the same laws of physics.

What keeps matter at rest or in uniform motion in the same straight line unless some external force acts on it?

Their models employed combinations of uniform circular motion and were centered on the earth.

Galileo formulated these concepts in his description of uniform motion.

Any two frames are in relative uniform motion.

The inertial frames move in all possible relative uniform motion.

Figure 1 illustrates velocity and acceleration vectors for uniform motion at four different points in the orbit.

The image at right shows the vector relationships for uniform circular motion.