# Local reference frame

1. Feb 17, 2012

### surajt88

According to this, if someone spins around at 2 revs per second when the moon is in the horizon, the moon seems to move at 4 times the speed of light. And this implies the moon is not in our local reference frame. And per this, local inertial frame applies to "small regions of a gravitational field". So the moon and earth are not in a "small region of a gravitational field"? I know all this boils down to the explanation what a reference frame is. Can someone clarify?

2. Feb 17, 2012

### Staff: Mentor

A frame which is spinning at 2 rev/s is not inertial even locally.

Essentially, a reference frame is a coordinate system*. You can imaging putting accelerometers at rest at each point in the coordinate system. If those accelerometers all read 0 all the time and are not changing distance wrt each other, then the reference frame is inertial. In GR, that can only be done locally.

*Technically, it is sloppy usage and a reference frame is what is called a "frame field", but the distinction is not usually important.

3. Feb 17, 2012

### harrylin

You seem to refer to reference frames in general relativity. However, special relativity is easier to understand although it ignores some small effects due to gravitational fields - effects that are of little relevance for your issue. SR uses the same "reference frames" as the reference systems that Newton's theory referred to: coordinate systems with clocks that are in rectilinear, uniform motion. The laws of physics (such as about the speed of light) do not refer to a spinning reference system.