The reference frame we know best

Graphic - foucault
Foucault Pendulum as observed in sourthern Hemishpere. Source

The earth is not a perfect inertial reference frame. One reason is its rotation about its own axis. Although the associated effects are generally small, they are evident (and sometimes important) in a number of contexts.

The behaviour of the Foucault pendulum provides one example. The only thing that is special about this pendulum (similar to the one in the picture above) is that it is large enough to keep oscillating freely over a long period. Then it becomes evident that its plane of oscillation does not remain fixed, as it would if the earth were truly inertial, but rotates relative to the earth. You can view a Foucault pendulum in NSW Australia (requires Flash Player 6).

We will return to consider some further consequences of these effects, in particular for wind patterns, at the end of this section.