Cecil Moore wrote:

The definition is relative, not absolute.

It's as absolute as anything we have. Name something absolute we
could have used instead, Cecil.

If the
relative speed of the earth is changing, then the
length of a second is also changing and we would
have no way of knowing.

Not to worry. Any relativistic motion on our part will only effect
the clocks in the other reference frames. And we can't even
communicate with any of those people. :-)

ac6xg