On Wed, 17 May 2017 05:57:45 -0700 (PDT),
wrote:

On Wednesday, May 17, 2017 at 10:29:28 AM UTC+1, Gerald Kelleher wrote:

There is no external reference for rotation as an isolated motion at a rate of 15° per hour and consequently a full 360° rotation in 24 hours.

Correct, but there are thousands of external references called "stars" which all show that the Earth rotates once every 23 hours, 56 minutes, 4 seconds.

The structure of the Universe itself provides a reference for
rotation, which is observed in centrifugal force, oblate rotating
spheres, Focault pendulums, and numerous other examples. We need no
visible external reference at all to measure how fast something is
rotating. Certainly we have instruments which measure the rotation of
the Earth via inertial properties to very high precision... and
clearly demonstrate that its rotation rate is less than one rotation
every 24 hours.