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.