How do scientists determine the speed of earth's rotation?
Observe a star when it is at 90 degree altitude at equator.at mid night.
Again observe the same star next night same ti me when it is at zenith.
THe time difference will be 23 hours 56 minutes and 4 seconds.
Circumference of earth is about 40,000 kilometers..Divide by time taken for one rotation..THat will give earth's rotational speed at equator.