There are two possible definitions of a non-rotating body. The first is that it is fixed with respect to the distant galaxies. The second is that is non-rotating when measured by an ideal inertial system such as a gyroscope. According to general relativity, these two are not exactly the same. -- Gene On Tuesday, October 24, 2017, 10:56:45 AM PDT, Brent Meeker <meekerdb@verizon.net> wrote: I don't understand that. The celestial sphere is defined as not rotating and is empirically fixed by distant galaxies. Brent On 10/23/2017 9:06 PM, Keith F. Lynch wrote:
(Copernicus, Galileo, Kepler et al believed that the celestial sphere stood still and Earth rotated. They were mostly right, but if they had kept close track, they would have noticed that the celestial sphere rotates by about 0.04 seconds of arc per year due to frame dragging.)