Relative Motion of the Earth
A rather simple experiment could be carried out, to accurately determine the relative motion of the Earth to fixed space.
If an electronic signal were sent from a satellite directly over the axis of the North Pole, and the moment of departure was known and synchronized with receiving stations here on the planet. It is travelling at light speed, away in all directions from a singular, static point. While the satellite IS in motion, at the instant of tramsmission, that unique point from which the signal originated does not change its fixed point location over time.
It takes a short time to first hit the Earth, at the North Pole. We note that receipt time. Light travels at a constant light speed, so we now know the distance the satellite was from the Pole at transmission time... distance = ( receipt time - tranmission time )/ c.
Armed with this info, the receipt times for ALL other stations is therefore predictable/defined. [ and that you know the latitude and longitudes of your receipt stations]
If the Earth was fixed in relative motion to that fixed transmission point, then the ACTUAL receipt times would exactly match the PREDICTED time for the signal's arrival.
This difference, between the predictable times for arrival and the actual times, contains sufficient information to precisely determine the Earth's relative motion.