The reason why this [diurnal motion] has not been looked into is probably the difficulty of finding a proper standard to measure it by; since it is itself the standard by which we measure all the other motions.

Astronomical Observations on the Rotation of the Planets Round their, Axes made with a View to determine whether the Earth's Diurnal, Motion is perfectly Equable, Philosophical Transactions of the Royal Society of London, Volume 71, 1781 (p. 116)