Hilchos Kiddush HaChodesh 11:14
If a person were to measure the speed of a heavenly body against a sphere that has Earth as its center, namely the sphere of the constellations, then the rate at which it travels would seem to change. Its rate of travel compared to the constellations could appear slower or faster on one day than on the day before or after.
Hilchos Kiddush HaChodesh 11:15
The set speed at which a heavenly body, the sun or the moon proceeds is called its average rate of motion. The progress of a heavenly body against the sphere of constellations sometimes appears faster and sometimes appears slower than this; this is called its true rate of motion. This is used to determine the actual position of the sun or the moon.