How can the moon move at the same speed in the geocentric and heliocentric systems?
A viewer asks how the moon could be moving at the same speed in both the geocentric and heliocentric systems.
Answer
In the heliocentric system the moon takes 27 to 28 days to go around the earth, whether or not the earth is rotating, and in the geocentric system it also takes 27 to 28 days, whether or not the earth is rotating, because the moon has an independent speed from the earth, sun, and stars; it moves independently. So it really does not matter which system you use. The only difference is why the sun and moon appear to cross the sky each day: in the heliocentric system the earth is rotating, and in the geocentric system the whole universe is rotating around the earth, both at 23 hours, 56 minutes, 4.09 seconds. So the moon goes the same speed no matter what.