How can stellar parallax be explained equally well in the geocentric and heliocentric systems?
A viewer asks how parallax can be totally equivalent in the two systems, noting that the position of the Earth and stars seems to change every day in the geocentric system versus yearly in the heliocentric system.
Answer
In the heliocentric system it's changing every day because the Earth is rotating; it's not in the same place it was an hour before, and it rotates once in 23 hours, 56 minutes, and 4.09 seconds, which is sidereal time. So it's changing every second, and what's changing with it is the view of the cosmos. The reciprocal of that is the geocentric system, in which the Earth is not rotating and the stars are moving around it, so their positions are changing every second. That's why you can go out your door at 8:00 when it's dark, at least in wintertime, and see Orion in the eastern part of the sky, and if you come out at 4 o'clock in the morning it's over on the west side. It moved. The question is what moved: was it the Earth rotating, or the stars moving around the fixed Earth? So it would be wrong to say everything's changing in the geocentric system but not in the heliocentric; they're both changing, they just have different elements that are changing. As for parallax, you can figure this out yourself. Put two quarters on the table. Put your left forefinger on the left quarter and your right forefinger on the other. Now take the quarter on the left and move it in a semicircle around the one on the right, making the right one the center. That's position one. For position two, put a quarter under each index finger again, but now instead of moving the left finger, move the right index finger in a semicircle to the left around the other quarter. What do you see? You see the exact same thing in both systems. All you've done is change what moves. That's why parallax can be the same in the geocentric or heliocentric system; they're exactly the same. But you won't find anybody in the school system saying, here's a case of relative motion. All they show you is the one where the left finger goes in a semicircle around the right; that's all they'll teach. You can't say, wait a minute, I can move my right index finger in a semicircle around my left and get the same result, because that's geocentric, and that's what the Catholic Church taught, because they read it in the Bible.