Why Don’t We Feel the Earth Move?
Showing only #5792 and its comments.
See chronicleThe Earth stands still at the center of the universe. The Moon, the Sun, and the planets circle it.
Every so often, a planet seems to stop, drift backward among the stars for weeks, and then move on. A planet that simply circled the Earth would never do that.
Each planet rides on a small circle, an epicycle, whose center travels along a larger one around the Earth. On the near side of its epicycle, the planet moves against the larger circle’s motion, so it seems to drift backward.
No arrangement of circles fits Tycho’s measurements of Mars. My best one still misses some of them by eight minutes of arc, far more than Tycho’s errors.
Ptolemy’s system has to set by hand that Mercury and Venus never stray far from the Sun, and that the outer planets loop backward just when they’re opposite the Sun in the sky. It never explains why. If the Earth and the planets all circle the Sun, both follow.
Venus shows phases like the Moon’s, from nearly full to a thin crescent, and looks largest when it’s a crescent. In Ptolemy’s system, Venus always stays between the Earth and the Sun, so it could only ever show us a crescent.