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Light added to light can make darkness. Hold a thin card in a narrow beam of light, and bright and dark stripes appear in its shadow; block the light passing one side of it, and they vanish. Waves can cancel each other out. Streams of particles can’t.

Gamma Draconis, and the other stars I’ve measured, shift back and forth over the year by about 20 seconds of arc, always toward the direction the Earth is moving in at the time. Light takes time to arrive, and the Earth’s motion slants the direction it seems to come from. A motionless Earth can’t produce this.

Everything on the Earth shares its motion, just as everything on a smoothly sailing ship shares the ship’s. A stone dropped from the top of the mast lands at its foot, and a cannonball keeps the Earth’s motion too, so it flies as far to the east as to the west. Equal distances show nothing about whether the Earth spins.

Jupiter carries four moons around with it as it moves. So a moving Earth can keep its Moon, too.

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.

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.

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.

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.

The air near the Earth turns with it and carries the clouds along, and whatever rises or falls through the air turns with the Earth too, on top of its own motion. So we see them move every which way, as they would on a motionless Earth.

The air might turn with the Earth and carry the clouds and the birds along, so they wouldn’t be left behind.

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.

The stars are so far away that, next to their distance, the Earth’s orbit is like a point. Their shift is far too small to see.