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Light is a stream of tiny particles thrown off by shining things. They fly in straight lines, which is why shadows are sharp. Glass and water pull them in as they enter, which bends their path and speeds them up.

Light is a wave: a disturbance spreading through an invisible medium, the ether, as sound spreads through air. It travels more slowly in glass and water than in air, which is why its path bends as it enters them.

Every object pulls on every other object. The pull grows with their masses and weakens with the square of the distance between them. This one law explains the orbits of the planets and why things fall on Earth.

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.

If the Earth spun from west to east, a cannonball fired eastward would fly a different distance than one fired westward with the same force. I’m sure both would fly equally far, so the Earth doesn’t spin.

Even my instruments, precise to a minute of arc, show no shift of the stars. So they would have to be at least 700 times farther from the Sun than Saturn is, leaving an absurd empty gap in between.

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.

If the air carried them along, they would never seem to move at all. But we see them move in every direction.

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

If the Earth spun once a day from west to east, it would outrun everything not standing on it. Clouds, birds, and anything thrown would always seem to drift westward, and we would never see them move east.

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.

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 Earth stands still at the center of the universe. The Moon, the Sun, and the planets circle it.

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.

If the Earth circled the Sun, we would see the stars from different places over the year, so they would seem to shift back and forth. They don’t.