Why Don’t We Feel the Earth Move?
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The Earth is a planet. It spins once a day and circles the Sun once a year, as the other planets do, with the Sun near the center of it all.
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 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.
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.
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.
The parallax is there, only tiny. Over the year, 61 Cygni moves back and forth by 0.31 seconds of arc either way against two fainter stars beside it, just as it should if the Earth circles the Sun. The stars are simply that far away: 61 Cygni is about 660,000 times farther from us than the Sun.
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.
The air might turn with the Earth and carry the clouds and the birds along, so they wouldn’t be left behind.
If the air carried them along, they would never seem to move at all. But we see them move in every direction.
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 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.
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.