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breckenridgeback t1_iugbvtm wrote

Reply to comment by Pluto258 in ELI5: if Earth rotates so fast, why does it always look still from outer space? by ShesOver9k

> Imagine holding a ball at arm's length

This is a little misleading, because a tennis ball at arm's length would be a closer analogy to the view from geosynchronous orbit. A tennis ball has a diameter of about 2.5 inches, and the average human arm is ~25, so you're viewing from ~5 radii away. The Earth's radius is ~4,000 miles, so you'd be viewing from ~20,000 miles away (geosync orbit is 22k).

The view from the ISS is more like holding a ball (250 miles / 4000 miles) * 2.5 inches = 0.16 inches sorry, twice that, 0.32 (since one of those is a radius and one is a diameter) from your eye, at which point it would be brushing against your eyelashes.

But it turns out the rotation's still pretty slow even that close up.

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bartolemew t1_iugpm50 wrote

Their explanation is perfect and illustrates it well enough, considering this is EILI5.

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mfb- t1_iugss67 wrote

In videos taken from the ISS you see some motion, but that's mainly the orbit of the ISS around Earth (which takes just ~1.5 hours, so it's much faster than Earth's rotation).

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Pocok5 t1_iuh6ono wrote

From low orbit the rotation is basically unnoticeable. The ground moves under you at 400-something m/s but you yourself are flying by at 7000+ so you're just trying to spot the ground move by under you slightly slower than expected.

Of course at near GSO you'd observe the earth being almost completely motionless because you have almost the same rotation period over it as the surface (you'd get to watch the dusk/ dawn line move over the surface at the expected speed though)

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breckenridgeback t1_iui89cw wrote

> From low orbit the rotation is basically unnoticeable. The ground moves under you at 400-something m/s but you yourself are flying by at 7000+ so you're just trying to spot the ground move by under you slightly slower than expected.

Yes, all of which I already said in my own top-level comment. But that's not an issue of distance, really, it's an issue of Earth's mass - the same wouldn't be true of a proportional orbit around a tiny asteroid.

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