eli5: I’ve heard orbit described as continuously falling past or missing the Earth, how then do objects in geosynchronous orbit above a single point not fall out of the sky?
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The Earth is rotating as well at the rate of one rotation per day. In geosynchronous orbit, the satellite orbits the Earth keeping pace with the rotation.
They are only above a single point on the rotation surface of the earth. Evey point on Earth except for the poles move because of earth rotation.
If you look at orbit it does not matter if Earth rotates or not, the orbital period for a given distance would be the same if Earth was not rotating. Geostationary orbit is just that the time of an orbit one around Earth is the same as it takes Earth to rotate once.
So both the point on Earth and the satellite move with the same angular speed around the center of the Earth.
The orbital period will be one sidereal day, that is one rotation of Earth relative to faraway stars. that is around 4 minutes less than 24 hours. The extra 4 hours per day is from the earth’s orbit around the sun, consider the moment of the sun during an orbit around the sun if the earth was not rotation relative to faraway stars.
4 minutes a day for 365 days are 1460 minutes which is proximally the same as 24 hours = 1440 minutes. The difference is because neither 4 minutes nor 365 days are exact
Because the orbit is around the center of the earth in a plane the point of earth it is stationary above also needs to move around the center in a single plane. Only the equator of the earth does that.
So geostationary satellites are always directly above the equator, if they was not they would move relative to the ground, the moment might just be north and south around the equator but it still moves.
The closer you are to the surface of the Earth, the faster you have to be moving sideways in order to “miss” the Earth as you fall.
As you get farther from the surface, the speed necessary to continuously “miss” the Earth drops.
The Earth is also rotating. Close to the surface, you have to be moving much faster than the surface of the Earth to stay in orbit. Very far away, you can move slower than the Earth turns and still stay in orbit.
Between those two points, there is a specific height where the speed necessary to stay in orbit results in you taking 24 hours to orbit the Earth. This is the same time it takes for the Earth to turn once on its axis, so assuming that the orbit is happening approximately above the equator, it will appear that the satellite is hovering above you to someone standing on the Earth.
But it’s not hovering. It’s still “falling past” the Earth. It’s just falling past at the same rate and in the same direction that the Earth turns.
Like your five.
Ok, grab a string and tie a weight to the end. Now spin around with the weight at the end of the string. This is “sorta” like an orbit.
Now imagine this string is actually gravity pulling it into you. There will be a distance away and a spinning speed that it will all balance out that weight will stay with you, just like the string, but if you spin too fast or too slow the weight will fly away or come crashing back to you.
Like your five.
Ok, grab a string and tie a weight to the end. Now spin around with the weight at the end of the string. This is “sorta” like an orbit.
Now imagine this string is actually gravity pulling it into you. There will be a distance away and a spinning speed that it will all balance out that weight will stay with you, just like the string, but if you spin too fast or too slow the weight will fly away or come crashing back to you.
Like your five.
Ok, grab a string and tie a weight to the end. Now spin around with the weight at the end of the string. This is “sorta” like an orbit.
Now imagine this string is actually gravity pulling it into you. There will be a distance away and a spinning speed that it will all balance out that weight will stay with you, just like the string, but if you spin too fast or too slow the weight will fly away or come crashing back to you.
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[Relevant xkcd](https://what-if.xkcd.com/58/)
The short version is that the earth is spinning too. There’s a distance from the earth where you can be going fast enough sideways to “miss” the earth when falling, i.e., be in orbit, but be moving at the same speed the earth is turning at. It’s pretty far out actually.
We call that distance “Geosynchronous Orbit”, since things in that position, moving sideways like that end up appearing to stay directly above the same point on the planet.
There is, of course, a much longer version, and it has to do with the satellite’s orbital velocity synchronizing with the gravitational force of the earth such that there’s an apparent relationship between a spot on the ground and the satellite, but it involves a lot of math, and is probably better understood by playing Kerbal Space Program.
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