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My husband and I are both in the Signal branch of the army but have very different understandings of how satellites move, if they move at all. Do they go around the earth with the rotation? Do they move at a higher rate of speed than the earth? How is it that when I use my compass, I can use the same azimuth to find the same satellite every time, day or night? Help me win this friendly debate!

In: Technology

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Satellites orbit the Earth. Gravity doesn’t hold anything stationary, it pulls objects in.

Satellites orbit the Earth at different speeds, depending on how far they are away from the surface:

* Satellites at [Low Earth orbit](https://en.wikipedia.org/wiki/Low_Earth_orbit) and [Medium Earth orbit](https://en.wikipedia.org/wiki/Medium_Earth_orbit) orbit the Earth fast than Earth revolves around its axis, therefore they complete one orbit in less than a day. The ISS for example orbits the Earth every 90 minutes.

* Satellites at [Geosynchronous orbit](https://en.wikipedia.org/wiki/Geosynchronous_orbit) orbit the Earth at the same speed that Earth revolves around its axis, so they actually remain above one spot on Earth all the time.

* Satellites at [High Earth orbit](https://en.wikipedia.org/wiki/High_Earth_orbit) orbit higher than geosynchronous orbit, so they orbit slower than Earth’s revolution around its axis.

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Satellites orbit their planets. The way this works (for circular orbits, which are the relevant ones here) (in Newtonian theory) is that they move around so fast that the centrifugal force (it’s a real force in a rotating F.O.R. . Don’t @ me) is equal to gravity, and as such they balance each other out. Because centrifugal force is proportional to the distance form the centre of the planet, this means depending on the distance from the planets centre of gravity satelites need to orbit at different speeds. In low earth orbit (~200-800km), which is where most earth observation, spy and scientific satellites are, this speed is around 7 km/s. A satellite in LEO will orbit around the entire earth once in around 70-110 min.

However there is also a type of orbit called geostationary. This is an orbit where a satellite moves at such a speed that it orbits earth once in 23hours56min (siderial day), and since the earth also spins around itself once in the same time, a satellite in an equatorial GEO Orbit will always remain above the same precise point on the ground. This orbit is used for communication and TV Satelites, because it allows you to aim your Satelite dish at them, and not have to move the dish to track the satellite because it’s completely stationary relative to a point on earths surface

There are different types.

Some orbit at a rate which causes them to move in relation to the surface, broadly East West. ISS, etc.

Some are in polar orbits, going North South, independent of rotation.

Some are geostationary, and hold position over the equator. Satellite TV, etc. This is why all sat dishes above the equator point South, and those below it point North

It depends on the satellite.

Most stuff in orbit will move across the sky so fast that they are basically constantly falling towards earth and constantly missing because they moved sideways so much.

There is however a sweet spot where the satellites are so far away that the speed at which they orbit the earth is exactly the same as the speed at which the earth turns.

This is the geostationary orbit and satellite in such an orbit always will remain in the same place above the equator.

You can point a satellite dish to a point above the equator and won’t have to move it to remain connected to such a satellite.

This is the type of orbit where most satellites used for communication are.

The main downside of this is that they are really far away, which is bad for things were latency matters. They also don’t prove as good coverage if you are far away from the equator.

The starlink service that has been in the news recently uses satellites much, much closer to earth. Because they are so close they appear to be constantly moving all the time in respect tot he ground. This requires constantly switching between satellites to stay connected.

The orbit of a satellite has very little to do with how fast the earth is spinning. Especially for satellites smaller than something like the moon.

The lower something is the faster it orbits.

Low Earth Orbit is defined as taking less than 128 minutes to go around the earth.

This would be true if days lasted 12 or 48 hours as it is 24 hours. The spin doesn’t matter, just the mass.

Satellites that are higher orbit slower. There are orbits that just so happen to have a orbital period of about 24 hours, which just so happens to be the same time the earth takes to spin.

These are called geosynchronous orbits.

Because of they were launched into these orbits, they come back to the same position at the same time every day.

There’s nothing special about them in a physics stand point. It’s just that rocket scientist did the math and launched them that way precisely so you and your dad could point a dish at a point and always get access to the satellite.

Now there is a precise band of geosynchronous orbits that has the same period as a day, is perfectly circular, and is also the direction the earth is spinning (IE directly over the equator).

This is called a geostationary orbit, and it’s where a lot of comm satellites are put. Because from the perspective of a person on the ground the thing isn’t moving.

Think of it this way. If you throw a ball along the rotation of the earth, it stays above it for some time, but it eventually falls to the Earth.

But what if you threw it so hard that the curvature of the Earth starts to fall away just as fast? Well then you have an orbit.

An object in orbit is moving sideways so fast that, even though gravity is pulling it back towards the Earth, the Earth is falling away just as fast as the object is falling towards it.

Something in orbit is falling all the time. It is moving sideways so fast that it keeps missing the Earth.

Imagine artillery that fires over the horizon. The shell follows the curve of the Earth until it lands. Now imagine that it fires the shell so hard that it follows the curve of the Earth all the way around until it hits itself. That’s orbit.

The higher the orbit, the longer it takes for the shell/satellite to go all the way around the Earth. There’s a couple of reasons for this but mostly it just has to go farther. The ISS (at 400 km elevation) takes 90 minutes to go around the Earth but the moon (at 384,000 km) takes about 4 weeks per lap. The satellites you are targeting are in a special orbit at 35,786 km where it takes almost exactly 24 hours to go around the Earth once. That means that they only need tiny adjustments to always be in the exact same place in the sky.

Think of it as firing a cannonball into space. If you don’t put enuf speed into the cannonball it drops back to earth. Add too much speed and it flies off into space. If you put just the right amount of speed, the cannonball will fly up and gravity will pull it down but the ball is flying fast enuf that when the ball falls back down, it literally misses the earth. That’s how you get into orbit. The earth spins at around 1000 mph at the equator. If the cannonball is in orbit at that same speed along the equator, it looks like it’s just hanging there from earth perspective but it is actually traveling at great speeds. That’s called geosynchronous orbit.

Depends on the orbit of the satellite. A geostationary orbit will mean that the satellite is always in the same position relative to the ground while a geosynchronous orbit will mean it passes over the same points constantly throughout its orbits.

>Do they go around the earth with the rotation? Do they move at a higher rate of speed than the earth?

Depends. The closer to the earth they are, the faster their rate of orbit is. Close to earn, the ISS for instance orbits once every 90 minutes. Further away you have things like Geosynchronous orbit which is a special distance from earth where one orbit is one day on earth. And further away, say the moon, orbits once every 30 days.

The math governing orbits does not care about how fast the earth is spinning below the orbit. The fact that geosynchronous orbit exists just comes from the fact that the closer you are to earth the faster the orbit, the further you are the slower the orbit, and that this progressively goes from 90 minutes near earth to infinite time the further out you go, thus passing 24 hours as some point.

If you want an intuitive understanding of orbits, there is an excellent game called Kerbal Space Program which really makes you understand what is going on. Its a bit silly but the physics there makes a lot of sense after playing it.

I have no idea how your compass works, if I were to guess, it uses the fact that orbits are predictable and it has somewhere saved the orbital data for the orbit, figuring out its location whenever needed, this at least is how GPS works.