# 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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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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There’s only one spot out there where the speed at which the object is falling is perfectly aligned with the speed of the rotation of the earth. This is geosynchronous orbit. It’s no different from any other or it except for this matching of the earths rotation.

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.

There’s only one spot out there where the speed at which the object is falling is perfectly aligned with the speed of the rotation of the earth. This is geosynchronous orbit. It’s no different from any other or it except for this matching of the earths rotation.

There’s only one spot out there where the speed at which the object is falling is perfectly aligned with the speed of the rotation of the earth. This is geosynchronous orbit. It’s no different from any other or it except for this matching of the earths rotation.

It’s a kinda shitty description of what’s happening, but has a popular foothold for some reason.

A stable orbit is when the centripetal force to keep you moving in a circle is equal to the gravitational force at the same altitude. It’s about balancing forces, not about throwing yourself at the ground and missing.

Geosynchronous orbits are achieved by calculating what altitude has an orbital period equal to the rate of earth’s rotation, then you stick a satellite at that altitude, at the appropriate velocity.

Edit: For those who think this *isn’t* about balancing forces please share a non-force based derivation of how you calculate orbital velocity at a given altitude.

For reference the force-based derivation is:

GM(earth) /2 = V^2 /r
The left side is gravitational force at a given altitude
The right side is centripetal force required for a velocity at that altitude
This rearranges to
V = sqrt (GM(earth) / r)

It’s a kinda shitty description of what’s happening, but has a popular foothold for some reason.

A stable orbit is when the centripetal force to keep you moving in a circle is equal to the gravitational force at the same altitude. It’s about balancing forces, not about throwing yourself at the ground and missing.

Geosynchronous orbits are achieved by calculating what altitude has an orbital period equal to the rate of earth’s rotation, then you stick a satellite at that altitude, at the appropriate velocity.

Edit: For those who think this *isn’t* about balancing forces please share a non-force based derivation of how you calculate orbital velocity at a given altitude.

For reference the force-based derivation is:

GM(earth) /2 = V^2 /r
The left side is gravitational force at a given altitude
The right side is centripetal force required for a velocity at that altitude
This rearranges to
V = sqrt (GM(earth) / r)

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 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.

It’s a kinda shitty description of what’s happening, but has a popular foothold for some reason.

A stable orbit is when the centripetal force to keep you moving in a circle is equal to the gravitational force at the same altitude. It’s about balancing forces, not about throwing yourself at the ground and missing.

Geosynchronous orbits are achieved by calculating what altitude has an orbital period equal to the rate of earth’s rotation, then you stick a satellite at that altitude, at the appropriate velocity.

Edit: For those who think this *isn’t* about balancing forces please share a non-force based derivation of how you calculate orbital velocity at a given altitude.

For reference the force-based derivation is:

GM(earth) /2 = V^2 /r
The left side is gravitational force at a given altitude
The right side is centripetal force required for a velocity at that altitude
This rearranges to
V = sqrt (GM(earth) / r)

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.