# Orbits of satellites

For a satellite orbiting the earth, if there is only gravitational force acting on the satellite, does it mean it can only orbit the earth around the equator or through north and south poles? (only in these two orbit FG=Fc hence it allows a constant circular motion in the same pathway)

• The two main types of satellites are geostationary satellites (around the equator) and low earth orbit/low polar orbits, which is around the axis of the poles. They serve different functions, and they consume different amounts of fuel due to the prescence of different gravitational forces of attraction. Commented Mar 2, 2019 at 4:52
• How would an orbiting satellite know about which great circle of the earth it is aligned with? Gravity (as far as Newton is concerned) doesn't care about how the Earth rotates on its axis. Commented Mar 2, 2019 at 5:45
• @AaronStevens - The Earth's gravity field is not quite that of a point mass. The Earth has an equatorial bulge, plus other non-spherical aspects. The bulge makes orbits inclined with respect to the equator precess. The polar satellites to which the previous comment alluded aren't quite polar orbits; they're instead inclined by about 98° or so. This inclination is very carefully chosen so as to make the orbital plane precess by 360° in one year. The Earth's oblateness can also induce an axial precession. Molniya and tundra orbits use this to their advantage. Commented Mar 2, 2019 at 7:48

It's not clear why you stated:

(only in these two orbit FG=Fc hence it allows a constant circular motion in the same pathway)

because that is not correct.

A satellite's orbit can be inclined in any direction. For most practical purposes, the earth can be treated as a point mass. Its rotation around its axis has no measurable effect on satellites; and its oblateness only has a very small effect.

Satellites do not need to be in circular orbits; elliptical orbits are not unusual.

In simple terms a satellite moves around the Earth in a circular or elliptical orbit under the action of the force of gravitational attraction between the satellite and the Earth.

Gravitational attraction is a central force which means that its line of action is through the centre of mass of the Earth and of the satellite.

This force causes the satellite to accelerate and maintain its orbit.

The obit of the satellite is “centred” on the centre of mass of the Earth which is the centre of the circular orbit or at a focus of the elliptical orbit.

Here are two orbits around the Earth with $$C1$$ and $$C2$$ the centre (or focus) of an orbit.

For orbit $$1$$ the gravitational attractive force $$F1$$ is towards the centre of the Earth $$C1$$ and $$C1$$ is located in the plane of the orbit.
It is an allowed orbit.

For orbit $$2$$ the gravitational attractive force $$F2$$ does not point towards the centre of the orbit $$C2$$ rather it points towards the centre of the Earth $$C1$$.
The centre of the Earth $$C1$$ is not in the plane of the orbit and so there is a component of force $$F2$$ which is not pointing towards the centre of the orbit $$C2$$.
This is not an allowed orbit.

I have not included an indication of where the North and South poles are to illustrate the fact that the allowed orbits are not necessarily over the Equator or exactly over the poles although the majority of satellites have obits over the equator (geostationary) or in a plane which is slightly inclined to the $$NS$$ line (polar orbits).

• so if the earth can be considered as a uniformed sphere, then its true that the satellite can only orbits in these two pathway( if FG=Fc)? Commented Mar 2, 2019 at 8:40
• @WangRui There are an infinite number of orbits some of which are over the equator and some over the poles in all cases it is the gravitational force of attraction which is causing the acceleration of the satellite. Commented Mar 2, 2019 at 9:14
• Oh yeah, i forgot these can just be different planes intersecting with the center Commented Mar 2, 2019 at 21:46