# Homework Help: Question about earth

1. Nov 19, 2008

### phys1618

1. The problem statement, all variables and given/known data

why does satellites orbit on a point above the equator and not anywhere else on the earth's surface???

2. Relevant equations

3. The attempt at a solution
I know satellite orbits around the earth, but i don't know why? can someone give me a clear explanation ??? thanks in advance.

2. Nov 19, 2008

### mgb_phys

It's the only way can remain in the same place above the earth and so you can point a stationary satelite dish at them. Most satelites aren't in this geo-stationary orbit - as it's called.

3. Nov 19, 2008

### Mentallic

Satellites orbit the Earth because with the high speeds they are travelling at, they always reach passed the horizon while Earth's gravitational field makes them "fall" back around. This way, they cannot escape the Earth or fall to the Earth. (I know this is a terrible explanation).

Not all satellites orbit above the same point on the Earth's surface at all times. These special cases of satellites are in a geostationary orbit.
In order for an object to orbit a large mass such as the Earth, it needs to be travelling at a certain speed to maintain this orbit. If it goes too slow, it will crash into the Earth, too fast and it will create an elliptical orbit or even escape the Earth's gravitational field completely.

The speed at which a satellite must travel to stay in orbit depends on the height above the ground (to be more specific, distance from the centre of mass). The higher a satellite is, the less speed it requires to stay in orbit. At a certain height, the speed the satellite needs to travel is correspondent with the Earth's rotation. i.e. It takes the satellite 24 hours to make a complete revolution around the Earth, while the Earth (at the equator) also takes this long.

A satellite can only follow the same point on the Earth if it is directly above the equator and high enough so as to be in the geostationary orbit.

4. Nov 19, 2008

### phys1618

like why the satellite has to orbit along with the EQUATOR and no where else?? like why it cant be a certain part of the earth's surface like usa or canada, or even the south or north pole...why equator??? is it less gravity??? or what..??

thank you for all responses.

5. Nov 19, 2008

### Mentallic

The Earth spins, right? Well yes, but not everywhere.
Imagine having a ball and marking the north and south poles on this ball. Now shove a stick through it so it passes through both poles. If you spin the ball around the stick while you hold the stick firmly in place, can you see how the "equator" will be spinning fastest, while the poles will not be moving at all?
Well this sort of happens with the Earth too. There is no spinning at the poles, and the spinning speed increases as you approach the equator.

Now, if you put a satellite into orbit, this means it will be travelling in a straight line around the Earth (while bending around the Earth due to gravity). This means no matter where the satellite is, it will have to travel around the circumference of the Earth. This means in order to go in a straight path and not turn along the journey, the satellite will be travelling the longest possible route around the Earth to end up back where it began.

Say you place a satellite above the northern coast of antarctica. You cannot make the satellite orbit above and follow the entire coast of antarctica until it ends up back at the northern coast. The satellite will need to travel entirely around the Earth (in this case, passing by the north pole) in order to stay in its straight line motion.

Now putting these 2 ideas together, can you see how for a satellite to orbit just above the same point on the Earth, it will have to be above the equator moving with the Earth's rotation.

6. Nov 19, 2008

### mgb_phys

You are going to need a globe to understand this (or an orange will do).
The orbit is a ring around the planet - but it must go around the other side of the earth
That is if you made the orbit a disk it would cut through the centre of the earth.
It cannot just go in a smaller circle around the north pole.

A satelite could orbit over N. America but it would go in a circle that would go around the earth and over Australia.
It would only be over N. America for part of the time.
The trick is to go around the equator and at the correct speed that it goes around at the same speed of the earth and so it appears to be over the same point on the equator all the time. This means it is at a constant position in the sky.

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