# Why are geosynchronous orbirs in equatorial plane circular?

• beserk

#### beserk

Anybody can answer my query ?

thanks !

Is this a homework question? Ask yourself: what are the differences between a circular and an elliptical orbit?

This isn't a homework problem.
Difference between circle and ellipse is that their eccentricities are 0 and (0,1) respectively.That doesn't seem to strike anything to me or am I too dumb ??

No, the difference between the two would not be zero, it would be whatever is the eccentricity of the elipse is. Circular orbits have no eccentricity, so eccentricity of elipse minus zero is eccentricity of elipse.

Research into Arthur Clarke who was a big pusher of geosynch circular orbits as early as the 40's. He published a book in '68 called Promise of Space which explains the benefits and how this may be achieved. But do not read anything past this...be fore warned.

beserk said:
Anybody can answer my query ?
thanks !
They don't have to be circular. They can have as much eccentricity as you want, as long as their period is 24 hours.

But they are circular anyways. Like Plastic Photon said, its because they're manmade, and that's the way we want them. Otherwise, we would need motors on our satellite dishes to track them as they traced analemmas around their average positions.

Well, what I was getting at is if the orbits weren't circular, they wouldn't be stationary.

russ_watters said:
Well, what I was getting at is if the orbits weren't circular, they wouldn't be stationary.

True, but the OP wasn't asking about geostationary orbits, but geosynchronous orbits. The first has to be circular and in the equatorial plane, the second doesn't. Of course, it is possible that the OP isn't aware of the difference or said one when he meant the other.

Janus said:
True, but the OP wasn't asking about geostationary orbits, but geosynchronous orbits. The first has to be circular and in the equatorial plane, the second doesn't. Of course, it is possible that the OP isn't aware of the difference or said one when he meant the other.
Could be...like me!

Is there much use for geosynchronous orbits as oposed to geostationary? Ie, are there applications where a little bit of back-and-forth doesn't make much difference?

Janus said:
True, but the OP wasn't asking about geostationary orbits, but geosynchronous orbits. The first has to be circular and in the equatorial plane, the second doesn't. Of course, it is possible that the OP isn't aware of the difference or said one when he meant the other.

Geostationary orbit(s?, can there be more than one geostationary orbit?) are geosynchronous orbits in equatorial plane.So obviously I mean geostationary orbit.

Hope I'm right

Since Eric Weisstein is 10x smarter than me, I don't want to say he's wrong, but I'm not sure I understand what is in the Scienceworld link for Geosynchronous Orbit ( http://scienceworld.wolfram.com/physics/GeosynchronousOrbit.html )
A geosynchronous orbit is one that has the same orbital period as the Earth's sidereal rotation (23 h 56 m 4 sec), but does not have an orbital inclination and eccentricity of zero. A satellite in such an orbit would weave figure-eights around a point on the ecliptic when viewed from the ground. It is often used to mean geostationary orbit and is sometimes also called a Clarke orbit.
Isn't a geostationary orbit a special case of a geosynchronous orbit. The wording of the article suggests not.
Wouldn't it weave figure-eights around a point on the celestial equator, rather than the ecliptic?
Any thoughts?

The wikipedia suggests that you are right about at leat the first point

http://en.wikipedia.org/wiki/Geosynchronous_orbit

A geosynchronous orbit is a geocentric orbit that has the same orbital period as the sidereal rotation period of the Earth. It has a semi-major axis of 42,164 km [1].

Synchronous orbits exist around all moons, planets, stars and black holes —unless they rotate so slowly that the orbit would be outside their Hill sphere. Most inner moons of planets have synchronous rotation, so their synchronous orbits are, in practice, limited to their leading and trailing Lagrange points. Objects with chaotic rotations (such as Hyperion) are also problematic, as their synchronous orbits keep changing unpredictably.

If a geosynchronous orbit is circular and equatorial then it is also a geostationary orbit, and will maintain the same position relative to the Earth's surface.

I think you're probably right about the second point too

russ_watters said:
Could be...like me!
Is there much use for geosynchronous orbits as oposed to geostationary? Ie, are there applications where a little bit of back-and-forth doesn't make much difference?
Geostationary orbits are a special case of geosynchronous orbits, just as a square is a special case of rectangles, and a circle is a special case of ellipses. (Wow, notice how I managed to come full circle? )

Geosynchronous orbits are just as useful as geostationary orbits (within limits, of course, since you wouldn't want the satellite to disappear below the southern horizon). The difference is the expense of tracking the satellite - if the antenna has to move, it will be more expensive to build and it will require more training to operate.

There's not going to be a big market for satellite TV if the user has to pay a lot for the antenna and has to attend a class to learn how to operate his antenna. You net a bigger profit by enduring the cost on the satellite end and widening the amount of customers you can appeal to.

Being receivers only (for the most part), your satellite dishes still can take a little bit of play in the angle. Since no orbit could ever be truly geostationary for more than instant, that's a good thing. Every geosynchronous satellite orbit actually traces out figure-8 ground tracks - some satellites just have very, very small figure 8's.

The Sun, Moon, planets, and non-uniform density of the Earth changes every orbit constantly and satellite operators have to make periodic orbit adjustments to keep each orbit nearly circular and nearly geostationary. The Sun's gravity will tend to pull every orbit towards matching the plane of the Earth's orbit around the Sun. The Moon's orbit, having been around for a long time, is already within 5 degrees of the plane of the ecliptic.

Your commercial satellites used for satellite TV usually maintain inclinations of less than half a degree. The orbits are oriented so the Sun and Moon will pull the satellite's inclination towards zero until the orbit lies exactly in the equatorial plane. Then the orbit's inclination slowly increases until the upper limit is reached and the orbit is adjusted to set the cycle over again.