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Speed of satellites in a geostationary orbit

  1. Feb 2, 2010 #1
    We're doing orbits and such in physics at the moment, and out teacher said "if a geostationary satellite gets further away, it has to go faster". I get this, because with a bigger orbit it would have to move faster to stay above the same point on earth, more distance to travel in the same time.

    However, seeing as it's above the same point on earth, relative to the earth it isn't moving at all. Same as the satellite with the smaller orbit.

    What I'm wondering is what do you reference this speed to, to say that it's going faster? Space itself? But I'm sure that would just bring in loads of complications.

    Thank you.
  2. jcsd
  3. Feb 2, 2010 #2
    I would be inclined to reference it to the center of the earth. Or, the surface of the earth if it was not spinning...
  4. Feb 2, 2010 #3
    So does the earth's core spin at the same rate as the earth, but in the other direction? Because that would cancel them out wouldn't it?
  5. Feb 2, 2010 #4


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    This needs some clarification. While the satellite would need to go faster to maintain the same position over the Earth, it can't do so and maintain an orbit. Higher orbits are slower orbits (a higher orbit would take longer than 24 hrs to complete). Conversely, lower orbits are faster orbits( they would take less than 24 hrs to complete. Thus there is only one altitude at which you can have a geostationary orbit.
    Generally, you measure the orbital speed with respect to a non-rotating point located at the center of the Earth.
    Another way of looking at it is to measure how long it takes the satellite to complete one orbit around the Earth with respect to some fixed star and then divide that into the circumference of the orbit measured with respect to the center of the Earth. For instance, for a geostationary orbit, the period is 24 hrs and the circumference of the orbit is 264,830 km. This gives an orbital speed of 11,035 kilometers per hour.
  6. Feb 2, 2010 #5
    Kepler's (1571-1630) Third Law described the satellite orbit period as "The squares of the period of the different [satellites] are proportional to the cubes of their respective major semiaxes."

    See http://en.wikipedia.org/wiki/Kepler's_laws_of_planetary_motion

    Kepler should have lived until October 1957, when the Russians launched Sputnik (90 minute period).

    Bob S
  7. Feb 2, 2010 #6
    Ah, that makes a lot more sense to me. Thanks a lot :)
  8. Feb 2, 2010 #7
    Can you see Satellites from the naked eye on ground at night? I sometimes see brief objects glowing & moving quiet fast compared to a plane then the light dims off. Thought it might be the reflection of the sun? No way its a meteor or anything, they move much faster across the sky.
  9. Feb 2, 2010 #8
    I've heard of people seeing them, I haven't though.
  10. Feb 2, 2010 #9


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    There are lots of visible satellites in Low Earth Orbit. Spend a couple of hours out on a clear evening and you can very often see them. As it happens, I was sleeping out last week on Lanzarote and saw one 'bustling' across the sky. You can't see them over the whole hemisphere though because they go down behind the Earth's shadow (they're eclipsed) or then they can be between you and the Sun and they are faint - like a New Moon - illuminated on their far side.
    They're easier to spot than meteorites because they're visible for longer.
  11. Feb 3, 2010 #10
    The ISS is sometimes the third brightest object after the sun and moon. Satellites produce no light, so you can only see them when they reflect sunlight. Thus normally you have the best chance of seeing them close to sunrise/sunset.
    http://www.heavens-above.com/" [Broken]
    Last edited by a moderator: May 4, 2017
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