If I am walking, the ground is stationary, and I move X units/hour relative to the ground.

If I am in low-earth orbit, then I am still moving X units/hour relative to the ground, only its a much bigger X.

If I am in geo-stationary orbit, I am moving 0 units/hour relative to the ground. But relative to the axis of the earth, I am moving as fast as the earth spins. So am I moving at 0 units/hour or the speed of the earth's rotation (plus a little since its away from the surface)?

If I go to the moon, the earth will be rotating so my speed would have to be relative to the distance from the axis of the earth, and not the position around the axis like in geo-stationary

Now imagine I am leaving earth's orbit, my speed can't be measured relative to the distance I am from the earth's axis because it's moving around the sun. So at what point does my speed change from being relative to earth to being relative to the sun and is there a spike in the numbers from this transition?

I guess I am confused because from everything I read and hear about spacecraft, the speed is always brushed over like its really obvious and intuitive. But I don't really think it is.

  • 2
    $\begingroup$ You get to pick the frame of reference to use, depending on what kind of calculation you are doing. $\endgroup$ – Organic Marble Jul 20 at 11:31
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    $\begingroup$ "If I am walking, the ground is stationary, and I move X units/hour relative to the ground." – The ground is only stationary because you define it to be. It actually moves at ~40000km/24hr around the axis of the Earth (at the equator), it also moves at a considerable speed around the Sun. $\endgroup$ – Jörg W Mittag Jul 20 at 14:34
  • $\begingroup$ At the "transition" there isn't a "spike in the numbers." It's just a change in which set of numbers you're using. $\endgroup$ – Camille Goudeseune Jul 20 at 15:27

Your concerns are all perfectly valid - giving just a number doesn't tell a lot. So, in all proper publications, the reference system has to be mentioned.

Typically, the reference is the body the spacecraft is mainly influenced by. I.e. in a Earth orbit it is the center of Earth, in lunar orbit the Moon. For interplanetary probes in transit it's usually the center of the Sun. Close to lift-off and landing one usually switches to a reference frame fixed on the surface of the planet. For most calculations it is the best to have a fixed, non-accelerating coordinate system (or at least one with a negligible acceleration, like Earths center in respect to LEO)

In other cases, but usually not when talking about velocity, the reference system might be a rotating one - e.g. the typical 8-shaped plot of the trajectory of the Apollo missions is drawn in a coordinate system centered on Earth, but rotating with the Moon.

  • $\begingroup$ Some images would be nice, for instance the trajectory of the Apollo missions in differentt coordinate systems, rotating with the Moon and rotating with Earth as well as not rotating. $\endgroup$ – Uwe Jul 20 at 14:02
  • $\begingroup$ @Uwe Please feel free to add some - I don't have any in stock and don't have the software to create them. On the other hand rotating reference frames are slightly out of scope of this question. $\endgroup$ – asdfex Jul 21 at 11:50
  • $\begingroup$ As an addotional point the body doesnt have to be a planet either! Even in a redevouz frame of reference you can express the speed of your spacecraft in terms of the other spacecraft. For instance- once a module is docked to the ISS its moving 0mph relative to the ISS. During docking its flying around the earth blisteringly fast but its approaching the ISS at a relative speed of 5-10m/s. Speed can be defined relative to anything! $\endgroup$ – Magic Octopus Urn Jul 21 at 14:39
  • $\begingroup$ i guess i'm most confused about LEO and GSO. Are GSO typically referenced as traveling 0 units/hour relative to a fixed point on the axis of the earth, or ~1000 mph relative to the axis of the earth. When/how does their speed matter $\endgroup$ – user1886419 Jul 24 at 8:29

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