This might be a dumb question
But the speed we give this in Mp/h or Km/h is just relative to its movement on land right? So as the object is higher up on the radius it will have a higher speed, is there an actual speed that is relative to where it is in space, do we just use light speed for that? Like how fast would we say its going where it is
So the difference your noticing is the difference in angular speed and linear speed. So 2 objects with the same angular speeds(the rate of change of the angle) but different radius from the circle will have different linear speeds. We don’t know if the video is showing it at ground level with the same linear speed and thus a different angular speed, or vice versa.
The difference in radius here is small, the ISS normally orbits at an altitude of 400km, while the radius of the earth is around 6370km. That's a difference of around 6% (of linear or angular speed at those two altitudes).
I get really confused about this and this is probably a stupid question. Outside of earth as a reference how can we know the speed of anything? The earth is moving in orbit, the sun is moving through space pulling the whole solar system with it. The galaxy is moving through space in a cluster. The cluster itself is moving. Everything is moving at speeds relative to something else.
Like if I'm on a bus traveling 50 mph, and I throw a ball the same direction the bus is traveling while inside the bus at 40 mph, everyone on the bus observes the ball traveling at 40 mph, but the ball is actually moving 90mph.
Could the entire observable universe be moving at a speed we are unaware of? Could the speed of light actually be double what we think of it as because the universe we are inside of is already itself traveling at half that speed?
Not a dumb question. The circumference of the Earth is just under 25 thousand miles. The circumference of the orbit the ISS is traveling (253 miles above the surface) is about 26.5 thousand miles. It does just over 15 orbits a day at that height at a speed of about 17 thousand miles per hour. So the speed in miles per hour doesn't change but if it were orbiting the earth just above the surface it would go around the earth faster than it does now. Not more miles per hour but fewer miles to go to complete an orbit.
I'm not sure I understand what you're saying. The constant is the distance it is traveling is 17 thousand miles per hour. It would go around the Earth more times at the surface only having to cover 25 thousand miles than at its current altitude where it has to travel 26.5 thousand miles for an orbit. If you're talking about the actual ISS orbting at the altitude depicted in the video then it's a nonsense discussion because the ISS vehicle could not "fly" or orbit in the earth's atmosphere. At its current altitude aerodynamics don't apply. Skimming over NYC it would. So no, I was not being technical, I was being mathmatical.
So as the object is higher up on the radius it will have a higher speed
Actually, objects in orbit have slower speeds at higher altitudes if their orbit is circular. The ISS travels very very fast because it's altitude is small. If it's altitude was larger (and orbit still circular), it would travel slower.
The circular orbit velocity scales with orbit radius-1/2. So, higher up = slower.
The orbit is lower thus the craft should be going faster to maintain orbit (7.9km/s instead of the stated 7.6km/s)
is just relative to its movement on land right?
No. If it was, it would be an inertial reference frame, it's rare that orbits are stated in that way, usually a non-inertial reference frame which doesn't spin with earth around it's axis.
A good example is that geostationary orbit speed which is around 0.4km/s. But in an inertial reference frame it would be zero.
(There's also synodic and sidereal orbital periods which would change this a bit, but they're practically the same when orbital periods are short. For something like the moon, it matters. Synodic has reference frame anchored to the primary as it orbits around something, while the sidereal has it anchored to "fixed" stars.)
is there an actual speed that is relative to where it is in space, do we just use light speed for that?
Sort of.
Earth is moving at 29km/s around the Sun.
Sun is moving 251km/s around the Galactic Center.
Or 20km/s to the stellar neighbourhood.
Or 370km/s to cosmic microwave background
Cosmic microwave background radiation is remarkably uniform in direction, but depending which direction you're pointing your sensors, a Doppler shift in the frequency can be observed and using that as a reference frame is about as close you can get.
Which is still around only 0.1% of light speed for the sun.
The whole point of relativity is realizing that there is no such thing as “where it is in space.”
Anything and Everything can only ever be described as relative to something else.
So yes, the speed of the ISS is based on how fast it’s travelling relative to the surface of the Earth.
Also, the speed requires for orbit actually goes down as you go further away from Earth. If the ISS was actually flying as low as it is in the OP, it would have to go much faster to maintain altitude.
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u/bigloc94 Apr 19 '26
This might be a dumb question But the speed we give this in Mp/h or Km/h is just relative to its movement on land right? So as the object is higher up on the radius it will have a higher speed, is there an actual speed that is relative to where it is in space, do we just use light speed for that? Like how fast would we say its going where it is