when NASA etc talk about things moving at X speed in space, what is that compared to? What’s the relative point in space used and how can you know yours inert against it?

Question

This is a weird one, but it boils down to people saying you can’t travel faster than light. But speed is only relative to another object isn’t it? I mean, I’m sat on the sofa now doing 0mph, but compared to the sun, we’re travelling thousands of mph.

So if speed is only relative to another object, does that mean two objects travelling close to SoL are actually going faster than light relative to each other?

Edit: there’s been some incredible responses which – to my endless surprise – have made perfect sense somehow! Thanks for the response!

In: 20

Answer 1

Speed is always relative to something. A plane’s airspeed is relative to the air it’s passing through; its groundspeed is relative to the ground it’s flying over. (The difference is the speed of the *air* over the ground.)

For rockets, there’s speed relative to the launch site; relative to the center of the Earth; relative to the Moon, Mars, Jupiter, or whatever other body it’s approaching; and relative to the Sun.

Any of these are valid, so choose whichever is convenient.