Reference frames are crucial.

For example if you carry a cup of tea up the aisle of a jet plane in flight, you might say your speed is 2 miles per hour. Someone on the ground, however might say you were moving at 572 miles per hour. Someone watching from the surface of our Sun might say you were travelling at about 18 miles per second, not to mention envying your air-conditioning.
Quoted from Stephen Hawkings' The Grand Design.
Sent from my Nokia phone!
Giantsitel