2017-04-29 03:36:01 UTC
If the rocket circling the earth was traveling west at 200 MPH, while the earth rotated east at 1,000 MPH, you could say it was really moving at 800 MPH. That's relativity. Light being the only constant, and space and time not just empty nothingness. Perhaps space and time were relative too!
Fast forward. Two atomic clocks fly on jets in opposite directions around the Earth. It turns out they come back at different times.
The next step may be testing if time moves forward with the earth going east, and backwards if a jet flew fast enough from east to west.