I'm not sure if this has been asked/discussed before or not. I tried to look but couldn't find a thread, so if there is one please direct me to it. In this book I'm reading it states that if two planes start off at say the US Naval Observatory, if one goes eastward & the other westward, time will move slower on the plane going eastward. Both planes travel at the same speeds, same height, etc. The reason the book states is because the plane traveling westward goes against the rotation of the Earth. If the US Navel Observatory is the reference frame I don't understand why time one way is slower than the other. If the reference frame was in outer space I could see why (you'd see the plane going eastward at say 100kph + the speed of the earth's rotation). Perhaps I'm over thinking this one or perhaps I don't understand as much as I thought I did. Any thoughts?