Fly from one Capital City to another without losing any 'Time'
Fly westwards from one Capital to Another. If you measure change in longitude and distance, you can find the required speed so that when you arrive it's the same "time of the day" as when you left. For which such journeys is the required speed lower?!
Example: Capital B is 30º west of Capital A and is 3,000 km away. 30º corresponds to 2 hours of Earth rotation, so you need to fly at 1,500 km/h