I know that the frame of reference determines that it is the stay-at-home twin who ages faster than the twin travelling at some fraction of light speed. Some years back, an experiment in which one of a pair of synchronized atomic clocks was placed on a commercial airliner and flown around the world, resulted in a discrepancy between the two clocks (a matter of tiny fractions of a second) precisely confirming the time dilation predicted by special relativity. However, I have another experiment to propose, along the lines of the twin atomic clock experiment. This time, the stay-at-home twin with one clock is positioned on the surface of the earth at the equator somewhere. At this point, viewed from the perspective of Polaris, the pole star, due to the rotation of the earth on its axis, the twin and his clock are moving counterclockwise in a circle with a radius of about 8000 miles (the radius of the earth). Now, from the same point, the travelling twin takes off in an airplane and flies westbound (that is, counter to the rotation of the earth) at a speed of about 2000 miles per hour--exactly twice the rotational speed of the earth at the equator. Viewed from Polaris, with a mighty telescope, the travelling twin moves in a path that is the mirror image of his stay-at home twin -- a circular arc that is 8000 miles in diameter. Also, since he flies counter to the direction of the earth's rotation, the earth's velocity (of which the Polaris viewer is presumed to be ignorant) must be subtracted from his apparent velocity. His speed, as seen from Polaris, is exactly the same as his stay at home twin. Now, one twin with an atomic clock stays put on the surface of the earth for twelve hours, while his brother with a synchronized clock flies completely around the globe and returns to the same spot twelve hours later. They have moved with respect to one another at a velocity of some 2000 miles per hour--which should produce a detectable time dilation along the lines of the experiment I described from many years ago. Yet, to the Polaris observer, the two brothers followed two identical, mirror-image semi-circular paths at exactly the same speeds. Which twin ages faster (and why)? You surely can't say that neither one does, since this experiment has actually been performed -- I really do remember hearing about it!