"for planets to orbit their stars requires constant acceleration doesn't it, to balance the centrifugal force? So, are we aging slower and slower in comparison to the sun, and the moon in comparison to us?"<br />------<br /><br />not so, on Mercury you would age slower relative to aging on Earth and if you could live on the Sun's surface you would be aging slowest relative to everywhere in the solar system (due to the effect of gravitational acceleration induced slowdown of aging - general relativistic phenomenon - that we talk about)<br /><br />it helps to consider part of the Sun's surface matter equivalent say to that of the Earth as being in orbit around the Sun at that distance... and conversely you could view the Earth for example as just a part of Sun except that it moves around it at differing rate and is at some distance above its surface - in fact you could think of all the planets of the solar system as being part of one huge system, one planet that has huge caverns (interplanetary space) with planets forming extension of the Sun's distant surface matter<br /><br />thus if we lived suspended in space outside the solar system, we would age fastest<br />given we live on Earth we age a bit slower due to its local gravitational acceleration and to that is added the acceleration due to the Sun's matter which further increases this slowdown of aging<br /><br />however such slowdown due to gravitational acceleration is really negligible and while I think it would be measurable if someone did experimental measuring of it, it is really too small to make any practical difference - point is the gravitation within solar system is way too weak as far as special relativistic effects go<br /><br />I think that if you lived on Jupiter you might be aging slower than living on Earth due to its huge mass even though Jupiter is much farther from Sun, in this case the local effect might override the general rule that if one is closer to the Sun one ages slower (not sure if that is so, it <div class="Discussion_UserSignature"> </div>