Aberration of Light

In 1725 James Bradley, an English amateur astronomer, was attempting to observe stellar parallax when, through meticulous observation he observed that stars did trace out small ellipses in the sky as expected for parallax, but that all stars traced out precisely the same ellipses! This didn't make any sense at all; the only way the parallax of all stars could be same would be if all the stars were the same distance from the Earth — a regression to the Earth-centred universe with a vengeance! After a while, the apparent motion Bradley observed was discovered to be the result of the aberration of light. On each point in its orbit, the Earth is travelling with a velocity with respect to the distant stars that differs by 60 kilometers per second from the velocity at the opposing point — where the Earth will be six months hence.

Now 60 kilometers a second is pretty fast compared to freeway speed limits or even the velocity of Earth satellites, but it's only 0.02% of the speed of light. However, by Bradley's time the art and science of measuring the positions of stars had advanced to such an extent that he was able to observe the mere 41 arc-second displacement of the apparent position of stars due to aberration.

This effect leads to the bizarre consequence that, in order to view a particular star, you must actually be looking slighty to the side of the star's "true" location. Your view is therefore aimed at empty space, but what you end up seeing is the image of the star.

It wasn't until 1837 that astronomers managed to measure the much smaller parallax of the nearest stars. The classical aberration of light discovered by Bradley must be taken into account whenever accurate positions of objects in the sky are calculated.

From John Walker's Website

Wikipedia entry