Aberration of Starlight

You have to tilt your telescope to catch the star light just as you have to tilt your umbrella to keep off wind-driven rain. It is a matter of relative velocity.
 


Common experience leads you to expect that the velocity of the light relative to the Earth would be greater than the velocity of the light relative to the star. That is, you expect the velocity of the light with respect to the Earth to be the hypotenuse of the triangle and therefore greater than c. But observations are consistent with the hypotenuse of the triangle being just equal to c. Actual experiments involve measurements of the change in the apparent positions of stars at different times of the year.

Relativity concepts

Apparent Star Positions
You have to tilt your telescope to catch the star light just as you have to tilt your umbrella to keep off wind-driven rain. It is a matter of relative velocity. As the Earth moves around the Sun, the star will appear to trace out a small ellipse. The shape of the ellipse depends on the orientation of the star to the ecliptic.

When a distant star is viewed from the Earth at different times of the year, its apparent position will change as a result of the Earth's velocity through space. This apparent ellipse of motion of the star is called the aberration of starlight. Measurements of the size of this ellipse offer one piece of evidence that the velocity of light is independent of observer motion. This effect is independent of the parallax which permits the determination of the distance of the closest stars by triangulation. </TD< tr>