James Bradley discovered a phenomenon called "stellar aberration." If you were to watch a
star over the course of a year, you'd note that it moves in little ellipses. The paths of the
stars over the poles (or more precisely, above the plane of the Earth's orbit) will be almost
circular, while the paths of those near the equator will be flat. This effect is called stellar
aberration. Unlike parallax, this affects all stars equally, no matter what their distance from
Here's the effect you see over the course of a year, highly exaggerated (that big yellow circle with the ring
around it represents the Solar System, whether it be Heliocentric or Geocentric).
This diagram compares stellar parallax (left) and stellar aberration (right). Bradley observed the phenomenon
on the right.
As the Earth moves from one point to the next, the apparent positions of the star are shifted in the direction
of the Earth's motion. However, it is not the change in the position of earth that causes this aberration. It is
its change in motion or speed (centripetal acceleration as it orbits the sun).
Consider a beam of light entering approximately due south towards earth. Due to the transverse velocity of
earth as it orbits earth, this beam of light will appear to be coming slightly east of its true position.
The effect he was seeing is due to changes in the Earth’s (directional) velocity.
The tangential velocity is
In sum, it's important to realize that stellar aberration was and still is proposed to be an apparent
aberration of the star's position. It is not considered to be an actual refractive phenomenon independent
of what observer's appear to see.