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James Bradley discovered aberration of light while making precise repeated measurements of stellar directions. The annual displacement did not have the timing expected for stellar parallax. He explained it by combining the observer's orbital velocity with the finite speed of light.

For a transverse observer speed $v\ll c$, the characteristic aberration angle is about $v/c$ in radians. Its dependence on the observer's velocity, rather than the star's Euclidean distance, distinguishes it from parallax. The effect supplied evidence for the orbital motion of Earth and a way to relate terrestrial orbital speed to the speed of light.^[ref]