The Birthday of James Bradley (1693) – Aberration of Light

James Bradley discovered the aberration of light (1728): the apparent shift in star positions due to Earth’s motion. This was the first direct observational evidence for the Earth’s revolution around the Sun. The aberration angle is v/c where v≈30 km/s (Earth’s orbital speed), c≈300,000 km/s, giving ≈20 arcseconds. Bradley used the law of sines and vector addition. He also discovered nutation (wobble of Earth’s axis). His work is a classic example of applying geometry (the parallelogram of velocities) to astronomy.