Skip navigation

Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

How did Bradley determine the speed of light in 1779 from stellar parallax?

Based on information from Isaac Asimov's Biographical Encyclopedia of Science and Technology:

"In 1727, while looking for evidence of parallax, James Bradley discovered stellar aberration, another annual effect, but much larger. For a star in a direction normal to the Earth's orbit around the Sun, this aberration angle in radians, as seen from the Sun, is essentially (orbital velocity/speed of light) = v/c, and turned out equal to 10(-4). Using a value for Earth's orbital velocity close to 30 km/sec, Bradley arrived at a value for the speed of light close to the modern value of 300,000 km/sec and better than the 227, 000 km/sec that had been announced by Olaus Roemer in 1676."

Return to the Special & General Relativity Questions and Answers page.

All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.

This answer provided by Isaac Asimov's writings.