#22 1919 · Dyson, Eddington and Davidson (1919 eclipse expedition) · Science / experimental physicsredesign-the-moment
Eddington tested Einstein's theory by waiting for the Moon to do the engineering for him
the problem
Testing whether the Sun bends starlight meant photographing stars the Sun's own glare erased
background
Einstein's 1915 general theory of relativity predicted that the Sun's gravity would measurably bend the path of starlight passing close to it — a testable, falsifiable claim that would distinguish his theory from Newtonian gravity, which predicted exactly half the deflection. But testing it meant photographing stars that appear, from Earth, extremely close to the Sun in the sky — precisely the position where the Sun's own overwhelming brightness makes any nearby star impossible to see or photograph on an ordinary day.
Building equipment powerful enough to filter out the Sun's glare while still resolving faint background stars right next to it was far beyond what any telescope of the era could do, and no artificial shield could replicate blocking a light source that bright at that scale. What the experiment needed was something to physically block the Sun's disc itself, precisely and briefly, without also blocking the stars around it.
the move
Astronomers Arthur Eddington and Edwin Cottingham traveled to the island of Príncipe off West Africa, while Andrew Crommelin and Charles Davidson traveled to Sobral, Brazil, to observe the total solar eclipse of 29 May 1919 — using the Moon's own disc, precisely aligned by nature, as the free shield that made the stars near the Sun visible for the few minutes of totality. They photographed the star field around the eclipsed Sun and compared the stars' apparent positions to reference photographs of the same field taken months earlier at night, without the Sun anywhere nearby.
the payoff
The measured deflection matched Einstein's predicted 1.75 arcseconds rather than the 0.875 arcseconds Newtonian gravity predicted, and the results were announced to a joint meeting of the Royal Society and Royal Astronomical Society on 6 November 1919, instantly confirming general relativity and making Einstein a global celebrity almost overnight.
what came after
The 1919 eclipse expeditions are recognized as one of the most consequential experimental confirmations in the history of physics, and the episode remains a standard teaching example of designing an experiment around a naturally occurring, precisely timed event rather than trying to engineer an equivalent artificial condition from scratch.
references
- [1]Eddington experimentWikipedia, 2025en.wikipedia.org
- [2]The 1919 solar eclipse that proved Einstein rightEarthSky, 2023earthsky.org