Eddington tested Einstein by waiting for an eclipse

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.

Scheduled the test around a total solar eclipse, using the Moon as a free shield that blocked the Sun's disc and revealed the stars for the few minutes of totality.

The Moon blocks the Sun's disc completely during totality, revealing the surrounding stars.

The eclipse was predictable years in advance, allowing planning of expeditions.

Comparing eclipse photos to night-time reference photos isolated the bending effect.

The predicted deflection was large enough to clearly distinguish Einstein's theory from Newton's.

When a test seems to require new equipment to overcome a fundamental obstacle, check whether a rare but predictable natural event already removes it for free.

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.