Involves

Moment

Stars beside a darkened Sun

Sobral and Príncipe, May 29, 1919; eclipse expeditions test light deflection

During a total solar eclipse, British expeditions photographed stars near the Sun from Sobral in Brazil and the island of Príncipe. The darkened Sun made nearby stars visible. Comparing their apparent positions with reference photographs allowed the teams to investigate how strongly the Sun's gravity bent the stars' light.

Date
May 29, 1919
Occasion
Solar eclipse
Test
Apparent star positions

The caveat

The 1919 measurements had substantial uncertainties, and the results were not the last word on general relativity. Later observations tested the prediction more precisely. The lesson is that a theory offered a discriminating test, not that one celebrated experiment made further scrutiny unnecessary.

Albert Einstein's general relativity predicted a specific deflection. The published results favored it and brought the theory international attention. For Popper, a prediction that risked disagreement with observation exemplified a different intellectual posture from explanations able to accommodate anything after the fact.

How it connects

The eclipse made a precise prediction vulnerable to measurement.

This moment appeared in Giving an idea a chance to fail, the Involves connection for September 12, 2026, which asked: What would you have to observe to change your mind?

Check yourself

What did the 1919 eclipse allow astronomers to compare?

Observed stellar positions with the deflection predicted by relativity. Exactly. Stars visible near the eclipsed Sun provided a way to test a prediction about gravitational light deflection.

What the sources establish

  • The 1919 expeditions used a solar eclipse to test relativity through apparent stellar deflection, with later observations improving precision.

Sources