Solar eclipses occur every few years. But the one that occurred on May 29, 1919 is considered one of the high points of 20th century science.
That day and thanks to that phenomenon, the British astrophysicist Sir Arthur Eddington performed the experiment that scientifically verified Albert Einstein's theory of relativity.
Both Newton's old theory of gravity and Einstein's new one predicted that light doesn't necessarily travel in straight lines, but can be deflected when it passes near something as heavy as the Sun.
But the German genius predicted that he would deviate further. Enough so that the apparent positions of the stars behind the Sun were detectably displaced from known and true positions.
From Oxford, Arthur Eddington observed the position of the stars in January and February 1919. The eclipse would be visible from both sides of the Atlantic so, to ensure good weather at at least one observation point, Frank Dyson, the Astronomer Royal , sent an observation team to Sobral (Brazil) and to Eddington to Sao Tome and Principe (Africa).
The skies were clear at both locations and, for a total of five minutes, both teams managed to take varying sharp photos of the stars.
When Eddington returned home and compared the apparent positions of the stars behind the Sun with the real ones, both data sets confirmed that Einstein's theory was correct. The discovery was published inPhilosophical Transactions of the Royal Society.
The phenomenon predicted by Einstein and verified by Eddington is known today as gravitational lens. The effect is used on a daily basis by astronomers, and much of the research on dark matter and energy is based on it.