The May 29, 1919, total solar eclipse occurred because the Moon aligned between the Sun and the Earth in which they appeared overlapped to a certain population of observers on the Earth. The Moon covered the Sun's light, which led to an absence of light for a small period of time. The solar eclipse of May 29, 1919, was the longest solar eclipse that had been observed and recorded up until June 8, 1937. This eclipse was visible through locations like southeastern Peru and northern Chile. This specific total solar eclipse was significant because it helped prove Einstein's theory of relativity.^[1] The eclipse was the subject of the Eddington experiment: two groups of British astronomers went to Brazil and the west coast of Africa to take pictures of the stars in the sky once the Moon covered the Sun and darkness was revealed.^[1] Those photos helped prove that the Sun interferes with the bend of starlight.^[1]

A total solar eclipse occurred on Thursday, May 29, 1919. With the duration of totality at maximum eclipse of 6 minutes 50.75 seconds, it was the longest solar eclipse that occurred since May 27, 1416. A longer total solar eclipse would later occur on June 8, 1937.

As the eclipse of 1919 occurred only 0.8 days after perigee (May 28), the Moon's apparent diameter was larger than usual.^{[clarification needed]}

It was visible throughout most of South America and Africa as a partial eclipse. Totality occurred through a narrow path across southeastern Peru, northern Chile, central Bolivia and Brazil after sunrise, across the Atlantic Ocean and into south central Africa, covering southern Liberia, southern French West Africa (the part now belonging to Ivory Coast), the southwestern tip of the British Gold Coast (now Ghana), Príncipe Island in Portuguese São Tomé and Príncipe, southern Spanish Guinea (now Equatorial Guinea), French Equatorial Africa (the parts now belonging to Gabon and R. Congo, including Libreville), Belgian Congo (now DR Congo), northeastern Northern Rhodesia (now Zambia), the northern tip of Nyasaland (now Malawi), German East Africa (now belonging to Tanzania) and northeastern Portuguese Mozambique (now Mozambique), ending near sunset in eastern Africa.

Newton's laws of physics ran on the belief of absolute time and three dimensions of space.^[2] This idea meant that time had only one dimension, and that it was universal.^[2][3] Einstein had the idea of combining space and time to make a four-dimensional world that worked together.^[4][5] Einstein's idea meant that extremely small matter particles could produce massive amounts of energy.^[4] If Einstein's theory was correct, matter and radiation would be connected to energy and momentum,^[6] meaning that when light was passing a large mass there would be an observable bend to the light.^[6]

Einstein's prediction of the bending of light by the gravity of the Sun, one of the components of his general theory of relativity, can be tested during a solar eclipse, when stars with apparent position near the Sun become visible. The stars cannot be seen without a solar eclipse because stars passing the sun are drowned by solar glares.^[7]

Following an unsuccessful attempt to validate this prediction during the Solar eclipse of June 8, 1918,^[8] two expeditions were made to measure positions of stars during this eclipse (see Eddington experiment). They were organized under the direction of Sir Frank Watson Dyson. One expedition was led by Sir Arthur Eddington to the island of Príncipe (off the west coast of Africa), the other by Andrew Claude de la Cherois Crommelin and Charles Rundle Davidson to Sobral in Brazil.^[9][10][11] The stars that both expeditions observed, the Hyades, were in the constellation Taurus.^[12]

The solar eclipse of May 29, 1919 allowed Einstein to finalize his theory of relativity.^[13] However, the May eclipse was almost missed, due to unexpected storms.^[14] The astronomers were almost unable to get photos of this eclipse due to a cloud.^[15][14] A thunderstorm happened during the morning of the eclipse, and it had been overcast that day and many of the days beforehand.^[15][14] Only thirty minutes before the eclipse did the clouds begin to dissipate, and even then they were taking many photos through gaps in the clouds.^[14]

The photographs taken during the eclipse of May 29, 1919, proved Einstein correct and changed ideas of physics.^[16] They also provided evidence that the Sun's mass did shift the way a star's light will bend.^[13] From the findings from these expeditions Dyson is quoted saying, "After a careful study of the plates, I am prepared to say that they confirm Einstein's prediction."^[16] He continued to explain that it left little doubt about light deflection in the area around the Sun and it was the amount Einstein demanded in his generalized theory of relativity.^[16]