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The May 29, 1919, Solar Eclipse

One hundred years ago today, a murky spot raced across the surface of the Earth at over twice the speed of sound. As the Moon completely blotted out the Sun for a small strip of our planet, indigenous peoples from Peru all the way to Mozambique witnessed one of the most awe-inspiring spectacles that one could ever hope to see — a total solar eclipse.

Solar eclipse
The May 29, 1919, total solar eclipse in Sobral, Brazil. Credit: F. W. Dyson, A. S. Eddington, and C. Davidson

Among the spectators was a team of astronomers, led by Arthur Stanley Eddington of Britain, who was hoping to photograph the eclipse during the moment of totality. The object was not to obtain a pretty picture or even to see the fine details of the ghostly white solar corona, which was visible for about seven minutes. The goal was to obtain high-quality images that revealed faint stars that only appeared while the Sun was totally eclipsed — the closer the stars were located to the silhouetted moon, the more valuable they were for science. Why is that?

Four years earlier, Albert Einstein published a revolutionary treatise that would change how humankind thinks of space and time. Einstein’s General Theory of Relativity (GTR) essentially tied together space and time into a single entity – spacetime – and explained why things like planets orbit the Sun. We often think of the fundamental force of gravity as, well, just that — a force. Einstein figured out that gravity is really a manifestation of a mass’ effect on spacetime. Anything with mass — you, me, the screen on which you are reading this — bends spacetime. The greater the mass, the more that spacetime is bent. The classic analogy is the bowling ball and marble on a rubber sheet. Place the marble on the sheet and you see it makes a very subtle dimple. But place the bowling ball on the rubber sheet and you get a very deep indentation, which makes sense as it has more mass. The rubber sheet represents spacetime, and the divots in it from the balls represent its curvature as a result of the presence of mass.

The mass of the Sun and Earth bend spacetime; as a result, the Earth’s motion in this curvature gives it its orbit. The Sun’s larger mass creates a larger depression in spacetime (visualized here as a dip in the imaginary green grid). Credit: T. Pyle/Caltech/MIT/LIGO Lab

Not only will more mass affect how spacetime is curved, but the amount of mass in a given volume will alter the how warped it is. For example, a beach ball would make a very wide but very subtle indentation in the rubber sheet, while a steel ball the size of a baseball would be a deeper, more confined dent. These effects on the rubber sheet are, of course, MANY orders of magnitude greater that how the masses of these objects bend actual spacetime. A star like our Sun, for example, has about 332,000 times as much mass as the Earth, and about 2×10^28 times as much mass as a person (that is 20,000,000,000,000,000,000,000,000,000 times as much mass). A star will produce a larger curve in spacetime – its divot in the rubber sheet would be very wide and very deep. Now let’s imagine that we compressed the Sun, which is not too far from a million miles in diameter, to make it into a smaller, denser ball. As we do so, the divot it creates in spacetime is getting narrower, but it is also getting deeper. Eventually, if we compress the Sun enough, gravity will take over and form it into a black hole – all of that mass will be compressed into a single point of, as far as we know, infinite density. The curve in spacetime has now become infinitely deep!

In the above diagram, a star is hidden by the Sun. As a result of the Sun’s mass bending spacetime, the starlight that would normally pass by the Sun and completely miss us follows a curved path and is redirected towards Earth, allowing observers to see it at the “apparent location.” The amount of curvature shown is significantly exaggerated. Credit: GSFC/NASA

So, back to the 1919 eclipse. Eddington was hoping to acquire images of the background stars surrounding the Sun during totality, and this eclipse was ideal in many ways. This was the longest eclipse in 500 years, with totality lasting nearly seven minutes! In addition, the Sun was located in the constellation Taurus during this time of the year, and the Sun was nestled among the Hyades star cluster, which provided a rich background of brighter stars that might be captured in the images. But what would those stars tell us? If Einstein was right, then stars that appeared near the eclipsed Sun would look like their positions had shifted slightly as a result of the starlight following warped spacetime. The closer the stars appeared near the Sun, the greater the shift. When it was all said and done, Eddington found that the stars appearing near the Sun had indeed deviated from the positions shown on celestial charts — Einstein was right!

This experiment has since been repeated during other total eclipses, such as the 2017 eclipse, and even with interplanetary spacecraft. For example, in 2002, as the Cassini mission (which studied the Saturnian system from 2004 to 2017) was on its way to Saturn, it just so happened it would be on the opposite side of the Sun from Earth. As radio waves were sent to and from the spacecraft, the full distance they traveled was altered slightly as they followed a curved path around the Sun. Given that we know to an extremely high precision both the speed of light and the time it took the signals to come from and go to Cassini, the total light-travel distance can be derived and compared to what the straight-line distance would have been. The experiment yielded results about the curvature of spacetime that were 50 times more precise than results from any previous experiment, which were well in agreement with the predictions of Einstein (learn more here).

Experiments continue to test the predictions of GTR. In fact, the first image ever of a black hole that was released this past April also provided a confirmation of its predictions. Even after more than a century, GTR is still holding up under the scrutinizing eyes of science.