One of the most incredible natural phenomenon on Earth has got to be a total solar eclipse. This incredible sight is one that hasn’t been seen within the continental United States for almost an entire century. This is about to change, because on a year from last Sunday, Americans will once again be able to witness the incredible event.

Photo by Buddy_Nath (Pixabay)
Photo by Buddy_Nath (Pixabay)

Due to the elliptical, inclined orbits of the Sun, the Moon, and Earth, these special alignments in which the Moon’s shadow passes over the Earth’s surface are incredibly rare. In fact, on average only one total eclipse occurs on Earth every year. Obviously, as the Earth is much bigger than the Moon, and also because of how distant the Moon is from Earth, a total solar eclipse happens only over an incredibly narrow area on the Earth’s surface at any given time.

The continental United States set for an incredible solar eclipse next year

On August 21, 2017, the “Great American Eclipse” will occur, passing over from the Oregon coast all the way to the Atlantic, finishing on South Carolina’s coast. While these sorts of eclipses occur in cycles, the continental United States haven’t experienced one sine 1918, an eclipse which almost confirmed Albert Einstein’s greatest achievement, his Theory of Relativity, to the world.

Einstein first put forth his general theory of relativity in 1915. Einstein predicted that the presence of matter and energy would curve the fabric of space-time itself, and what humanity perceived as gravity was merely matter and energy following the path of this curved space.

Previously, gravity was thought of as an invisible, instantaneous force that massive objects exerted on one another. Einstein’s theory gave rise to a new set of predictions for celestial orbits, black holes, an expanding Universe, and most incredibly, the bending of starlight.

Einstein’s theory tested through scientific ingenuity

In the early 20th century, space telescopes, such as the one named after Sir Edwin Hubble, did not exist. Without modern technology, scientists used instead a clever substitute: as the positions of stars were known, one was able to observe stars close to the Sun during the day, where the Sun’s gravity would then act on those light rays.

Considering that Einstein’s and Newton’s predictions were different for how that light would be bent (Einstein predicted double the amount Newton did), comparing observations during the day with observations at night of the same starts allowed one to prove one of the two correct.

Since the Sun is so bright, obviously one cannot view stars during the day with a naked eye. During a total solar eclipse, however, the Sun’s light is temporarily blocked from reaching Earth, revealing, for many observers, stars during daylight hours.

The first solar eclipse to occur after Einstein’s theory of relativity was published occurred only months after, on February 3, 1916. However, since World War I was fully underway, it was difficult to organize a scientific expedition. Einstein was lucky, however, as the next total solar eclipse was to occur on June 8, 1918, and would allow scientists a chance to fully test Einstein’s theory against Newton’s.

At this point in history, the relative orbits of the Sun, the Moon, and Earth were known very precisely. Predictions for eclipse times were known to the second and to closer than a kilometer, allowing for scientists to be in the right place, at the right time, to properly view an event.

Scientists head out to test the general theory of relativity

A team of physicists, sent by the United States Naval Observatory, traveled to Baker City, Oregon, where the eclipse’s duration would be the greatest. The team was led by John C. Hammond, and its lead physicist was Samuel Alfred Mitchell. Mitchell was an expert on solar eclipses who had been observing the for the U.S. Navy since 1900. The team also included physicist and artist Howard Russell Butler, who was in charge of painting the solar eclipse as it occurred.

As the Moon slowly moved over the Sun, the sky was cloudless, and the team was incredibly excited, as such conditions would allow for the observation of stars near the Sun. However, the eclipse’s totality only lasted two minutes, and unfortunately clouds covered the Sun during the important moments where the sky darkened.

Less than five minutes after it began, the eclipse was over. Though Butler drew an impressive painting, no starts were observed through the clouds, and Einstein’s general theory of relativity remained, unfortunately, untested.

So close, yet so far

However, the next total solar eclipse occurred in 1919, and was observed by two scientific teams: one in South America and one in Africa. Through combining the data of both expeditions, many start were not only successfully observed, but were observed to be deflected by the Sun’s gravity by an amount in agreement with Einstein’s predictions, not Newton’s. Thus, Einstein’s theory was officially confirmed.

The continental United States wouldn’t see another total solar eclipse until the present day, an incredible streak that will finally end almost a year from now, on August 21, 2017. If clouds block your view, don’t get too upset, as another is scheduled to return in 2024, and then again in 2052. When you see the moon’s shadow pass over the Earth, remember that you are bearing witness to a natural example of the most important physics theory of all time.