Abstract

Explore the significance of the 1952 solar eclipse glass plate housed at Yerkes Observatory, which was taken by astronomer George Van Biesbroeck to put Einstein’s general relativity to the test. In this article, we uncover the enduring impact of this artifact, surviving generations and serving as a catalyst for renewed eclipse expeditions.

Sunlight often shines through the glass plate of the 1952 total solar eclipse, where it sits nestled in a window between the domes of the 24″ and 41″ reflecting telescopes at Yerkes Observatory (Figure 1). Some keen-eyed visitors ask about the fingerprints in the upper right corner, wondering which astronomer left behind their mark on the plate. Most ask about the Sun’s corona, which makes its ghostly appearance to human eyes only during a solar eclipse’s peak totality. Though remarkable, the corona is not the true centerpiece of this plate. Instead, it is the tiny points of light you see in the background, each a star in the constellation Aquarius, whose position in the sky provided one of the first refined measurements supporting Einstein’s theory of general relativity.  

General relativity has three main testing points as outlined in Einstein’s 1915 paper, “The field equations of gravitation” often denoted as the classical tests of relativity (Einstein, 1915). The first testing point is based on previous observations that could not be explained by Newtonian gravity, particularly the precession of the planet Mercury. In the 19th century, astronomers observed that the point of Mercury's closest approach to the Sun, or perihelion, was slowly shifting over time (Le Verrier, 1859). Classical Newtonian gravity predicts this shift, or precession, in Mercury’s orbit, but the observed shift exceeds Newtonian predictions. Within the framework of general relativity, the gravitational interaction between the Sun and Mercury accounts for the additional precession observed in Mercury's orbit. This successful explanation is considered one of the early triumphs of general relativity.

The second classical test focuses on measurements of gravitational redshift where light loses energy as it emerges from a strong gravitational field. Testing for gravitational redshift involves comparing the observed energy of light from a source in a strong gravitational field to the expected energy in the absence of gravity. These measurements can be made with spectral observations of dense objects and were first accomplished in 1954 by Daniel Popper with his observations of the white dwarf 40 Eridani B (Popper, 1954).

The third testing point is the predicted bending of light as it passes by a strong gravitational field. As outlined in the theory of general relativity, massive objects bend the path of light from background light sources. How much a background object’s light bends depends on two things: the mass of the intermediary object and how close the background object’s light passes by the intermediary object. This bending of light leads to a shift in the background object’s position compared to its position in the sky when not passing near the massive object. For example, Einstein calculated that a background star near the Sun’s edge, or limb, will have an apparent shift of 1.75 arcseconds (arcsec) — less than one-thousandth of a degree in the sky (Einstein, 1915.) 

Astronomers need to make two observations to observe this effect. The first is a baseline observation of the background star’s location, or where a star appears in the sky without the influence of the Sun’s gravity. The second observation is where solar eclipses come into play. Because a star’s light must pass very close to the Sun’s limb for the shift in position to be observed, the observations required to confirm Einstein’s prediction are possible only during a total solar eclipse, or when the Moon blocks the brightest light from the Sun and allows one to uniquely see distant stars that appear close to the Sun’s edge. Indeed, at every total solar eclipse following the publication of general relativity, observers set out on eclipse expeditions hoping to capture this phenomenon. 

Though attempts were made at two previous eclipses, the 1919 solar eclipse is considered a definitive milestone for general relativity. The story is well-known: Arthur Eddington, among other observers at the time, traveled to Sobral, Brazil, and the island of Principe off the coast of western Africa to observe a total solar eclipse. With the Hyades star cluster well-positioned near the Sun at the time of the eclipse, the timing was perfect to put general relativity to the test. Despite inclement weather and less than ideal observing conditions, the shift in stellar position was observed. As reported by the observers to the Royal Astronomical Society in November of 1919, one eclipse yielded an observed shift of 1.98 ± 0.12 arcsec, and the other, 1.61 ± 0.30 arcsec (note that both measurement errors are reported as probable errors as opposed to the more modern standard deviation)(Dyson, Eddington, & Davison, 1920).  

Despite attempts in the 1980s to undermine these results with accusations of selection bias (e.g., Earman & Glymour, 1980; Collins & Pinch, 1998), the 1919 observations provided clear evidence that the Sun indeed bends background starlight and solidified general relativity as the modern theory of gravity. The observations and subsequent calculations did not suffer from selection bias (Gilmore & Tausch-Pebody, 2021), but more precise results through better observing conditions and the elimination of systematic errors would be a natural next step for observers. Perhaps satisfied with the results from Eddington, discouraged or impacted by the intrinsic difficulties surrounding eclipse observations, or focused on new ideas such as quantum mechanics and Hubble’s law, few astronomers took eclipse expeditions to test general relativity after the late 1920s. Only three expeditions took place between 1930 and 1951 that had publishable results related to general relativity (Von Klübe, 1960).

This brings us to 1952, nearly 35 years after the 1919 eclipse, and to the story of the glass plate still on display at Yerkes Observatory. George Van Biesbroeck, or Van B as many past and present Yerkes staff call him, was 72 years old — technically retired, but still very much an active observer at Yerkes and McDonald Observatories. Five years prior, in 1947, Van B traveled to Brazil to observe a total solar eclipse, but problems with the telescope’s mirrors and filters produced less than satisfactory, though publishable, results (Van Biesbroeck, 1950). Compelled by the predicted presence of some well-placed stars near the Sun in 1952 and knowledge gained from the almost-failed 1947 expedition, Van B made plans to travel to Sudan and observe using the same setup as in 1947. This time, despite almost being foiled by a dust storm, the observations were a success (Figure 2 and Figure 3), and Van B left Sudan with two viable photographic plates of the eclipse ("Decision in Khartoum", 1952). 

Six months after the eclipse, Van B returned to Sudan to take the comparative image of the stars in the constellation Aquarius without the influence of the Sun (Figure 4). These data, paired with his eclipse observations, were all that was needed to begin his calculations. Recall that the theory of general relativity predicts that the Sun will bend starlight at its limb by about 1.75 arcsec. Through his observations, Van B measured the Sun's influence to be 1.7 ± 0.1 arcsec, making this one of the first precise measures of relativity that was also well-aligned with Einstein’s predictions (Van Biesbroeck, 1953).

From 1952 onward, general relativity has been tested countless times and remains our prevailing theory of gravity. While many still take eclipse expeditions in pursuit of science, such expeditions are no longer only for well-funded scientists with the objective to test fundamental theories of physics. The chance to observe the corona by eye and see the sky go dark at the moment of totality — the visceral, almost instinctual reaction to fall silent as the Sun disappears — still sets people around the world on a search for the next total solar eclipse. Our ability to connect and share experiences over long distances, too, make experiencing an eclipse more accessible than ever to those who can’t travel to the path of totality. Almost 70 years after Van B’s last eclipse expedition, we at Yerkes are planning eclipse expeditions of our own; the first expeditions since the observatory became a non-profit in 2020. We are traveling not solely in pursuit of science but in the hope that we can share the moment of totality with as many people as possible. Until then, we hope for clear skies in April and are excited to share our eclipse expeditions with you all.