May 29, 2020
Thanks to Peter Ponzio, HMU Tutor, for today’s post.
The introductory course at Harrison Middleton University is called “The Great Conversation: The Cornerstone Course.” It includes short readings on a wide range of subjects and serves as an introduction to our methods. One of these readings, The Expanding Universe, by Sir Arthur Eddington, is the subject of today’s blog.
In this selection, Eddington made three observations, quite radical at the time, that are the subject of this piece. His first observation was that objects which he termed “spiral nebula,” were in fact other galaxies distinct from the Milky Way. The second observation was that these galaxies seem to be receding from us at a constant rate of speed. The third observation he made was that the Newtonian theory of gravity was no longer valid, having been supplanted by Einstein’s theory of spacetime (Eddington 253 – 264).
Before commenting on these three observations, I’d like to make a digression in order to explain my interest in Eddington and astronomy in general. I had read Milton’s Paradise Lost a number of times before embarking on my Master’s and Doctoral programs at Northwestern University and Harrison Middleton University. In Book VIII, Adam speaks to Raphael about the heavens, saying:
When I behold this goodly Frame, this World
Of Heav’n and Earth consisting, and compute,
Thir magnitudes, this Earth a spot, a graine,
An Atom, with the Firmament compar’d
And all her numberd Starrs, that seem to rowle
Spaces incomprehensible (for such
Thir distance argues and thir swift return
Diurnal) meerly to officiate light
Round this opacous Earth, this punctual spot,
One day and night. . . (Milton 232)
This passage, along with several others in the poem, sparked my interest in astronomy which led to a desire to learn more about astronomy in general and the purchase of several telescopes and related gear to explore “this goodly Frame, this World.”
To return to Eddington’s work, his first observation regarding the existence of “spiral nebula” as being distinct galaxies has proven true. At the time of his writing, Eddington speculated that there might be one hundred billion galaxies in the universe and that they might be from one to 150 million light years distant from Earth. We now believe that Eddington’s estimate of the number of galaxies is directionally correct, and scientists estimate that there are between 100 to 200 billion galaxies in the universe. His estimate of the distance of these galaxies, although stated with the disclaimer that “they doubtless extend far beyond the latter distance” (Eddington 253), is far less than what we now estimate the galactic distances to be. The Hubble Space Telescope has captured “Ultra Deep Field” observations which extend back to approximately 13.5 billion light years, or 500 light years after the big bang (Christensen, Lars and Bob Forsbury 96 – 103).
The second observation that Eddington discussed in his writings is that the universe is expanding at a constant rate. Not only is it expanding in what we perceive to be a direction away from us, but it is expanding at a constant rate in every direction. This constant rate is known as the cosmological constant, and was predicted in Einstein’s General Theory of Relativity. The logical conclusion to be drawn from this observation is that, if this expansion occurs for the foreseeable future, the universe will expand forever, and each galaxy will be an island existing in a vast, desolate, unimaginable expanse of space. As scientists have studied the cosmological constant, they now realize that the calculations may be subject to slight errors. Even a slight error in the calculation would yield surprising results: on the one hand the universe would go on expanding forever in the scenario presented above; or, it would expand and then began contracting until it collapsed into a single point only to start the big bang over again.
Eddington’s third observation, regarding the law of gravity, is based on Einstein’s theory that gravity is not a “Newtonian force . . . just the curvature of spacetime” (Skuse 28 – 33). Einstein estimated that the sun’s mass caused a shift of 1.75 arc-seconds in the observable light of stars; in other words, a star’s light would shift by 1.75 arc-seconds compared to the apparent position of the star. Einstein’s theory appeared elegant, but his theory needed to be verified by observations. Eddington theorized that during a solar eclipse, the amount of deflection of light caused by the sun’s mass could be measured. In 1919, Eddington traveled to Brazil to observe the solar eclipse and measure the shift in the light of the stars during the solar eclipse. Eddington’s observations resulted in a shift of 1.75 arc-seconds, just as Einstein predicted.
Nearly 100 years later, during the solar eclipse of 2017, Donald Bruns, a retired physicist, made a series of observations using a Tele Vue NP101 refractor telescope and a Finger Lakes CCD camera to take a series of photographic exposures of 20 stars that appeared during the eclipse. After calibrating and analyzing these images, he was able to calculate the shift in the star’s light due to the sun’s mass, of 1.7512 arc-seconds, verifying Einstein’s and Eddington’s results (Bruns 22 – 25)
Eddington could not have foreseen the advances in astronomy that allow professional and amateur astronomers to make observations such as the one undertaken by Donald Bruns mentioned above. Today, amateur astronomers can take Earth-based photographs of astronomical objects only dreamed of in the 1920s. As an example, the following is a photograph of the Globular Cluster located in Hercules, which is known as M13 (Messier 13), taken by the author using an 80mm achromatic reflecting telescope and an Orion G3 CCD Camera mounted on a Sky Watcher HEQ5 equatorial mount.
An image of M13. Photo credit: Peter Ponzio.
Eddington’s observations, like Einstein’s, have stood the test of time and have added to our understanding of our place in the universe. His inclusion in the Great Books is merited not only by his observations of celestial phenomena, but also by his ability to relate these phenomena in a way that is readily understandable by the general public. For this, we owe him a debt of gratitude.
Works Cited
Bruns, Donald. “A Picture Perfect Eclipse Experiment.” Sky and Telescope, vol. 136, no.2, 2018, pp. 22 – 25.
Christensen, Lars and Bob Fosbury. Hubble, 15 Years of Discovery. Springer Science + Business Media, Inc., New York, 2006.
Eddington, Arthur. The Expanding Universe. Great Books of the Western World. vol. 56, 2007, pp. 253 – 295.
Milton, John. Paradise Lost. Great Books of the Western World. vol. 29, 2007, pp. 93 – 333.
Skuse, Benjamin. “A Relatively Important Eclipse.” Sky and Telescope, vol. 137, no. 5, 2019, pp. 28 – 33.
