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Age of the Universe: Size of the Universe: Infinite 300,000 Light Years Cosmic1919 Times MT. WILSON ASTRONOMER ESTIMATES SUN’S GRAVITY BENDS STARLIGHT MILKY WAY TEN TIMES BIGGER THAN THOUGHT Einstein’s Theory Triumphs But Disputes Suggestions that Spiral Nebulae are Other “Island Universes” “One of the greatest—perhaps the great- actly as arranged, but out of the 16 plates taken, The Milky Way is a “discoidal” (disc-shaped) Dr. Shapley has concluded that “our galactic est—of achievements in the history of human only two showed as many as five stars each. galaxy of stars 10 times bigger than astronomers universe appears as a single, enormous, all-com- thought” was what Sir Joseph Thomson, Presi- Prof. Eddington was also unable to stay several had previously conceived, according to Mt. Wil- prehending unit, the extent and form of which dent of the Royal Society of London, called Dr. more months to take check-photographs of the son astronomer, Dr. Harlow Shapley. Moreover, seem to be indicated through the dimensions of Albert Einstein’s prediction, which was appar- star field. he claims, the Sun exists nearer to its edge than the widely extended assemblage of globular clus- ently verified during the total eclipse of the Sun Sir Frank explained in detail the apparatus to its center. But he disputes the hypotheses of ters.” The center of our discoidal sidereal system May 29 last. both expeditions had employed, the way the other astronomers that scores of spiral nebulae “is distant from the Earth some twenty thousand Sir Joseph made his pronouncement during photographic plates were measured back at the seen in the starry heavens are other galaxies, parsecs”—more than 60,000 light-years—“in the a discussion of the results from observations of Greenwich Observatory, the corrections that had or “island universes”, that resemble the Milky direction of the constellation Sagittarius,” Dr. the solar eclipse at a joint meeting of the Roy- to be made for various disturbing factors, and the Way. Shapley continued. al Society and the Royal Astronomical Society methods by which comparison between the the- In his tour-de-force series of papers through- His conclusions fly in the face of generally ac- in London on Thursday evening, November 6, oretical and observed positions had been made. out 1918 and 1919, the prolific Dr. Shapley ex- cepted astronomical wisdom. “Until the last year before a large attendance of astronomers and He convinced the meeting that the results were amines other recent astronomical work in aston- or so, most students of stellar problems believed physicists. The excitement in the air was almost definite and conclusive, and that deflection did ishing detail, as well as presenting the results of rather vaguely that the Sun was not far from the palpable as it seemed generally accepted that take place. He also asserted that the measure- his own astronomical photography using the 60- center of the universe, and that the radius of the the observations were decisive in verifying the ments showed that the extent of the deflection inch reflector of the Mount Wilson Observatory galactic system was of the order of 1,000 par- prediction of Dr. Einstein, Professor of Physics was in close accord with the theoretical amount in southern California. His particular subject of secs,” he said (1,000 parsecs is more than 3,000 at the University of Berlin and Director of the predicted by Dr. Einstein, as opposed to half of interest is globular star clusters—nearly spheri- light-years). Some astronomers thought the Kaiser Wilhelm Physical Institute. that amount, the amount that would follow if the cal clusters of hundreds of stars that have long galactic system might be as large as 10,000 to principles of Newton were correct. puzzled astronomers because of their peculiar 20,000 light-years across. But according to Dr. The prediction “After a careful study of the plates I am pre- positions in only certain parts of the sky. When Shapley, the positions of globular clusters in the According to the gravitational principles pared to say that there can be no doubt that they Dr. Shapley began his study in 1914, 69 globular arrangement of sidereal objects suggest “that the enunciated by Sir Isaac Newton in his classic confirm Einstein’s prediction,” Sir Frank de- clusters were known; by the time he completed actual diameter of the galactic system is of the work Optics some two centuries ago, a ray of clared. “A very definite result has been obtained his work in 1918, he had added another 17 to the order of 100,000 parsecs”, This is a staggering light from a distant star just grazing across the that light is deflected in accordance with Ein- list. distance, larger than 300,000 light-years across, edge of a massive object should be bent by an stein’s law of gravitation.” In addition to pinpointing the exact position more than 10 times larger than any other astron- amount that depends on the object’s mass and “For the full effect that has been obtained, of each globular cluster in the sky, he also spread omer had hypothesized. thus its gravitational field. Newton thought of we must assume that gravity obeys the new law out their light into spectra to determine their mo- “This newer conception greatly embarrasses the gravity as a force that pulls things toward an ob- proposed by Einstein,” added Prof. Eddington. tions, specifically whether they were approach- interpretation of spirals as stellar organizations of a ject: the bigger the object, the stronger the pull. “This is one of the most crucial tests between ing the Sun or receding from it. From these data, size comparable to that of the Galaxy,” Dr. Shapley The most massive object in the vicinity of Newton’s law and the proposed new law.” Dr. Shapley sought to calculate the gravitation- said, because such a size would imply that the spi- the Earth is the Sun. So according to Newtonian al forces on the clusters, to learn whether they rals were inconceivable distances away in space. principles, a light ray from a distant star graz- WHY A TOTAL SOLAR ECLIPSE? were revolving around a common center, and if “For example,” he pointed out, “if any bright spiral so, the location of that center. He also sought to of 10 minutes of arc in angular measure has an ac- ing the edge of the Sun should be attracted or Herr Einstein in Berlin bent by the Sun’s gravity by an amount equal According to predictions by both Sir Isaac determine the distances of the globular clusters tual diameter directly comparable with that of the to 0.87 seconds of arc. To be sure, that angle is the Sun and the stars to be photographed at the Newton and Dr. Albert Einstein, a ray of light from the Sun using the novel method of Cepheid galactic system, its distance must be greater than a very small, about equivalent to a human hair at same time. from a star nearly behind the Sun (as seen from variables pioneered by Miss Henrietta Leavitt hundred million light-years.”. Similarly, the aver- 75 feet; but it is actually measurable on today’s Prof. Eddington himself decided to lead an Earth) will be deflected—bent toward the Sun— of Harvard Observatory. He also looked at ir- age proper motions suggested by the careful ob- astronomical photographic plates if adequate care expedition to the island of Principe, in the Gulf as it passes by the limb (edge) of the Sun. Such a regularly-shaped clusters of stars, the so-called servational measurements of several astronomers is taken. of Guinea close to the coast of West Africa, deflection would make the star look slightly far- “open clusters”, as well as other individual stars “would indicate appalling velocities in space.” Dr. Einstein’s general theory of relativity, near the end of the path of totality (see map). ther away from the edge of the Sun than it really and types of objects. In short, Dr. Shapley concludes, many observa- however, conceives of gravitation as indistinguish- He also convinced the Astronomer Royal—Sir is. After four years of diligent study, often assisted by tions “all seem definitely to oppose the “island uni- able from inertia. The “force” of gravity one feels Frank Dyson, Director of the Royal Observato- Dr. Einstein’s theory of relativity, however, his wife Martha B. Shapley, Dr. Shapley has pub- verse” hypothesis of the spiral nebulae.” pressing one down into a chair is the same as the ry, Greenwich—to send another expedition else- predicts that the amount of the deflection should lished a number of astonishing conclusions. “force” one feels when pulled forward in an auto- where, to minimize the chances of clouds inter- be double that predicted by Newtonian mechan- mobile when the driver brakes. fering with the observations. Led by Dr. Andrew ics. The maximum shift, for a star whose ray of According to Dr. Einstein, gravity, like in- Crommelin from the Royal Observatory, it set light just grazes the limb of the Sun, would be 1.75 ertia, doesn’t pull. Instead, a mass warps or up instruments at Sobral in northern Brazil, near seconds of arc, twice the amount Newton predicted EXPANDING OR CONTRACTING? curves space and time surrounding the object. the beginning of the path of totality. (0.87 arcsecond). The apparent positions of stars Einstein’s Theory Predicts Universe Must be Doing One or the Other The amount of curvature is proportional to the At each of these places, if the weather were pro- closer to the Sun’s limb would be shifted more than amount of mass. The curvature of space then pitious on the day of the eclipse, it would be pos- those of stars farther away. Einstein Says Neither curves the paths taken by rays of light. sible to take during totality a set of photographs of The more stars around the Sun during a solar Dr. Einstein’s theory, which is highly math- the obscured Sun along with a number of bright eclipse, and the more photographs astronomers In 1917, Albert Einstein and the Dutch astronomer Willem de Sitter showed that Einstein’s ematical, predicts that the curvature of space stars which happened to be in the vicinity. can take, then the more accurately the differences general theory of relativity could describe a highly simplified universe. around the Sun should bend starlight by twice between Dr. Einstein’s predictions and Sir Isaac’s But when it was applied to the real universe full of stars, there was a difficulty. as much as Newton’s theory predicts: 1.75 sec- Results discussed could be measured. Each year on May 29, as the Dr. Einstein’s model predicted that either all the stars would be expanding or moving apart Earth revolves around the Sun, the Sun appears to onds of arc. Thus, Dr. Einstein predicts that a At the joint meeting, Sir Frank described the from one another, as if from a gigantic explosion. Or they had to be contracting, as if they were all pass in front of the Hyades in Taurus, a cluster of ray of light from a distant star, grazing the edge work of the two expeditions. Their purpose was collapsing upon one another. stars so bright they are clearly visible to the naked of the Sun on its way to the Earth, would suf- to ascertain whether the light from these stars But Dr. Einstein, a devotee of the book Ethics by Dutch philosopher Spinoza, quoted from eye in winter. fer twice the deflection predicted by Newtonian as it passed by the Sun came as directly toward memory Corollary 2 of Proposition 20: “It follows that God is immutable or, which is the same thing, The only time stars are visible in the sky principles. the Earth as if the Sun were not there, or if there all His attributes are immutable.” Dr. Einstein was troubled by the concept of a universe that was not along with the Sun is during a total solar eclipse, The amount by which starlight is deflected was a deflection due to the Sun’s presence. “The immutable or unchangeable. when the Moon blocks most of the Sun’s blind- by the Sun is thus regarded by astronomers and effect of the predicted gravitational bending of “We can add, on the left side of the field equation—for the time being— an unknown universal ing light. And by unusual good fortune, the total physicists as one of the crucial tests in determin- the ray of light is to throw the apparent position constant,” Dr. Einstein said, explaining the quantity he designates by the Greek letter lambda, and solar eclipse of 1919 took place right on May 29. ing the validity of the Dr. Einstein’s Theory of of the star away from the Sun,” said Sir Frank. If calls the “cosmological constant.” He explains: “Not much harm is done thereby… the proposed new Relativity versus Newtonian physics. a deflection were to occur, measurements would universal constant determines the average density of the universe that can remain in equilibrium.” be made of how far the stars would appear on According to Dr. Einstein’s beautiful immutable universe, the presumably spherical universe May’s solar eclipse the photographic plates from their theoretical would be neither expanding nor contracting. Dr. Einstein made his prediction in a paper positions. published in 1916, in the middle of the late The Royal Observatory party arrived in Bra- Great War between England and Germany. But zil in ample time to prepare for the eclipse and a neutral Dutch astronomer smuggled a copy of photograph stellar fields. The day of the eclipse Dr. Einstein’s published paper through war-torn opened cloudy but cleared later, and the obser- Europe to England. There it was read by Profes- vations were carried out with almost complete In Their Own Words sor Arthur Stanley Eddington, Plumian Profes- success. The observers stayed on in Brazil until The path of totality for the eclipse of May 29, 1919 spanned sor of Astronomy and Experimental Philosophy July to secure the star field in the night sky at the Atlantic (dark line); the eclipse was very long — nearly Periods of 25 Variable Stars in the Small The Relation of the System of Stars to at Cambridge University—the same chair and the altitude of the eclipse epoch and under iden- seven minutes at its maximum duration. The two dots indicate Magellanic Cloud - Miss Henrietta Leavitt the Spiral Nebulae - G. F. Paddock university where Newton pioneered his great the positions of the expeditions led by Dr. Crommelin for the A remarkable relation between the brightness of Endeavors have recently been made to present tical instrumental conditions. The photographic Royal Greenwich Observatory (left) and Prof. Eddington for the studied variables and the length of their periods a comparative list of average radial or line-of-sight theory of gravity. plates were measured at Greenwich immediate- Cambridge University (right). has been noticed. There is a simple relation between velocities of the several different kinds of objects in the Although astronomers who had read earlier ly after the observers’ return, each plate being the brightness of the variables and their periods: the sky, and to discuss the relation of the spiral nebulae to unpublished drafts of Einstein’s paper tried to measured twice over. brighter variables have the longer periods. other objects. The average radial velocities of all except test his prediction during the total solar eclipses The Cambridge University party arrived on the spirals range in increasing magnitude from zero to of 1912 and 1914, they were foiled by clouds Principe on April 23. The island is about 10 miles Spectroscopic Observations of Spiral fifty kilometersper second. But a considerable jump is noticed from the fifty kilometers to 400 kilometers for and by the start of the Great War. But a study long by 4 miles wide. “We soon realised that the Nebulae - V. M. Slipher of the conditions of the 1919 solar eclipse prospects of a clear sky at the end of May were the average of the spirals. This suggests the question: The average radial velocity of spiral nebulae is +400 Are the spirals dissociated from the star system? The showed that the Sun would be very favourably not very good,” recounted Prof. Eddington. The km/sec. Radial velocity is the speed along an observer’s average velocity is decisively positive, which means placed amongst a group of bright stars at that sky was completely cloudy at the beginning of line of sight; positive radial velocity means an object is that they are receding not only from the observer or star time. Moreover, the Sun’s light would be totally the eclipse, but about half an hour before total- receding, while negative radial velocity means an object system but from one another. is approaching. As well may be inferred, the average blocked by the Moon for over five minutes (see ity they caught glimpses of the Sun’s crescent velocity of the spirals is about 25 times the average “Why a Total Solar Eclipse?”), allowing both through the clouds. They took photographs ex- stellar velocity.

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