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The 1919 solar eclipse expedition
Vistas in Astronomy, 1976, Vol. 20, pp. 207-209.
33.
Pergamon Press.
Printed in Great Britain
THE 1919 SOLAR ECLIPSE EXPEDITION
P.A.H. SEYMOUR
National Maritime Museum
I would really like to retitle this paper the "Big G" in Greenwich, because the 1919 eclipse expedition represents one of the highlights in a long association between gravity and Greenwich, "Big G" itself being measured b y Maskelyne in 1775 and b y Airy in 1854. However, I would like to start by considering a very minor aspect of this association, namely the perihelion shift of Mercury. Even in Halley's time it was already evident that Mercury had an anomalous motion. Le Verrier was later to show that this anomaly could not be explained by the application of Newtonian mechanics to the solar system as it stood at that time. In 1859, Le Verrier proposed that the perihelion shift could be explained if one supposed that there was an intermercurial planet. Later on this proposed planet was to be given the name Vulcan, and it was with the search for Vulcan that some of the astronomers at Greenwich were associated. Hind in 1872, Airy in 1876 to 1877, and Dyson in 1901 all showed an interest in the search that never produced anything convincing. Other theories to explain this shift consisted of the non-sphericity of the Sun and an extended mass of diffused matter between the Sun and Mercury. The most fruitful class of theories consisted of mathematical modifications to Newton's Law of Gravity. At least part of the shift could be explained by replacing Newton's Law of Gravity using analogies with laws of electrodynamics that were then current. 1 The accepted explanation was, in fact, going to come from a modification of the Law of Gravity and the equation of motion in a gravitational field. However, this was going to stem from a rather different direction. In 1905, Einstein published his Special Theory of Relativity from which he deduced that energy has inertial mass given by the famous equation E = m c 2. In 1911 Einstein postulated the principle of equivalence. This principle proposes that a system of reference in uniform acceleration cannot be distinguished from a homogeneous gravitational field by the performance of any physical experiment. This led to a demonstration that all energy has both gravitational and inertial mass, and the two types of mass are always equal to each other. From this he deduced that " A ray of light going past the Sun would accordingly undergo deflection to the amount of .83 seconds of arc." However, in this paper he did point out that, "The relations here deduced, even if the theoretical foundation is sound, are valid only to a first approximation."2 In 1916 Einstein published the General Theory of Relativity. This incorporated the Principle of Equivalence, which gave physical content to the theory, and the Principle of General Covariance, which was a statement concerning the mathematical form of the laws of physics. This theory required a modification to Newton's field equations and the equations of motion in a gravitational field. When the theory was applied to the motion of Mercury it explained the perihelion shift. Another consequence was that a ray of light passing close to the Sun would be deflected by 1.75 seconds of arc.
3
In 1917 Dyson suggested that the theory could be tested on the occasion of the 29 May, 1919 eclipse. This experiment consists of photographing the star field around the eclipsed Sun and comparing the photographs so obtained with one of the same star field taken about 6 months later. Two British expeditions were sent out; one from Cambridge (led by Eddington) went to Principe off the West Coast of Africa, and another from Greenwich went to Sobral in Brazil. The results were presented at a joint meeting of the Royal Society and the Royal Astronomical Society. The paper by Dyson, Eddington and Davidson has this to say, "Thus the results of the expeditions ... can leave little doubt that a deflection of light takes place in the neighbourhood of the Sun and that it is of
9N~
208
P.A.H. SEYMOUR
the a m o u n t d e m a n d e d by Einstein's generalised t h e o r y of relativity ...,,4 However, the t h e o r y did have its detractors and one a s t r o n o m e r wrote to Dyson saying: "I cannot yet feel that the relativity theory is established or even highly probable. But all the more reason for leaving no stone u n t u r n e d to test its consequences so long as m a n y good minds believe it possible. ''s In any case, such an important result needed further verification, and more expeditions were necessary. There are two points that I would like to consider with regard to subsequent expeditions. Firstly, are these results comparable with later results? Secondly, h o w did these expeditions affect subsequent a t t e m p t s to measure the same effect? The answer to the first e q u a t i o n can be seen in Table 1. The results are TABLE 1. Comparative Results of Various Solar Eclipse Expeditions to Measure Light Deflection. Date
1919
Group & References Dyson et al. (4)
No. of stars
rmin (solar radii)
7 5
2 2
Result (seconds of arc) 1,98 -+0.12 1.61 -+0.30
1919DA
Freundlich (7)
2.0 to 2.2
1919DA
Hopmann (6)
2.16 -+ 0.14
1919DA
Danjon (8)
2.06
1919DA
Mikhailov(9)
1.95 -+ 0.09
1922 1928
Campbell (10a) & Trampler (10b)
92 145
2.1 2.1
1922
Dodwell (11)
14
2
1.18 to 2.35
1922
Chant (12)
18
2
1.42 to 2.16
1929
Freundlich (7)
17
1.5
2.24 -+0.10
1936
Mikhailov(18)
25
2
2.71 -+ 0.26
1936
Matukuma (14)
8
4
1.28 to 2.13
1947
Biesbroeck (15a)
51
3,3
2.01 -+0.27
1952
Biesbroeck (15b)
10
2,1
1.70 _4-0.10
2.1 2.07
rmin = Distance of closest star from the centre of the Sun, in Solar Radii. DA = Different analysis. comparable, but less weight can be attached to the 1919 results because fewer stars were measured compared to later attempts. On the second point, it can be claimed that the 1919 e x p e d i t i o n affected subsequent observations in two i m p o r t a n t respects. First of all at least one of the observational techniques used by the British Expeditions, was used by the very successful Lick e x p e d i t i o n of 1922. To quote: "Following the example of the British Expedition of 1919 an intermediate plate was taken at Tahiti with the glass side turned towards the objective. This intermediate plate, being right and left reversed could be placed film to film with any of the eclipse photographs or the night comparison plates so that corresponding star images on the two plates fell close together. ''(1Ua)
The 1919 solar eclipse expedition
209
The other important effect was that the 1919 expedition led to a thorough discussion of possible sources of error and this was important to all subsequent expeditions. The possible sources of error are: 1. 2. 3. 4. 5.
refraction effects in the Earth's atmosphere due to a temperature drop in the Moon's shadow; refraction of light in the Sun's corona; displacement of star images due to a gradient in the background blackening of the negative; distortions introduced b y the shrinking of the emulsion while it was drying; distortions in the optical system due to temperature changes during the eclipse.
A major difficulty with this experiment is that the scales of the two photographs to be compared are unavoidably different. Several astronomers have pointed out that one should not extract the scale factor from the results themselves but one should make an independent measure of this factor. Another source of controversy has been the method of fitting the theoretical curve to the observational results. It is for these reasons that the results of the 1919 eclipse and later expeditions were, in fact, reanalysed b y some authors. The results of these analyses are somewhat larger than that obtained by Dyson et al. ; however they are still comparable with later expeditions. In conclusion one can say that these expeditions yield results that were in good agreement with the General Theory and they were comparable to the results of subsequent expeditions. F r o m the point of view of techniques and discussion they had an important effect on subsequent observations. One can also see that the interest of astronomers at Greenwich in the problems of gravity spans the gradual transition from Newton to Einstein.
REFERENCES 1. 2. 3. 4. 5.
Whittaker, E., A History of Tbeories of tbe Aetber and Electricity. Einstein, A. (1911), Annal. Phys. 35. Einstein, A. (1916), Annal. Phys. 49. Dyson, F.W., Eddington, A.S. and Davidson, C. (1920), Mem. Roy. Astron. Soc. 62, 291. Meadows, A.J. (1975), Greenwicb Observatory, vol. 2, p. 94 (Taylor and Francis). (Letter from C.W. Perfineto F.W. Dyson, 7 February, 1920). 6. Hopmann, J. (1923), Phys. 24, 476. 7. Freundlich, E.F., Klilber, H. v. and Brunn, A. v. (1931), Abbandlungen Preussiscbe Akad. der Wiss., Nr. 1. 8. Danjon, A. (1932), Compt. rend. 194, 252. 9. Mikhailov, A.A. (1956), Astron. J. U.S.S.R. 33,912. 10a. Campbell, W.W. and Trumpler, R. (1923), Lick. Obs. Bull. 11, 41. 10b. Lick. Obs. Bull. 13, 130. 11. Dodwell, G.F. and Davidson, C.R. (1924), M.N.R.A.S. 84, 150. 12. Chant, C.A. and Young, R.K. (1924), Publ. of tbe Dominion Astropbys. Obs., vol. II, 275. 13. Mikhailov, A.A. (1940), Dokl. Akad. Nauk. U.S.S.R. 29, 189. 14. Matukuma, T., Onyki, A., Yosida, S. and Iwana, Y. (1940), Jap. J. Astron. Geopb. 18, 51. 15a. Biesbroeck, G. (1950), Astron. J. 55, 49, 247. 15b. (1953), Astron. J. 58, 87.
33.
Pergamon Press.
Printed in Great Britain
THE 1919 SOLAR ECLIPSE EXPEDITION
P.A.H. SEYMOUR
National Maritime Museum
I would really like to retitle this paper the "Big G" in Greenwich, because the 1919 eclipse expedition represents one of the highlights in a long association between gravity and Greenwich, "Big G" itself being measured b y Maskelyne in 1775 and b y Airy in 1854. However, I would like to start by considering a very minor aspect of this association, namely the perihelion shift of Mercury. Even in Halley's time it was already evident that Mercury had an anomalous motion. Le Verrier was later to show that this anomaly could not be explained by the application of Newtonian mechanics to the solar system as it stood at that time. In 1859, Le Verrier proposed that the perihelion shift could be explained if one supposed that there was an intermercurial planet. Later on this proposed planet was to be given the name Vulcan, and it was with the search for Vulcan that some of the astronomers at Greenwich were associated. Hind in 1872, Airy in 1876 to 1877, and Dyson in 1901 all showed an interest in the search that never produced anything convincing. Other theories to explain this shift consisted of the non-sphericity of the Sun and an extended mass of diffused matter between the Sun and Mercury. The most fruitful class of theories consisted of mathematical modifications to Newton's Law of Gravity. At least part of the shift could be explained by replacing Newton's Law of Gravity using analogies with laws of electrodynamics that were then current. 1 The accepted explanation was, in fact, going to come from a modification of the Law of Gravity and the equation of motion in a gravitational field. However, this was going to stem from a rather different direction. In 1905, Einstein published his Special Theory of Relativity from which he deduced that energy has inertial mass given by the famous equation E = m c 2. In 1911 Einstein postulated the principle of equivalence. This principle proposes that a system of reference in uniform acceleration cannot be distinguished from a homogeneous gravitational field by the performance of any physical experiment. This led to a demonstration that all energy has both gravitational and inertial mass, and the two types of mass are always equal to each other. From this he deduced that " A ray of light going past the Sun would accordingly undergo deflection to the amount of .83 seconds of arc." However, in this paper he did point out that, "The relations here deduced, even if the theoretical foundation is sound, are valid only to a first approximation."2 In 1916 Einstein published the General Theory of Relativity. This incorporated the Principle of Equivalence, which gave physical content to the theory, and the Principle of General Covariance, which was a statement concerning the mathematical form of the laws of physics. This theory required a modification to Newton's field equations and the equations of motion in a gravitational field. When the theory was applied to the motion of Mercury it explained the perihelion shift. Another consequence was that a ray of light passing close to the Sun would be deflected by 1.75 seconds of arc.
3
In 1917 Dyson suggested that the theory could be tested on the occasion of the 29 May, 1919 eclipse. This experiment consists of photographing the star field around the eclipsed Sun and comparing the photographs so obtained with one of the same star field taken about 6 months later. Two British expeditions were sent out; one from Cambridge (led by Eddington) went to Principe off the West Coast of Africa, and another from Greenwich went to Sobral in Brazil. The results were presented at a joint meeting of the Royal Society and the Royal Astronomical Society. The paper by Dyson, Eddington and Davidson has this to say, "Thus the results of the expeditions ... can leave little doubt that a deflection of light takes place in the neighbourhood of the Sun and that it is of
9N~
208
P.A.H. SEYMOUR
the a m o u n t d e m a n d e d by Einstein's generalised t h e o r y of relativity ...,,4 However, the t h e o r y did have its detractors and one a s t r o n o m e r wrote to Dyson saying: "I cannot yet feel that the relativity theory is established or even highly probable. But all the more reason for leaving no stone u n t u r n e d to test its consequences so long as m a n y good minds believe it possible. ''s In any case, such an important result needed further verification, and more expeditions were necessary. There are two points that I would like to consider with regard to subsequent expeditions. Firstly, are these results comparable with later results? Secondly, h o w did these expeditions affect subsequent a t t e m p t s to measure the same effect? The answer to the first e q u a t i o n can be seen in Table 1. The results are TABLE 1. Comparative Results of Various Solar Eclipse Expeditions to Measure Light Deflection. Date
1919
Group & References Dyson et al. (4)
No. of stars
rmin (solar radii)
7 5
2 2
Result (seconds of arc) 1,98 -+0.12 1.61 -+0.30
1919DA
Freundlich (7)
2.0 to 2.2
1919DA
Hopmann (6)
2.16 -+ 0.14
1919DA
Danjon (8)
2.06
1919DA
Mikhailov(9)
1.95 -+ 0.09
1922 1928
Campbell (10a) & Trampler (10b)
92 145
2.1 2.1
1922
Dodwell (11)
14
2
1.18 to 2.35
1922
Chant (12)
18
2
1.42 to 2.16
1929
Freundlich (7)
17
1.5
2.24 -+0.10
1936
Mikhailov(18)
25
2
2.71 -+ 0.26
1936
Matukuma (14)
8
4
1.28 to 2.13
1947
Biesbroeck (15a)
51
3,3
2.01 -+0.27
1952
Biesbroeck (15b)
10
2,1
1.70 _4-0.10
2.1 2.07
rmin = Distance of closest star from the centre of the Sun, in Solar Radii. DA = Different analysis. comparable, but less weight can be attached to the 1919 results because fewer stars were measured compared to later attempts. On the second point, it can be claimed that the 1919 e x p e d i t i o n affected subsequent observations in two i m p o r t a n t respects. First of all at least one of the observational techniques used by the British Expeditions, was used by the very successful Lick e x p e d i t i o n of 1922. To quote: "Following the example of the British Expedition of 1919 an intermediate plate was taken at Tahiti with the glass side turned towards the objective. This intermediate plate, being right and left reversed could be placed film to film with any of the eclipse photographs or the night comparison plates so that corresponding star images on the two plates fell close together. ''(1Ua)
The 1919 solar eclipse expedition
209
The other important effect was that the 1919 expedition led to a thorough discussion of possible sources of error and this was important to all subsequent expeditions. The possible sources of error are: 1. 2. 3. 4. 5.
refraction effects in the Earth's atmosphere due to a temperature drop in the Moon's shadow; refraction of light in the Sun's corona; displacement of star images due to a gradient in the background blackening of the negative; distortions introduced b y the shrinking of the emulsion while it was drying; distortions in the optical system due to temperature changes during the eclipse.
A major difficulty with this experiment is that the scales of the two photographs to be compared are unavoidably different. Several astronomers have pointed out that one should not extract the scale factor from the results themselves but one should make an independent measure of this factor. Another source of controversy has been the method of fitting the theoretical curve to the observational results. It is for these reasons that the results of the 1919 eclipse and later expeditions were, in fact, reanalysed b y some authors. The results of these analyses are somewhat larger than that obtained by Dyson et al. ; however they are still comparable with later expeditions. In conclusion one can say that these expeditions yield results that were in good agreement with the General Theory and they were comparable to the results of subsequent expeditions. F r o m the point of view of techniques and discussion they had an important effect on subsequent observations. One can also see that the interest of astronomers at Greenwich in the problems of gravity spans the gradual transition from Newton to Einstein.
REFERENCES 1. 2. 3. 4. 5.
Whittaker, E., A History of Tbeories of tbe Aetber and Electricity. Einstein, A. (1911), Annal. Phys. 35. Einstein, A. (1916), Annal. Phys. 49. Dyson, F.W., Eddington, A.S. and Davidson, C. (1920), Mem. Roy. Astron. Soc. 62, 291. Meadows, A.J. (1975), Greenwicb Observatory, vol. 2, p. 94 (Taylor and Francis). (Letter from C.W. Perfineto F.W. Dyson, 7 February, 1920). 6. Hopmann, J. (1923), Phys. 24, 476. 7. Freundlich, E.F., Klilber, H. v. and Brunn, A. v. (1931), Abbandlungen Preussiscbe Akad. der Wiss., Nr. 1. 8. Danjon, A. (1932), Compt. rend. 194, 252. 9. Mikhailov, A.A. (1956), Astron. J. U.S.S.R. 33,912. 10a. Campbell, W.W. and Trumpler, R. (1923), Lick. Obs. Bull. 11, 41. 10b. Lick. Obs. Bull. 13, 130. 11. Dodwell, G.F. and Davidson, C.R. (1924), M.N.R.A.S. 84, 150. 12. Chant, C.A. and Young, R.K. (1924), Publ. of tbe Dominion Astropbys. Obs., vol. II, 275. 13. Mikhailov, A.A. (1940), Dokl. Akad. Nauk. U.S.S.R. 29, 189. 14. Matukuma, T., Onyki, A., Yosida, S. and Iwana, Y. (1940), Jap. J. Astron. Geopb. 18, 51. 15a. Biesbroeck, G. (1950), Astron. J. 55, 49, 247. 15b. (1953), Astron. J. 58, 87.