The 1919 solar eclipse expedition is a famous test of Einstein's general relativity theory. It is a story with many aspects: physics, astronomical measurements, scientific method, science and World War I.

1. The 1919 Eclipse and Relativity
A story with many aspects
Dennis Miller
25.11.2019
Presented to the Arbeitskreis Philosophie Kelkheim. Originally in German as Naturwissenschaften und
Gesellschaft: Sonnenfinsternis 1919 und die Relativitätstheorie.

2. The 1919 Solar Eclipse Expedition
Aspects of the story
Or as an adventure story:
100 years ago, immediately after the first world war, it was very difficult to undertake such an
expedition. There were many risks - logistics, problems with apparatus, weather. Considering how
many things could go wrong success seems unlikely. Nevertheless, the heros finally triumph with a
spectacular result.
Theoretical physics
space, time, light, gravity
Astronomical measurements
very difficult: small effects, many uncertainties
Science & society
research landscape, political & international aspects
reaction of media and general public
Scientific method
nature of scientific theories and how are they confirmed ?
data analysis
Physics
Philosophy,
socienty

3. Relativity Theory - Overview
Special Relativity Theory
- Speed of light in free space is always the same. Does not depend on speed of source or observer.
- The laws of physics are the same for all observers moving with constant speed.
→ At very high speeds length and time different for different observers: surprising consequences that
contradict everyday experience!
→ Mathematical combinations of space and time that are the same for all observers (invariants).
Equivalence of gravity and acceleration
- This is not a new idea, but Einstein brought it into focus and discussed the consequences
General Relativity Theory:
- Extension of previous theory to include acceleration
- Gravity: geometrical phenomenon in curved space-time
- Difficult mathematics

4. Three tests of general relativity
Einstein's suggestion
● Light deflection by sun's gravitational field
● Solar red shift
Spectral lines shifted to longer wavelenths by sun's gravity
● Perihelion of mercury
The orbit of mercury shows small deviations from predictions of Newton's theory
(unexplained precession 43″ per century). General relativity gives quantitative
explanation.

5. Light deflection by gravity
The deflection is very small and decreases with the distance of the light ray from the sun. Stars near the sun cannot
normally be seen except during a total eclipse. Einstein wrote to George Hale, a leading American astronomer, to ask if
there might be some other method to observe the deflection. His answer was no: the only possibility would be to take
accurate photographs during an eclipse.
Earth
Sun
True position
Apparent position

6. Special
Relativity
General
Relativity Eclipse
Results
announced
a = 0.87" a = 1.75"
Public interest in Relativity
Einstein becomes famous
WWI
a is the deflection angle for a star near the edge of the sun. There are two theoretical values:
in 1915 Einstein saw that General Relativity indicates the angle should be twice as large as
his original prediction made in 1911.
The predictions for a are the ones quoted by Eddington. Einstein originally gave a slightly lower value, 0.83", for the
simple theory.
Theory
Timeline Relativity Theory

7. Light deflection: three theoretical possibilities
Value of a
1.75″ Einstein. General Relativity.
0.87″ Newton. This is Einstein's first prediction. It is compatible with Newton's theory together
with the gravity / acceleration equivalence principle. This value was derived at the beginning
of the 19th century, but the work had been forgotten and Einstein was not aware of it until
about 1922.
0″ No deflection. At the time most most scientists did not expect gravity to deflect light rays.
Theoretical physicists, however, increasingly saw problems with this simple idea.

8. Einstein and Eddington both were both pacifists. They
belonged to minority religious groups (Eintstein Jewish,
Eddington Quaker).
Photos: Wikipedia
Eddington and Dyson knew eachother well. Both had
been very successful in mathematics as Cambridge
undergraduates. Dyson would have heard about Relativity
from Eddington.

9. Solar Eclipse 29 May 1919
Sobral
Principe
A good opportunity because:
- Bright stars near the sun
- Long totality (about 5 min at observation stations)
Stations selected
- Sobral, Brazil
- Principe, a volcanic island,
in 1919 Portuguese colony

10. 1912: Argentinian expedition to Brazil.
Rain – no observations
1914: German expedition: Eclipse in south Russla
Outbreak of WWI – no observations
The astronomers were interned but returned to Germany as part
of a prisoner exchange after a few weeks.
1918: Eclipse in USA.
Problems with equipment
In spite of considerable effort, the data evaluation did not give
a reliable conclusion. No publication in a scientific journal.
Previous attempts to measure light deflection during an eclipse
- all unsuccessful -

11. Planning the eclipse expedition
* Meetings of Joint Permanent Eclipse Committee
Britain had the advantage of a permanent committee to organise eclipse expeditions. In other countries observatories
decided individually whether to observe eclipses. The committee decided to start planning for the 1919 eclipse in spite
of the war. In Nov. 1917 nobody knew how long the war would last or how it would end. Planning in wartime was
difficult, in particular as many of the technical staff were serving in the armed forces.
The expedition was considered a matter of national importance: a counterweight to Germany's strong position in
theoretical physics.
During the war there was no direct communication between British and German scientists. However, both Einstein and
Eddington had contacts to colleagues in neutral Holland.

12. Dept.
Liverpool
Arr.
Madeira
Arr.
Brazil
Arr.
England
Dept.
Brazil
Comparison
Plates
Madeira
Arr.
Principe
Dept.
Principe
Arr.
England
Eclipse
Feb. Mar. Apr. May Jun. Jul. Aug.
Sobral Team
Principe Team
1919
Expedition timeline

13. Results: timeline
12.09.1919 Bournemouth meeting of British Association for Advancement of Science:
Eddington and Cottingham show photo with protuberance.
Brief, preliminary report: light deflection has been measured.
Sept 1919 Einstein receives news via H. A. Lorentz (NL) that results confirm general
relativity.
06.11.1919 Official presentation of results at meeting of Joint Eclipse Committee.
Jan 1920 The November presentation published as a scientific paper.

14. How large is the light deflection ?
There are many pictures like this in
the internet. The angle is drawn larger
so that it can been seen properly. We're
not talking about 10 or 20 times larger:
the angle is about 30.000 times smaller
than shown!
Earth
Sun
True position
Apparent position

15. Apparent sizes: Moon and Sun approx. 30’
Stars approx. 3-4” (due to atmospheric turbulence)
--> Very small displacements: various small effects must be considered
- Stellar aberration: seasonal effect due to movement of the Earth
- Differential refraction: refraction by Earth’s atmosphere depends
on height above the horizon.
- Small changes of magnification (e.g. from temperature changes)
Angular units
1° = 60‘ = 3600“
Displacement smaller than
diameter of star on the
photo
Photo of stars
during eclipse
Photo of stars
at night
Precise measurement
of positions
Mathematical Evaluation
- Relativity effects
- Scale effects
Relativity: Deflection decreases with distance from Sun
Scale: Increase in magnification gives deflection that
increases with distance from the middle of the plate.
Evaluation
Additionally small effects due to inexact alignment of eclipse
and comparison plates are also be taken into account.

16. Principe: 1.61″ ± 0.30
Sobral: 1.98″ ± 0.12
Theory: 3 possibilities
1.75″ Einstein
0.87″ “Newton”
0″ no deflection
Deflection angle at sun's rim
Results of eclipse expedition: Nov. 1919 presentation
The presentation was published in Philosophical Transactions of the Royal Society a few weeks later.

17. Public reaction
The confirmation of Einstein's theory was a sensation - not just for the scientific world but for the general public too.
Numerous articles appeared in newspapers and magazines.
“Lights all Askew in the Heavens”
“Einstein Theory Triumphs”
“Revolution in Science”
“Newtonian Ideas Overthrown”
“A patriot fiddler-composer of Luton
Wrote a funeral march which he played with the mute on,
To record, as he said, that a Jewish-Swiss-Teuton
Had partially scrapped the Principia of Newton.”
New York Times
10.11.1919
The Times
07.11.1919
Punch
19.11.1919
Vossische Zeitung
18.11.1919
“Einstein und Newton”
Vossische Zeitung
30.11.1919
“Zum Siege der Relativitätstheorie”
(by Erwin Freundlich)
USA - Sensationalist headlines
Britain - Einstein versus Newton
Germany - Factual reports.
E. Freundlich complains of
too little public interest.

18. Why did Einstein and the theory of relativity became so
famous outside the arcane world of theoretical physics?
- Theoretical physics is difficult to understand.
- Little practical application. Quantum theory, developed about the same time, was technologically
more important, but aroused less public interest.
- No conflict with religion (unlike theory of evolution).
- Confirmation of the theory as dramatic event (most scientific theories are confirmed by the
accumulation of results).
- Aftermath of war: the public wanted something that captured the imagination.
- New ideas on space and time fitted revolutionary developments in the arts, such as cubism.
- Popular accounts of the theory did not use technical terms but strange and intriguing combinations of
familiar words: “curved space”, “fourth dimension”.
- Surprisingly, the difficult mathematics of general relativity. According to American newspapers only
twelve men understood the theory – an exotic elite group that aroused public interest.
Surprising at
first sight
Possible
explanations

19. Did Dyson and Eddington really show Einstein was right?
The positive reaction in the press was based on the conclusions as presented at the November 1919
meeting. But how certain were the results really? Many modern accounts are critical. Kragh, for
example, writes:
This was, in fact, a too-optimistic conclusion that could be obtained only by a treatment of the
available data that came close to manipulation, including rejection of data that did not agree with
Einstein’s prediction.*
The next slides show the background to this critical view.
* In Quantum Generations, his history of 20th century physics

20. Results for the three telescopes
Include in final
result ?
Sobral Astrographic
Poor quality measurements (focus problems). Result uncertain but no
quantitative error bar possible.
No
Sobral 4 Inch Satisfactory. Yes
Principe Astrographic
Less data than hoped for (some cloud during eclipse). Evaluation
procedure adjusted.
Yes

21. Criticism of the evaluation and conclusions
Criticism of Eddington and Dyson's conclusions
- The evaluation was insufficiently objective; a method that gave the desired result was preferred (before the
expedition Eddington was convinced that General Relativity was correct). Rejection of data not supporting this
conclusion (Sobral astrographic) is questionable.
- The result was accepted as a definite proof of Einstein's theory because of a good publicity campaign. Objectively,
this conclusion was uncertain.
- Eddington was considered an expert on general relativity. As a result his conclusions were accepted even when
supported by weak arguments.
Kennefick defends Eddington und Dyson
- Objective reasons for mistrusting the results of the Sobral astrographic telescope.
- Dyson led the evaluation of the Sobral results. Unlike Eddington, he was not a firm supporter of General Relativity
before the expedition.
 Nevertheless, the confirmation of Einstein's theory was less certain than claimed at the time.
In spite of popular acclamation, not all of Eddington's contempories were convinced. This criticism was revived
decades later when the eclipse expedition was studied from a historical and philosophical perspective. Earman and
Glymour's 1980 paper is considered a key contribution to this discussion.

22. Scientific Method
The criticism of Eddington and Dyson's presentation of their results raises a number of general points:
- When should data be rejected?
This is generally a difficult question. It must usually be decided on a case-by-case basis,
considering details of the data. Rejection of data should be transparent: reports should
state which data is rejected and why. Alternative evaluations including this data are also
desirable.
- How flexible should the evaluation method be?
There may be a variety of ways of evaluating the measurements, especially with complex
data sets. A flexible approach allows one to choose the most satisfactory method. On the
other hand, this could result in choosing the method that gives the best agreement with a
preferred conclusion.
Modern computer methods are a great advantage in comparing different evaluations. In
particular, a critical look at several statistical parameters and graphical presentations helps
select an appropriate evaluation.

23. Einstein's 3 tests of general relativity
● Light deflection by sun's gravitational field.
Prediction. Confirmation of theory by observation of a single event (eclipse).
● Solar red shift
Shift of spectral lines is due to a superposition of two phenomena:
- general redshift (relativity).
- Doppler effect caused by the movement of gas in the sun.
Relativity effects are found only by looking at all the data rather than individual
spectral shifts. Confirmation of theory by gradually building up a consistent picture.
● Perihelion of mercury
The orbit of mercury showed small deviations from predictions of Newton's theory
(first observed 1859). Attempts to explain this with classical mechanics had not been
successful.
Like the light deflection, this is a very small effect (unexplained precession 43″ per
century). General relativity gave a quantitative explanation.
Explanation of observed effect, not a prediction.

24. Conclusion
The 1919 Eclipse expedition to test Einstein's theory is a very interesting
piece of scientific research. It is a well-known topic in the History and
Philosophy of Science.
This expedition has many aspects: scientific, philosopical and historical.
It is a fascinating story, but the drama is not typical for the way science
works.
However, it provides important insights into scientific method and the
interaction of science and society.

25. Epilogue -1: later eclipse work
1979: New Analysis of the Sobral measurements with improved technology
Astrographic: a =1.55″ ± 0.34
4 Inch: a = 1.90″ ± 0.11
(Theor. 1.75″)
1973: Last professional solar eclipse expedition to measure light deflection by gravity
Univ. Texas; Mauritania
a =1.66 ″ ± 0.19
2017: Best results for light deflection
Amateur astronomer; USA
a =1.752″ ± 0.060 (Theor. 1.751″)
Retired physicist D. G. Bruns used modern technology. All the the equipment could be transported in a private
car. The setup was programmed to take a large number of photographs during the 2½ minutes of totality. Data
analysis indicated a 3.4 % uncertainty, but his result was in fact much closer to the theoretical value.

26. Epilogue -2
In 1913 Einstein asked American astronomer George Hale if there was a trick to measure the light
deflection in daylight. His answer was no – it could only be done in a total eclipse.
Today the answer is yes – use radio astronomy.
- Radio waves, like light, are electromagnetic waves: the same physical laws apply.
- The sun is a weak radio source, so strong sources can be seen even when they are nearby.
- Positions of radio sources can be measured very accurately.
→ The agreement with Einstein's theory is better than 99.9%

27. Literature
F.W. Dyson, A.S. Eddington, C. Davidson, A Determination of the deflection of light by the Sun’s Gravitational
Field from Observations made at the Total Eclipse of May 29, 1919, Phil. Trans. R. Soc. A, 220, 291-332 (1920)
J. Earman, C. Glymour, Relativity and Eclipses: The British Eclipse Expeditions of 1919 and Their Predecessors,
Historical Studies in the Physical Sciences, 11, 49-85 (1980)
M. Longair, Bending space–time: a commentary on Dyson, Eddington and Davidson (1920) ‘A determination of
the deflection of light by the Sun’s gravitational field’, Phil. Trans. R. Soc. A, 373: 20140287 (2015)
D. Kennefick, No Shadow of a Doubt, Princeton Univ. Press (2019)
D. Soares, The 1919 Eddington Eclipse, DOI: 10.13140/RG.2.2.33288.88321 *
R. Vaas, Der krumme Einstein-Beweis, Bild der Wissenschaft, 34-41 (Dec. 2019)
R. Vaas, Licht auf krummen Touren, Bild der Wissenschaft, 14-29 (Nov. 2019)
This is a small selection of the numerous publications on the 1919 eclipse expedition. Daniel Kennefick's
book was my main source of information.
* Preprint version available on Researchgate.

28. Thank you !
© Dennis Miller 2019