1) In 1919, two expeditions were sent to observe a solar eclipse in order to test Einstein's theory of general relativity, which predicted light would bend near massive objects like the sun. 2) The expeditions were led by Arthur Eddington and took place in Principe and Sobral, where they measured the positions of stars near the sun during the eclipse and found their results confirmed Einstein's prediction of light bending. 3) This made Einstein famous worldwide and was a landmark achievement that helped establish general relativity as the new theory of gravity, replacing Newton's theory.

1. `Lights all Askew in the
Heavens’
The 1919 Eclipse Expeditions
Peter Coles

5. “..the Supreme God is a Being eternal, infinite,
absolutely perfect…He endures forever and is
everywhere present; and by existing always and
everywhere, he constitutes duration and
space..”
Newton, Principia (General Scholium).
Absolute Space, Absolute Time

6. The Mystery of Mass
• Mass can represent the resistance of a
body to being accelerated (inertial)
• ..or the strength of gravity produced
(active)
• ..or the tendency to feel gravity
(passive)
• Are these masses the same?

8. The Einstein (R)evolution
• Albert Einstein was born in Ulm on 14 March 1879
• In 1894 he dropped out of school
• After failing the entrance exam the previous year, he
entered the Institute of Technology (Zurich) in 1896.
• “Too lazy” to continue as a scientist, he left in 1902 to
become a patent clerk in Bern.
• In 1905, he revolutionized science with the special
theory of relativity. “E=mc2” and all that.
• In 1915, he went even further, with the general
theory of relativity.

9. General Relativity
• The “Principle of Equivalence”
• Acceleration and Gravity
• The curving of space
• ..and the bending of light!

12. Light Bending..a comedy of
errors.
• Newton 1704 “Do not Bodies act upon Light at a
distance, and by their action bend its Rays?”
• 1804 Johann Georg von Soldner, calculates bending
of light by the Sun: 0.87 seconds of arc
• 1907 Einstein thinks about light bending, but then
shelves the idea.
• 1911 Einstein tries again using “E=mc2”; gets
Soldner’s answer: 0.87 seconds of arc.
• 1915 Einstein tries again, and finds a mistake - a
factor of two. The new value is 1.74 seconds of arc.

14. The Factor Two
222222
 drdrdtcds
222
1
2
22
2
2 2
1
2
1 













drdr
rc
GM
dtc
rc
GM
ds
Flat Space:
Schwarzschild:
Energy Momentum

15. Eddington and the Expeditions
• Arthur Stanley Eddington was born in 1882.
• In 1912 becomes Plumian Professor of Astronomy
and Experimental Philosophy at Cambridge.
• Earlier in 1912 Eddington had been involved in an
Eclipse expedition to Argentina. It rained.
• 1916 de Sitter tells him about Einstein’s prediction
and suggests the idea of light bending measurements
during an eclipse.
• 1917, Frank Watson Dyson, the Astronomer Royal
realises the eclipse of 29 May 1919 would be perfect.

17. The Eclipse of 1919
• Date: 29 May 1919
• Path of Totality is across the South
Atlantic from Sobral to Principe
• Duration is long…7 minutes or so at
Principe.
• Behind the Sun during totality was not
just a star, but a cluster of stars: The
Hyades

19. War and Peace
• BUT Eddington was a Quaker, and therefore
a pacifist.
• The First World War had started in 1914, but
conscription was not introduced in the British
Army until 1917.
• Eddington refused to be drafted…
• He was saved by a deal by Dyson, which
protected him on condition he agreed to lead
an expedition in 1919 if the war was over.

20. The Equipment
• Funding: £100 for equipment, £1000 for travel
and labour costs
• Two “astrographic” object glasses, one to
Principe (Oxford), Sobral (Greenwich), both
stopped down to 8 inches.
• A 4 inch telescope taken to Sobral as a
backup
• All were equipped with coelostats
• The two astrographic object glasses were
mounted in stainless steel tubes

22. The Irish Connection
• All the optical equipment was made by Grubb
in Dublin.
• The Oxford astrographic moved to Keele
University in 1962
• The RGO moved to Herstmonceux
• The 4-inch telescope and coelostat are on
display at Dunsink Observatory

29. Some Data Analysis…
DyEfeydx
DxEcbyax
y
x




Coordinates of stars Measured
deflections
Gravitational deflections
at star positions

30. The Results
• Eddington went to Principe, off the coast of (then)
Spanish Guinea
• Crommelin went to Sobral (Northern Brazil).
• Eddington was nearly rained out
“THROUGH CLOUD. HOPEFUL”
• Crommelin was luckier “ECLIPSE SPLENDID”
• After some controversy, Einstein was declared the
winner!

31. The Controversy
• Principe astrographic: 2 “poor” plates. ( =1.62 ±
0.45)
• Sobral astrographic: 18 “poor” plates ( = 0.86 ±
0.48)
• Sobral 4”: 8 “good” plates: ( = 1.98 ± 0.18)
• Eddington included the Principe results, despite not
really getting enough measurements for an
astrometric solution
• The Sobral astrographic suffered from serious optical
problems but plates were re-measured in 1979 with
the result:  =1.55 ± 0.34.

32. Where are the plates?

34. The Aftermath
• This made Einstein more
famous than any scientist
before or since.
• Reconciliation of Britain and
Germany
• What might have been…the
two expeditions of 1912 and
1914 failed to take
measurements when the
prediction was wrong!
• Much better measurements
were made in 1922, and
later using radio
observations.

35. Curiosities
• Einstein became the most famous
scientist ever - it was not 1905, but
1919, that made Einstein a household
name.
• What would have happened if the
measurement had been made when
Einstein had the wrong answer?
• Research on gravity is still a big field!