1) In 1919, expeditions led by Eddington to Principe and Crommelin to Sobral measured the bending of starlight by the Sun during a solar eclipse to test Einstein's theory of general relativity, which predicted light bending would be twice that of Newton's prediction. 2) The measurements found light bending of 1.61 ± 0.40 seconds of arc in Principe and 1.98 ± 0.16 seconds of arc in Sobral, consistent with Einstein's prediction of 1.74 seconds of arc. 3) This confirmed Einstein's theory of general relativity and made him famous worldwide, revolutionizing our understanding of space, time, gravity and the universe.

1. `Revolution in the Skies’
The Story of the 1919 Eclipse
Expeditions
Peter Coles
Department of Theoretical Physics

4. Newton’s Laws of Motion
• I. Every body continues in a state of rest or
uniform motion in a straight line unless it is
compelled to change that state by forces
impressed upon it.
• II. Rate of change of momentum is
proportional to the impressed force, and is in
the direction in which this force acts
• III. To every action, there is always opposed
an equal reaction.

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. Universal Gravitation
• “the attractive force between any two
bodies depends on the product of the
masses of the bodies and on the square
of the distance between them”
• “the attraction always is in a
duplicate proportion to the Distance
from the Center Reciprocall”

7. 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 Principle of Equivalence
maF  2
r
GMm
F 
Why should these masses be the
same?
Perhaps this Equivalence is because of
a some sort of Principle?

10. The Einstein (R)evolution
• Albert Einstein was born in Ulm on 14 March 1879
• 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.

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

15. Light Bending..a comedy of
errors.
• Newton 1704 “Do not Bodies act upon Light at a
distance, and by their action bend its Rays?”
• 1802 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.

17. 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

18. 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.

20. 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.
• Near the Sun during totality was not just
one star, but a cluster of stars: The
Hyades

22. 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 1916.
• 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.

23. 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

25. 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

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

33. 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”
• In the end Eddington got 1.610.40 seconds,
Crommelin 1.980.16
• After some controversy, Einstein was declared the
winner!

35. 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 remeasured in 1979: 
=1.55 ± 0.34

37. 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.

38. 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?