Talk given at "Origins of the Expanding Universe" in honour of Vesto M. Slipher, 13th-15th September 2012, Flagstaff Arizona

1. Einstein, Eddington
and the 1919 Eclipse
Expeditions
Peter Coles
(Cardiff University)
http://telescoper.wordpress.com

4. Absolute Space, Absolute Time
“..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).

6. General Relativity
• Published in 1915
• The “Principle of Equivalence”
• Acceleration and Gravity
• The curving of space
• ..and the bending of light!

9. α is the angular
deflection at grazing
incidence
α=4GM/ Rc 2

10. Light Bending..a comedy of errors.
• Newton “Do not Bodies act upon Light at a distance,
and by their action bend its Rays?”
• 1801 Johann Georg von Soldner, calculates bending
of light by the Sun: α = 0.87 seconds of arc: the
“Newtonian” Prediction
• 1907 Einstein thinks about light bending, but then
shelves the idea.
• 1911 Einstein tries again using “E=mc2”; gets
Soldner’s answer: α = 0.87 seconds of arc.
• 1915 Einstein tries again, and finds a mistake, a
factor of two. The new value is α = 1.74 seconds of
arc.

12. The Factor Two
Flat Space:
2 2 2 2 2 2
ds =c dt −dr −r d Ω
Schwarzschild:
−1
2GM 2 2 2GM
2
( )
ds = 1− 2 c dt − 1− 2
rc rc ( ) 2 2
dr −r d Ω 2
Energy Momentum

13. Eddington and the Expeditions
• 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.

15. 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 (max for a total eclipse is ~ 9 mins)
• Behind the Sun during totality was a cluster
of stars, The Hyades.

18. 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.

19. 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

21. The Measurements
• Eddington went to Principe (off the coast of
Spanish Guinea)
• Crommelin went to Sobral (Northern Brazil).
• Eddington was nearly rained out
“THROUGH CLOUD. HOPEFUL”
• Crommelin was luckier “ECLIPSE
SPLENDID”
• The results were presented as supporting
Einstein
• But it wasn’t quite as simple as that..

23. The Paper
• Dyson, Eddington and 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
• Philosophical Transactions of the Royal
Society of London. Series A, Containing
Papers of a Mathematical or Physical
Character, Volume 220, pp. 291-333
• 81 citations on ADS!

24. Some Data Analysis…
ax +by+c+αE x =Dx
dx+ey+f +αE y= Dy
Coordinates of stars Measured
deflections
Gravitational deflections
at star positions

25. The Controversy
• Principe astrographic: 2 “poor” plates. (α=1.62 ±
0.45)
• Sobral astrographic: 18 “poor” plates (α= 0.86 ± 0.48)
• Sobral 4”: 8 “good” plates: (α=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 remeasurement in the 1970s
gave results consistent with the Einstein value.

28. 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.