Talk given at the Irish National Astronomy Meeting in the Armagh Observatory and Planetatrium, September 6th 2019

1. `Einstein, Eddington and
the Eclipse Expeditions
of 1919’
Peter Coles
NUI Maynooth

2. “..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

5. 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.

6. 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

8. 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.

9. 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

11. 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.

12. 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

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

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

22. 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!

23. 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

24. 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.

25. 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?