They Wrote Down Three Answers Before They Left

The listener is a child; the reader is a parent. This page is here so you can answer the questions your child asks after the story. The script is available during playback via the script button.

What this is about

The 1919 eclipse expedition is usually told like this — "a British scientist proved a German scientist's theory."

This piece does not use that summary. It starts instead from the opening paragraph of the paper they wrote.

There, alongside the purpose of the expedition, are three answers set out in advance: nothing at all, or 0.87 seconds of arc, or 1.75. And the last two differ by exactly a factor of two.

That is the heart of this piece. Deciding what would mean what, before you look.

And the man named in the third answer has a story of his own that is not the one we are told. Einstein published a wrong theory first, worried that someone would reach the answer before him, and the first expedition sent to measure his prediction was stopped by a war.

It is where the three parts of this set lock together — Galileo gave away his numbers so others could check, Darwin wrote in words others would know, and here they fix the rule before measuring.

What actually happened

• The 1919 eclipse expedition went to Sobral in Brazil and to the island of Príncipe in the Gulf of Guinea, led by Dyson, Eddington and Davidson (PMID 25750149).
• The opening of their paper states the purpose: "The purpose of the expeditions was to determine what effect, if any, is produced by a gravitational field on the path of a ray of light traversing it." (openalex:W1973137999)
• It then sets out the three alternatives it was especially desired to discriminate between (openalex:W1973137999):
  1. The path is uninfluenced by gravitation.
  2. The energy or mass of light is subject to gravitation in the same way as ordinary matter. If the law of gravitation is strictly Newtonian, this leads to an apparent displacement of a star close to the Sun's limb amounting to 0″·87 outwards.
  3. The course of a ray of light is in accordance with Einstein's generalised relativity theory. This leads to an apparent displacement at the limb of 1″·75 outwards.
• And it adds: in either of the last two cases the displacement is inversely proportional to the star's distance from the Sun's centre, with the displacement under (3) being just double that under (2) (openalex:W1973137999).
• Einstein's own side of the story is not the one we are told. He arrived in Berlin in March 1914 and separated from his wife Mileva that year. More to the point, he realised that his earlier Entwurf theory was incorrect and embarked on exhaustive work to replace it — and while he did, he was worried that he would be beaten to the correct theory by Hilbert. In a series of presentations in November 1915 he showed that the new theory could successfully explain the orbit of Mercury. General relativity was born (DOI: 10.1093/oso/9780198794875.003.0012).
• 1919 was not the first attempt. The outbreak of the First World War stranded Freundlich in Russia as he waited for a solar eclipse (DOI: 10.1093/oso/9780198794875.003.0012). The 1919 expedition was doing, years later, what a war had stopped.
• The British were not the only ones there. At Sobral — the second city of the state of Ceará — Brazilian, American and British teams were to observe totality, and they had different observational agendas. The British team's test of Einstein's prediction was one among them (DOI: 10.1177/00732753261419603).
• Other people wanted other things. The Brazilian scientific community used the occasion to advance astrophysical knowledge about the Sun and to establish its scientific credentials internationally; the political elite seized the chance to project an image of Brazil's modernity abroad; and a large audience turned to newspapers to engage with the eclipse (DOI: 10.1177/00732753261419603).
• The expedition succeeded. And it became a turning point in the history of relativity not only because of its importance as a test of Einstein's theory, but also because of the intense public interest which was aroused by its success (PMID 25750149).
• What was checked then is an instrument now. The gravitational bending of electromagnetic waves by massive bodies is of the greatest importance for contemporary and future astronomy, astrophysics and cosmology — a key tool of modern observational astronomy (PMID 25750149).

Questions your child may ask

"Why write the answers down first?" So that you cannot say "I was right all along" no matter what comes out. Write it first and the result speaks for itself — 0.87 means one thing, 1.75 means another, and you decided before measuring. For a child: "Once you write it down first, you cannot change your words afterwards."

"Why does being double matter?" Because if two answers are close, you cannot tell them apart. Had it been 0.87 versus 0.88, both would sit inside the measuring error. A factor of two survives ordinary error. An experiment is only an experiment when the answers are far enough apart.

"How small is 1.75 seconds?" Here "seconds" means an angle, not time. A very small angle — a star appearing shifted just a touch to one side. They sailed to the other side of the world to measure a difference that small.

"Why only during an eclipse?" Because the Sun is too bright for the stars beside it to be seen. Only in the few minutes when the Moon covers it do those stars appear. They travelled for months for a few minutes.

"Did the British do all of it?" No. Brazilian, American and British teams were at Sobral, each wanting different things (DOI: 10.1177/00732753261419603). This piece asks not "who did it" but "what was layered on that one spot."

Ideas inside the story

• A rule fixed in advance — writing down "what would mean what" before measuring. This is the centre of the piece.
• A difference you can resolve — two answers must be far enough apart for observation to choose between them.
• Overlapping purposes — science, politics, the press and the public each had their own reason for the same event.

What is evidence and what is story

What the sources state explicitly — the 1919 Sobral and Príncipe expedition led by Dyson, Eddington and Davidson; the stated purpose and the three alternatives (0″·87 / 1″·75 / exactly double / inversely proportional to distance); the Brazilian, American and British teams at Sobral with differing agendas; the aims of Brazil's scientific community, political elite and public; the expedition's success and the intense public interest it aroused; and that gravitational light bending is now a key tool of observational astronomy.

Who Eddington was — the Royal Society obituary describes him as among the first to teach the unity of Nature working on the cosmic and the atomic scale, a view he preserved with characteristic courage. The same obituary frames the era by the discovery of X-rays in 1895 and the quantum theory of 1901 (DOI: 10.1098/rsbm.1945.0007) — the story uses that to link back to our own X-ray piece.

• Which of the three answers it was — the third. Eddington announced the successful measurement of the gravitational light bending predicted by Einstein's general theory of relativity (DOI: 10.36253/978-88-5518-467-0.34). The number itself is not given (see below).
• Einstein became world-famous overnight. "Albert Einstein abruptly rose to worldwide fame in November 1919" (same DOI). His initial fame came in late 1919; he was in Berlin, and afterwards became a symbol for international reconciliation during the early Weimar Republic (DOI: 10.1017/s026988971200004x). The closing of the story rests on that — "Not who said it. Whether it was true."

What this piece deliberately avoided

• ⚠️ We did not state the values they actually measured. The abstract of the original paper we read gives only the purpose and the three predictions, not the results. Our evidence says "it succeeded" and no more, so the story says no more. Not writing what we happen to know but cannot source is the rule of this line.
• The controversy over Eddington allegedly discarding plates that disfavoured him. We know work exists that addresses this head-on, but we could not obtain an abstract, so we could not build on it. It would be more honest to treat it once we can.
• ⚠️ We did not explain what general relativity says. Our three core sources name the theory but never state its content. The children's script says so outright — "Let me be honest with you. I cannot explain it properly today." A physics-education study reports that GR and cosmology remain largely inaccessible even to high-school physics teaching because of the conceptual and mathematical prerequisites (DOI: 10.1103/ss9c-lkpj). Not pretending to know beats inventing an analogy.

One more note for parents — the summary "a British scientist proved a German one" erases two things: the other nations' teams who were there, and the fact that the expedition's fame did not come from the test alone. The source places the importance of the test and the public interest side by side as reasons it was a turning point (PMID 25750149). Being confirmed and becoming famous are different events.

Sources and what followed

Core sources

• Arthur Stanley Eddington, 1882-1944 (Royal Society obituary). DOI: 10.1098/rsbm.1945.0007

• Traveling towards fame: Einstein and the Eddington eclipse expedition. DOI: 10.36253/978-88-5518-467-0.34

• Einstein and Relativity: What Price Fame? (Berlin; symbol of reconciliation). DOI: 10.1017/s026988971200004x

• A determination of the deflection of light by the sun's gravitational field (Dyson, Eddington & Davidson, 1920). openalex:W1973137999

• Bending space-time: a commentary on Dyson, Eddington and Davidson (1920). PMID 25750149

• Eclipse on paper: The 1919 total solar eclipse in Brazilian newspapers. DOI: 10.1177/00732753261419603

• Berlin, war, marriage breakup, and general relativity completed (1914–1915). DOI: 10.1093/oso/9780198794875.003.0012

• Cosmology and general relativity in upper secondary school through new targeted teaching materials. DOI: 10.1103/ss9c-lkpj

What followed

• What they measured then is today's telescope. That heavy bodies bend light is no longer something to test but something to use — the gravity in front is used as a lens to see what lies far behind (PMID 25750149).
• The eclipse was an event in Brazil too. Newspapers in Rio de Janeiro, Belém do Pará, Camocim and Sobral covered it in different registers, and that difference itself became a subject of study (DOI: 10.1177/00732753261419603).

Things to try together

• Write first, then measure. Before measuring anything, have your child write "if it comes out this much, then A; that much, then B." A bean sprout's height, the number of steps to school — anything. Writing before measuring makes it impossible to revise the claim afterwards, and they feel that themselves.
• Make it double. Talk about why two predictions that are too close cannot be told apart. Comparing a one-millimetre difference with a twofold one on a ruler makes it obvious.
• Count who was there. This story has teams from at least three countries, plus newspapers and the public. It is practice in counting how many people were actually present at an event summarised as one person's achievement — the same question as the two pieces before it.