Príncipe

The test was simple to state. Light from a star passing close to the Sun should be bent by its gravity. Newtonian reasoning predicted a deflection, at the Sun’s edge, of about 0.87 seconds of arc; Einstein’s completed theory predicted 1.75, twice as much. The only time stars can be photographed beside the Sun is during a total eclipse, and the eclipse of 29 May 1919 had the unusual advantage of placing the Sun in front of the bright Hyades cluster.

The Príncipe account, as published by Dyson, Eddington and Davidson in the Philosophical Transactions of the Royal Society in 1920 and summarised by Malcolm Longair for the Royal Society in 2015, is candid. Totality ran from 2h 13m 5s to 2h 18m 7s Greenwich time. The crescent Sun was seen continuously through cloud only from about 1.55. Conditions improved in the last third of the eclipse, a few suitable stars appeared on a few plates, and most weight in the reduction went on two of them. The Príncipe deflection came out at 1.61 seconds of arc, with an uncertainty of about 0.3.

Eddington had meant to measure all the plates on the island, but a strike by the steamship company meant leaving early, and most of the work was done back in Cambridge. He did measure one before he left. His notebook for 3 June records that it gave a result agreeing with Einstein.

At Sobral, the weather was better. The small four-inch telescope gave 1.98 seconds of arc, with an uncertainty of 0.12; the main instrument’s images were out of focus, its result depended on how the analysis handled that, and the authors set it aside as unreliable. The combined results were announced at a joint meeting of the Royal Society and the Royal Astronomical Society on 6 November 1919, and the newspapers did the rest.

The question has been put seriously, and it deserves a straight account rather than a verdict.

From 1980, when the philosophers John Earman and Clark Glymour published a critical account of the analysis, historians and sociologists of science asked whether Eddington, already persuaded of relativity, had discarded the inconvenient Sobral data to get the answer he wanted. Their account was used heavily in Harry Collins and Trevor Pinch’s much more widely read book The Golem, which, as the historian Daniel Kennefick notes, stops short of claiming that any data was fudged.

Two things answered it. For the Einstein centenary of 1979 the Sobral plates were remeasured at the Royal Greenwich Observatory with a modern machine, and the reanalysis, published by G. M. Harvey in 1979, supported the original conclusion. And the historian Daniel Kennefick, in Physics Today in 2009, examined the decisions made in 1919 and concluded that the team’s treatment of the data was defensible and, if anything, less biased than some later eclipse analyses.

What no one disputes is that the 1919 measurements were thin. Eclipse measurements in visible light stayed contested for decades; Kennefick notes that only radio telescopes, timing quasars as the Sun passed in front of them with no eclipse needed, confirmed Einstein’s value precisely. Príncipe’s place in the story is secure; the idea that it proved anything on its own is not.

Cocoa came to the islands from Brazil in 1819, and the first tree to reach maturity is recorded on Príncipe, in 1824 by one account and 1822 by another. By 1905 São Tomé and Príncipe had become the world’s largest producer and exporter of cocoa, and it kept that position until the British Gold Coast, now Ghana, overtook it in 1911.

The plantations, called roças, ran on imported labour. Portugal had abolished slavery in its colonies, and the estates recruited serviçais — contract workers — from Angola, Mozambique and Cape Verde; an estimated 67,000 were brought in between 1888 and 1908. In 1905 the British journalist Henry Nevinson published the first of a series of articles, collected in 1906 as A Modern Slavery, tracing people from the Angolan interior to the island plantations. A British Foreign Office report of 1906 concluded that slaves had been renamed serviçais. Portugal denied it.

The British chocolate makers who bought most of the islands’ cocoa — Cadbury, Fry’s and Rowntree’s — held out for years, and Cadbury began its boycott on 9 March 1909. Pests, disease and the loss of buyers then did the rest, and after independence in 1975 most of the great estates declined or were abandoned. Many of their buildings are now standing ruins in secondary forest.

Sundy was once the second-largest roça on Príncipe. It sits in the north-west of the island, above the sea, and it is where Eddington and Cottingham set up their telescope.

In 2019, the centenary year, the agroforestry and tourism company HBD Príncipe built a chocolate factory on the old estate, and the owner’s house was converted into a hotel. The long rows of workers’ buildings, some roofless, stand around the old square behind it.

It is possible to visit Sundy for the eclipse and never think about the rows. It is better not to.

The southern half of Príncipe is a different island: a massif of steep phonolite peaks between 500 and 948 metres, covered in forest that includes the island’s patch of primary rainforest. It has been protected since 2006 as the Obô Natural Park and is uninhabited.

Príncipe has a list of birds found nowhere else, among them the Príncipe kingfisher, the Príncipe sunbird, the Príncipe starling, Dohrn’s thrush-babbler and the Príncipe scops owl. UNESCO’s biosphere description records high rates of endemism in plants, molluscs, insects, reptiles and bats as well, and notes the island’s importance for nesting sea turtles, seabirds and whales.

The biosphere designation, in 2012, covered the whole island and its sea. In 2025 it was extended to São Tomé, which made São Tomé and Príncipe the first country designated a biosphere reserve in its entirety.