Most flags belong to a place. The Esperanto flag belongs to a language, which makes it one of the odder items in vexillology: a national-style banner for a nation that does not exist and was never supposed to. Its speakers are scattered across every continent, estimated anywhere from tens of thousands to two million depending on how you count, and the green flag with its white canton and five-pointed star is one of the few things they all share besides the grammar. They call it the Verda Stelo, the green star.
A language in search of a symbol
Esperanto was published in 1887 by L. L. Zamenhof, a Jewish eye doctor from Białystok, then part of the Russian Empire, under the pseudonym Doktoro Esperanto, "Doctor One-Who-Hopes." The pseudonym named the language, and hope became its official colour. Zamenhof's project was practical idealism: a deliberately easy, politically neutral second language that would let people talk across borders without anyone's mother tongue winning.
An invented language has an obvious problem: its speakers cannot find each other. In 1892 the movement's magazine, La Esperantisto, proposed a solution, a green star to be worn as a badge of mutual recognition. Zamenhof later explained where the pieces came from. The green had been suggested by Richard Geoghegan, an English-born Esperantist of Irish descent (green for hope, with Ireland's green as a nudge), and the five-pointed star shape had appeared on the cover of the French Esperantist Louis de Beaufront's grammar. The combination stuck. A small green star on a lapel meant, and still means, "you can speak Esperanto to me."
Boulogne-sur-Mer, 1905
The flag arrived when the movement first met itself in person. In August 1905 the French port town of Boulogne-sur-Mer hosted the Unua Universala Kongreso, the first World Esperanto Congress: 688 participants from around 20 countries, the first large-scale test of whether the language actually worked as living speech between strangers. By every account it did. Zamenhof gave an emotional opening address, and the congress passed the Declaration of Boulogne, which defined the movement as neutral in politics and religion, a definition that has anchored it ever since.
The local Esperanto club had made a flag for the occasion: green field, white canton, green star, the club's own emblem built from the movement's badge. The congress adopted it as the flag of the whole movement. Some accounts hold that the version approved at Boulogne carried a letter E superimposed on the star, a detail that quickly fell away if so, and the plain star has been standard ever since. There was no vote of a state, no gazetted law. A club flag was simply promoted to world flag by the several hundred people to whom it mattered most.
The Universal Esperanto Association, founded in 1908, later set out the conventional proportions: a 3:2 flag, the white canton a square with sides half the flag's height, and the star sized to fit a circle whose radius is roughly a third of the canton's side. Green is usually printed in a strong medium shade, though no single Pantone value has ever been fixed.
Reading the flag
The symbolism is compact. Green is hope, espero, the root of the language's own name. White is peace and neutrality, the movement's founding promises. The star's five points are conventionally the five inhabited continents, Europe, the Americas, Asia, Africa, and Oceania, joined at a single centre. The reading is tidy enough that it is sometimes retrofitted; the star began as a recognition badge, and the continental interpretation grew around it. Esperantists have worn the star far longer than they have argued about it.
The flag has also had real enemies, which is unusual for a language emblem. Esperanto's internationalism made it suspect to both twentieth-century totalitarianisms. Hitler had attacked the language in Mein Kampf as a tool of Jewish world conspiracy, and Esperantists, including Zamenhof's three children, were murdered in the Holocaust. Stalin's purges swept up Soviet Esperantists in the late 1930s as suspected cosmopolitans and spies. Under both regimes, the green star was evidence. The movement rebuilt after 1945, and the World Congress has met nearly every year since, green flags out front.
The star since 1905
The flag's national styling eventually bothered some of the people it represented. A movement defined by neutrality and statelessness was flying something that looked like a state flag, canton and all. For Esperanto's centenary in 1987, the movement gained an alternative emblem, the jubilea simbolo: two mirrored E shapes (often described as a Latin E facing a Cyrillic Э) forming a globe on white. Some organizations use it; others find it corporate and bloodless, and the debate between "verdstelanoj" and modernizers has trundled along harmlessly for decades. The old flag remains the one people actually sew.
You can see the Verda Stelo today at the annual World Esperanto Congress, which draws attendees from dozens of countries, on the covers of the movement's magazines, and on badges at polyglot gatherings. Esperanto never became the world's second language, but it never disappeared either, and its flag has now outlived the empires, and the objections, of the century it was born into. For a banner adopted by a few hundred optimists in a French seaside town, 120 years of continuous use is a respectable run.
References
[1] "Esperanto symbols," Wikipedia. https://en.wikipedia.org/wiki/Esperanto_symbols
[2] "World Esperanto Congress" and "Declaration of Boulogne," Wikipedia. https://en.wikipedia.org/wiki/Declaration_of_Boulogne
[3] Flags of the World (CRW Flags), "Flags of Esperanto." https://www.crwflags.com/fotw/flags/qy-eo.html
[4] Transparent Language Esperanto Blog, "Merits and Myths Behind Some Esperanto Symbols: The Esperanto Flag." https://blogs.transparent.com/esperanto/merits-and-myths-behind-some-esperanto-symbols-the-esperanto-flag/
[5] "L. L. Zamenhof," Wikipedia, on the creation of Esperanto and the fate of Zamenhof's family. https://en.wikipedia.org/wiki/L._L._Zamenhof
[6] Universala Esperanto-Asocio (UEA), on flag conventions and the 1987 jubilee symbol. https://uea.org