On this day: 20 May

The First Council of Nicaea, the first ecumenical council of the Christian Church, was formally opened by Constantine the Great.

Christopher Lekapenos is crowned Byzantine co-emperor by his father, emperor Romanos I Lekapenos, on the feast of Whitsun.

A major earthquake hits Syria, widely felt between Sicily and Iraq.

King Sancho IV of Castile creates the Estudio de Escuelas de Generales in Alcalá de Henares.

King Mohnyin Thado formally ascends to the throne of Ava.

The Battle of Alfarrobeira is fought, establishing the House of Braganza as a principal royal family of Portugal.

Portuguese explorer Vasco da Gama discovers the sea route to India when he arrives at Kozhikode (previously known as Calicut), India.

Ulysses, who would be famously depicted in a posthumous woodcut, landed in Lisbon, becoming the first of his species to visit Europe in a thousand years.

Hernan Cortés defeats Pánfilo de Narváez, sent by Spain to punish him for insubordination.

Cartographer Abraham Ortelius issues Theatrum Orbis Terrarum, the first modern atlas.

J. S. Bach led the first performance of his Pentecost cantata Erschallet, ihr Lieder at the chapel of Schloss Weimar.

The Mecklenburg Declaration of Independence is allegedly signed in Charlotte, North Carolina.

By the Law of 20 May 1802, Napoleon Bonaparte reinstates slavery in the French colonies, revoking its abolition in the French Revolution.

Napoleon Bonaparte leads his French troops into the Battle of Bautzen in Saxony, Germany, against the combined armies of Russia and Prussia. The battle ends the next day with a French victory.

American Civil War: The state of Kentucky proclaims its neutrality, which will last until September 3 when Confederate forces enter the state.

American Civil War: The State of North Carolina secedes from the Union.

U.S. President Abraham Lincoln signs the Homestead Act into law, opening eighty-four million acres (340,000 km2) of public land to settlers.

American Civil War: Battle of Ware Bottom Church: In the Virginia Bermuda Hundred campaign, 10,000 troops fight in this Confederate victory.

Levi Strauss and Jacob Davis receive a U.S. patent for blue jeans with copper rivets.

Representatives from seventeen countries signed the Metre Convention, which set up an institute for the purpose of coordinating international metrology and for coordinating the development of the metric system.

The Triple Alliance between the German Empire, Austria-Hungary and the Kingdom of Italy is formed.

History of cinema: The first public display of Thomas Edison's prototype kinetoscope.

Cuba gains independence from the United States. Tomás Estrada Palma becomes the country's first President.

Charles Lindbergh takes off for Paris from Roosevelt Field in Long Island, N.Y., aboard the Spirit of St. Louis on the first nonstop solo flight across the Atlantic Ocean, landing 33+1⁄2 hours later.

World War II: German paratroopers began the Battle of Heraklion on the island of Crete, capturing the airfield and port in Heraklion ten days later.

World War II: Battle of Crete: German paratroops invade Crete.

The Luttra Woman (skull pictured), a bog body from the Early Neolithic period, was discovered near Luttra, Sweden.

The first session of the National Diet opened in Tokyo, Japan.

Generalissimo Chiang Kai-shek wins the 1948 Republic of China presidential election and is sworn in as the first President of the Republic of China at Nanjing.

In the United States, the Armed Forces Security Agency, the predecessor to the National Security Agency, is established.

Discovery of the cosmic microwave background radiation by Robert Woodrow Wilson and Arno Penzias.

The Popular Movement of the Revolution political party is established in the Democratic Republic of the Congo.

The Battle of Hamburger Hill in Vietnam ends.

In a referendum in Quebec, the population rejects, with 60% of the vote, a government proposal to move towards independence from Canada.

Radio Martí, part of the Voice of America service, begins broadcasting to Cuba.

The first post-Communist presidential and parliamentary elections are held in Romania.

"One for the Road", the series finale of American television sitcom Cheers, was watched by 42.4 million American households on its original airing.

In deciding Romer v. Evans, the U.S. Supreme Court struck down a constitutional amendment in Colorado that prevented protected status under the law for homosexuals or bisexuals.

The independence of East Timor is recognized by Portugal, formally ending 23 years of Indonesian rule and three years of provisional UN administration (Portugal itself is the former colonizer of East Timor until 1976).

Mamata Banerjee is sworn in as the Chief Minister of West Bengal, the first woman to hold this post.

The International System of Units (SI): The base units are redefined, making the international prototype of the kilogram obsolete.