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1840 (MDCCCXL)was a leap year starting on Wednesday of the Gregorian calendar and a leap year starting on Monday of the Julian calendar, the 1840th year of the Common Era (CE) and Anno Domini (AD) designations, the 840th year of the 2nd millennium, the 40th year of the 19th century, and the 1st year of the 1840s decade. As of the start of 1840, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.
- January 3 – One of the predecessor papers of the Herald Sun of Melbourne, Australia, The Port Phillip Herald, is founded.
- January 10 – Uniform Penny Post is introduced in the United Kingdom.
- January 13 – The steamship Lexington burns and sinks in icy waters, four miles off the coast of Long Island; 139 die, only four survive.
- January 19 – Captain Charles Wilkes' United States Exploring Expedition sights what becomes known as Wilkes Land in the southeast quadrant of Antarctica, claiming it for the United States, and providing evidence that Antarctica is a complete continent.
- January 21 – Jules Dumont d'Urville discovers Adélie Land in Antarctica, claiming it for France.
- January 22 – British colonists reach New Zealand, officially founding the settlement of Wellington.
- February – The Rhodes blood libel is made against the Jews of Rhodes.
- February 5 – The murder of a Capuchin friar and his Greek servant leads to the Damascus affair, a highly publicized case of blood libel against the Jews of Damascus.
- February 6 – The Treaty of Waitangi, granting British sovereignty in New Zealand, is signed.
- February 10 – Queen Victoria of the United Kingdom marries her cousin Prince Albert of Saxe-Coburg and Gotha.
- February 11 – Gaetano Donizetti's opera La fille du régiment premieres in Paris.
- March 1
- March 4 – Alexander S. Wolcott and John Johnson open their Daguerreian Parlor on Broadway (Manhattan), the world's first commercial photography portrait studio.
- March 9 – The Wilmington and Raleigh Railroad is completed, from Wilmington, North Carolina to Weldon, North Carolina. At 161.5 miles (259.9 km), it is the world's longest railroad.
- July 4 – The Cunard Line's 700-ton wooden paddlewheel steamer RMS Britannia departs from Liverpool, bound for Halifax, Nova Scotia, on the first steam transatlantic passenger mail service.
- July 15 – The Austrian Empire, the United Kingdom, the Kingdom of Prussia, and the Russian Empire sign the Convention of London with the Sublime Porte, ruler of the Ottoman Empire.
- July 21 – August Borsig's steam locomotive, the first built in Germany, competes against a Stephenson-built locomotive on the Berlin–Jüterbog railroad; the Borsig locomotive wins by 10 minutes.
- July 23
- August 10 – Fortsas hoax: A number of book collectors gather in Binche, Belgium, to attend a non-existent book auction of the late Count of Fortsas.
- September 10 – Ottoman and British troops bombard Beirut, and land troops on the coast, to pressure Egyptian Muhammad Ali to retreat from the country.
- September 16 – Joseph Strutt hands over the deeds and papers concerning the Derby Arboretum, which is to become England's first public park.
- September 30 – The frigate Belle-Poule arrives in Cherbourg, bringing back the remains of Napoleon from Saint Helena to France. He is buried in Les Invalides.
The frigate Belle-Poule brings back the remains of Napoleon to France.
беларуская (тарашкевіца): 1840
srpskohrvatski / српскохрватски: 1840