1800

1800 (MDCCC) was an exceptional common year starting on Wednesday in the Gregorian calendar and a leap year starting on Sunday in the Julian calendar, the 1800th year of the Common Era (CE) and Anno Domini (AD) designations, .  Because of a leap day in the Julian calendar, the Gregorian calendar moved ahead by 12 days.

 Events 
 World population hits 1 billion.
 March 14 – Cardinal Barnaba Chiaramonti is elected pope as Pius VII.
 March 21 – Pius VII is ordained.
 April 24 – U.S. Library of Congress founded.
 May 5 – Great Britain passes the Act of Union to join Great Britain and Ireland into the United Kingdom of Great Britain and Ireland to take effect on 1 January 1801. The act is signed by King George III in August.
 May 15 – Napoleon Bonaparte crosses the Alps and invades Italy.
 June 14 – Battle of Marengo, Napoleon defeats the Austrian troops near Marengo, Italy.
 June 2 – First smallpox vaccination in North America, at Trinity, Newfoundland.
 June 27 – Pascha Jussuf Karamanli of Tripoli declares war on Sweden by having the flagpole on the consulate chopped down.
 September 5 – The island of Malta, that was occupied by the French, is conquered by British troops.
 November 1 – U.S. President John Adams becomes the first President of the United States to live in the Executive Mansion (later renamed the White House).
 November 17 – The U.S. Congress holds its first Washington, DC session.
 December 3 – Battle of Hohenlinden, the French army defeats the Austrian troops.
 December 24 – An assault on Napoleon Bonaparte fails in Paris.
 December 24 – Pierre Coudrin and Henriette Aymer de la Chevalerie found the Congregation of the Sacred Hearts of Jesus and Mary in Paris.
 Invention of the voltaic pile by Alessandro Volta: the first chemical battery
 The infrared radiation is discovered by Wilhelm Herschel.
 The Althing of Iceland, the world's oldest parliament, is abolished.

January 7 - Millard Fillmore, thirteenth President of the United States (d. 1874)


November 30 - Charles Adams, son of John Adams (b. 1770)