1986 – High levels of radiation resulting from the Chernobyl disaster are detected at Forsmark Nuclear Power Plant in Sweden, leading Soviet authorities to publicly announce the accident.

1973 – The Dark Side of the Moon by Pink Floyd, recorded in Abbey Road Studios goes to number one on the US Billboard chart, beginning a record-breaking 741-week chart run.

1986 – The Chernobyl disaster occurs in the Ukrainian Soviet Socialist Republic.

1933 – The Gestapo, the official secret police force of Nazi Germany, is established by Hermann Göring.

1954 – The first practical solar cell is publicly demonstrated by Bell Telephone Laboratories.

1953 – Francis Crick and James Watson publish "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid" describing the double helix structure of DNA; though much of the actual research was accomplished by Rosalind Franklin and others.

1933 – Nazi Germany issues the Law Against Overcrowding in Schools and Universities limiting the number of Jewish students able to attend public schools and universities.

1990 – STS-31: The Hubble Space Telescope is launched from the Space Shuttle Discovery.

1915 – The arrest of 250 Armenian intellectuals and community leaders in Istanbul marks the beginning of the Armenian genocide.

1800 – The United States Library of Congress is established when President John Adams signs legislation to appropriate $5,000 to purchase "such books as may be necessary for the use of Congress".

2005 – The first YouTube video, titled "Me at the zoo", was published by co-founder Jawed Karim.

1970 – The first Earth Day is celebrated.

1864 – The U.S. Congress passes the Coinage Act of 1864 that permitted the inscription In God We Trust be placed on all coins minted as United States currency.

2014 – The American city of Flint, Michigan switches its water source to the Flint River, beginning the ongoing Flint water crisis which has caused lead poisoning in up to 12,000 people, and at least 12 deaths from Legionnaires' disease, ultimately leading to criminal indictments against 15 people, five of whom have been charged with involuntary manslaughter.

1995 – Oklahoma City bombing: The Alfred P. Murrah Federal Building in Oklahoma City, USA, is bombed, killing 168 people including 19 children under the age of six.

1993 – The 51-day FBI siege of the Branch Davidian building in Waco, Texas, USA, ends when a fire breaks out. Seventy-six Davidians, including 18 children under age 10, died in the fire.

1927 – Mae West is sentenced to ten days in jail for obscenity for her play Sex.

1775 – American Revolutionary War: The war begins during the Battles of Lexington and Concord with a victory of American minutemen and other militia over British forces, later referred to as the "shot heard round the world".

 Lazy8 wrote:
It seems to be both representation and taxation:

Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons .