* Astronomy

The Mexborough & Swinton Astronomical Society was founded on 21 May 1978 
http://en.wikipedia.org/wiki/Hoober_Observatory

The William Miller Sperry Observatory was dedicated on May 21, 1967
http://en.wikipedia.org/wiki/William_Miller_Sperry_Observatory

The Kuiper Airborne Observatory was dedicated on May 21, 1975
http://en.wikipedia.org/wiki/Stratospheric_Observatory_for_Infrared_Astronomy

The GREGOR solar telescope was inaugurated on May 21, 2012
http://en.wikipedia.org/wiki/GREGOR_Solar_Telescope

Saint Helena is an island in the South Atlantic Ocean was discovered on 21 May 1502
http://en.wikipedia.org/wiki/Saint_Helena

On 21 May 1946, Louis Alexander Slotin (1 December 1910 - 30 May 1946) accidentally began a fission reaction
http://en.wikipedia.org/wiki/Louis_Slotin

The first scientific drifting ice station in the world, North Pole-1 was established on May 21, 1937
http://en.wikipedia.org/wiki/Soviet_and_Russian_manned_drifting_ice_stations

Robert Clyde "Bob" Springer (born May 21, 1942) is a retired American astronaut
http://en.wikipedia.org/wiki/Robert_C._Springer

Ruth Bancroft Law (May 21, 1887 - 1970) was a pioneer American aviator during the 1910s. 
http://en.wikipedia.org/wiki/Ruth_Bancroft_Law

Glenn Hammond Curtiss (May 21, 1878 - July 23, 1930) was an American aviation and motorcycling pioneer
https://en.wikipedia.org/wiki/Glenn_Curtiss

Sergei Konstantinovich Tumansky (21 May 1901 - 9 September 1973) was a designer of Soviet aircraft engines
http://en.wikipedia.org/wiki/Sergei_Tumansky

Giuseppe Mercalli (May 21, 1850 - March 19, 1914) was an Italian volcanologist. 
http://en.wikipedia.org/wiki/Giuseppe_Mercalli

Andrei Dmitrievich Sakharov (May 21, 1921 - December 14, 1989) was a Soviet nuclear physicist
http://en.wikipedia.org/wiki/Andrei_Sakharov

David Orlin Hestenes, Ph.D. (born May 21, 1933) is a theoretical physicist and science educator. 
http://en.wikipedia.org/wiki/David_Hestenes

Charles Cóckell (born May 21, 1967) is a Professor of Astrobiology
http://tinyurl.com/p2eejs8

Mary Anning (21 May 1799 - 9 March 1847) was a British fossil collector.
http://en.wikipedia.org/wiki/Mary_Anning#Ichthyosaurs

Albrecht Dürer (21 May 1471 - 6 April 1528) was a German painter
http://en.wikipedia.org/wiki/Albrecht_D%C3%BCrer

Gaspard-Gustave de Coriolis (21 May 1792 - 19 September 1843) was a French mathematician
http://en.wikipedia.org/wiki/Gaspard-Gustave_Coriolis

Nils Christoffer Dunér (May 21, 1839 - November 10, 1914) was a Swedish astronomer.
http://en.wikipedia.org/wiki/Nils_Christoffer_Dun%C3%A9r

NGC 6287 was discovered by German-British astronomer William Herschel on the 21st May 1784. 
http://astronomy.activeboard.com/t57622382/ngc-6287/

NGC 6369 was discovered by German-British astronomer William Herschel on the 21st May 1784. 
http://astronomy.activeboard.com/t57622371/ngc-6369/

NGC 6401 was discovered by German-British astronomer William Herschel on the 21st May 1784. 
http://astronomy.activeboard.com/t44198488/ngc-6401/