* Astronomy

World Art Day is an international celebration of the fine arts 
http://en.wikipedia.org/wiki/World_Art_Day

"I Wandered Lonely as a Cloud" is a poem by William Wordsworth.
http://en.wikipedia.org/wiki/I_Wandered_Lonely_as_a_Cloud

"A Dictionary of the English Language" was written by Samuel Johnson and published on 15 April 1755
http://en.wikipedia.org/wiki/A_Dictionary_of_the_English_Language

Georgy Timofeyevich Beregovoy (April 15, 1921 - June 30, 1995) was a Soviet cosmonaut who commanded the space mission Soyuz 3 in 1968.
http://en.wikipedia.org/wiki/Georgi_Beregovoi

Gregory J. Harbaugh (born 15 April 1956) is a former NASA astronaut. 
http://www.jsc.nasa.gov/Bios/htmlbios/harbaugh.html

Marsha Sue Ivins (born 15 April 1951) is a former American astronaut
http://en.wikipedia.org/wiki/Marsha_Ivins

John Lynch Phillips, PhD (born April 15, 1951) is a NASA astronaut.
http://en.wikipedia.org/wiki/John_L._Phillips

Soichi Noguchi (born 15 April 1965) is a Japanese aeronautical engineer and a JAXA astronaut. 
http://en.wikipedia.org/wiki/Soichi_Noguchi

Samuel Kurtz Hoffman (15 April 1902 - 26 June 1995) was an American engineer who specialised in rocket propulsion.
https://en.wikipedia.org/wiki/Samuel_Kurtz_Hoffman

Leonardo di ser Piero da Vinci (April 15, 1452 - May 2, 1519) was an Italian Renaissance polymath
http://astronomy.activeboard.com/t9402183/leonardo-da-vinci/

Pietro Antonio Cataldi (15 April 1548, Bologna - 11 February 1626, Bologna) was an Italian mathematician. 
http://en.wikipedia.org/wiki/Pietro_Cataldi

Leonhard Euler (15 April 1707 - 18 September 1783) was a pioneering Swiss mathematician and physicist. 
http://www.astronomy.activeboard.com/t14043970/leonhard-euler/

Sir Robert Sibbald (15 April 1641 - August 1722) was a Scottish physician and antiquary.
http://en.wikipedia.org/wiki/Robert_Sibbald

Friedrich Georg Wilhelm von Struve (April 15, 1793 - November 23, 1864) was a Baltic German astronomer
http://en.wikipedia.org/wiki/Friedrich_Georg_Wilhelm_von_Struve

Maurice (Moritz) Loewy (15 April 1833 - 15 October 1907) was a French astronomer.
http://en.wikipedia.org/wiki/Maurice_Loewy

Christian Pedersen Horrebow (April 15, 1718 - September 15, 1776) was a Danish astronomer
http://en.wikipedia.org/wiki/Christian_Horrebow

Charles Roger Alcóck (born June 15, 1951) is a British New Zealander astronomer. 
http://is.gd/PZu0g6

Timothy B. Spahr (April 15, 1970) is an American astronomer.
http://en.wikipedia.org/wiki/Timothy_B._Spahr

Larry W. Esposito (born April 15, 1951) is an American planetary astronomer 
http://en.wikipedia.org/wiki/Larry_W._Esposito

Extrasolar planet HD 83443 b was discovered on the 15th April 2000
http://en.wikipedia.org/wiki/HD_83443_b

Extrasolar planet HD 202206 b was discovered on the 15th April 2000
http://en.wikipedia.org/wiki/HD_202206_b

Extrasolar planet HD 169830 b was discovered on the 15th April 2000
http://en.wikipedia.org/wiki/HD_169830_b

Asteroid (43) Ariadne was discovered by N. R. Pogson on April 15, 1857
http://astronomy.activeboard.com/t42868577/asteroid-43-ariadne/

Asteroid (275) Sapientia was discovered by Johann Palisa on April 15, 1888 in Vienna.
http://en.wikipedia.org/wiki/275_Sapientia

On April 15, 1988, a U.S. satellite recorded the blast of a behemoth fireball above Indonesia. 
http://astronomy.activeboard.com/t57305065/airburst-over-indonesia-1988/