who discovered the orthocenter of a triangle? I tried to answer Is there a name for this result in planar geometry? and wanted to go back to the first mention of the orthocenter (or even the altitude of a triangle, but i did draw a complete blank.
Orthocenter or even altitude is not mentioned in Euclid's elements (Heath's translation) 
it is also not mentioned in 
Heath, T.L. (1921). A History of Greek Mathematics: From Thales to Euclid I. Oxford.
http://www.archive.org/details/cu31924008704219
So when was it discovered?
(please add references)
 A: The three altitudes of any triangle are concurrent at the orthocenter H (Durell 1928). This fundamental fact did not appear anywhere in Euclid's Elements.
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The name was invented by Besant and Ferrers in 1865 while walking on a road leading out of Cambridge, England in the direction of London (Satterly 1962).
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A: This web page discusses the matter of who first proved that a triangle's altitudes concur. It states that "The timing of the first proof is still an open question; it is believed, though, that even the great Gauss saw it necessary to prove the fact. The earliest known proof was given by William Chapple (1718-1781) in 1749." 
This web page on proofs of this result gives Chapple's proof and cites it as follows: "It comes from the Miscellanea Curiosa Mathematica, Number IX, edited by Francis Holliday and published by Cave - I understand that this publication was launched in 1745, with two issues a year, which would place the publication date in 1749, the year of publication of a collected edition".
