Born Jan Van Ostaeyen in the town of Loenhout (thus Loennouthesius is sometimes appended to his Latin surname) in Brabant, Stadius spent his youth in the Schaliënhuis, on the old Dorpsstraat and one of the oldest houses in Loenhout (today a tavern and restaurant). Not much else is known regarding his youth besides the fact that his mother was not the spouse of his father.
After receiving his education at the school of Latin at Brecht, Stadius studied mathematics, geography, and history at the University of Leuven, where he studied under Gemma Frisius. After his studies in Leuven, he became a professor (hoogleraar) of mathematics, but in 1554 he went to Turin, where he enjoyed the patronage of the powerful Duke of Savoy.
Stadius also worked in Paris, Cologne, and Brussels. In Paris, he debated with the trigonometrist Maurice Bressieu of Grenoble, and made astrological predictions for the French court. In his Tabulae Bergenses (1560), Stadius calls himself both royal mathematician (of Philip II of Spain) and mathematician to the Duke of Savoy.
During his stay in Brussels, his first work appeared: Ephemerides novae at auctae, first published by Arnold Birckmann of Cologne in 1554. An ephemeris (plural: ephemerides) (from the Greek word ephemeros, "daily") was, traditionally, a table providing the positions (given in a Cartesian coordinate system, or in right ascension and declination or, for astrologers, in longitude along the zodiacal ecliptic), of the Sun, the Moon, and the planets in the sky at a given moment in time; the astrological positions are usually given for either noon or midnight depending on the particular ephemeris that is used.
This work, read by Tycho Brahe and Nostradamus, posited a link between mathematics and medicine. Stadius had been encouraged to publish it by his old teacher Gemma Frisius, who in 1555 urged Stadius not to fear being accused of believing that the earth was not stationary while the sun stood still (as Copernicus had), or for abandoning the medieval Alfonsine Tables in favor of his own observations. In this 1555 letter from Frisius that was published in several editions of Ephemerides, Stadius' old teacher wrote that the system devised by Copernicus gave a better understanding of planetary distances, as well as certain features of retrograde motion.
Death and legacy
In Paris, he died and was buried. On his epitaph it indicates that he died on 17 June 1579 and that he had lived 52 years and almost 2 months. It is for this reason that the suspected birth date of Stadius is 1 May 1527.
- (Dutch) Prospector: Johannes Stadius
- (Dutch) Flapuit: Johannes Stadius
- (English) Bibliopolis: Ephemerides
- James H. Holden: A History of Horoscopic Astrology 
Steven Vanden Broecke: The Limits of Influence: Pico, Louvain, and the Crisis of Renaissance Astrology 
- Emalsteen, Jos (1927). Oudheid en Kunst. Brecht.
- Ernalsteen, Jozef A.U. Joannes Stadius Leonnouthesius 1527-1579. LZ Antwerpen-Brecht 938.1.
- Gingerich, Owen (1973). Proceedings of the American Philosophical Society 117 (N6): 513–522.
- Weyns, A.J. (1977). "Vlaamse Stam" 11. pp. 584–587.