Born Achleuthen near Krensmünster, (Austria), 28 May 1721
Died Krensmünster, (Austria), 27 August 1791
Placidus Fixlmillner was an observatory director and observer who worked on the orbit of the newly discovered planet Uranus. Fixlmillner was the nephew of Alexander Fixlmillner, the Abbot of Krensmünster. Placidus displayed a talent for mathematics while studying at the monastery school from 1729 to 1735. After studying philosophy, music, and mathematics at Salzburg from 1735 to 1737, he joined the Benedictines in 1737 and then studied theology and foreign languages from 1740 to 1745, during which time he received his doctorate in theology. Fixlmillner spent his entire professional career at the college associated with the Krensmünster abbey, where he served as professor of canon law (1746–1787) and dean of higher studies (1756–1787). For his work in the former position, he was named a notary Apostolic to the Roman Court, a position he held until his death. In 1756, Fixlmillner published the short theological work Reipublicae sacrae origines divinae.
A year after his interest in mathematics was rearroused by the transit of Venus that occurred in 1761, Fixlmillner was appointed director of the observatory (atop a nine-story building) that his uncle had established at the monastery, a position that he also held until his death. Fixlmillner described the observations that he made to establish the latitude and longitude of the observatory in his Meridianus speculae astronomicae cremifanensis (1765), and he summarized 10 years of observations in Decennium astronomicum (1776). Shortly after his death, his successor P. Thaddaeus Derflinger arranged for the publication of Fixlmillner’s Acta Astronomica Cremifanensia (1791), which among other things, described his observations from 1776 to 1781 and included essays on the parallax of the Sun, the 1769 transit of Venus, the occultation of Saturn in 1775, sunspots, stellar aberration, and planetary aberration and nutation.
Fixlmillner is best known for his work on the determination of the orbit of the planet Uranus after it was optically discovered by the English astronomer William Herschel on 13 March 1781. In 1784, Fixlmillner computed elements for its orbit based on both suspected prediscovery observations of the planet made by the English astronomer John Flamsteed on 13 December 1690 (an object designated 34 Tauri) and the German astronomer Tobias Mayer on 25 September 1756 (an object designated Mayer 964) and the postdiscovery opposition observations made by Pierre Méchain on 21 December 1781 and Fixlmillner himself on 31 December 1783. Taking account of aberration and nutation in reducing the heliocentric position of 34 Tauri to the time of Flamsteed’s observation, he then applied his computed elements to 140 observations and worked out the residuals. The residuals in both latitude and longitude were in general relatively small, except for five that were between 20 and 30s. The German astronomer Johann Bode , who also found a good agreement of Fixlmillner’s elements with observations, arranged for a seven-page set of the latter’s tables of the motion of Uranus to appear in the Berliner Astronomisches Jahrbuch for 1789 (published in 1786).
On 7 July 1788, however, Fixlmillner reported to Bode that his tables were showing greater deviation from observation: 33s at the opposition of 13 January 1787 and an even larger amount at the one of 18 January 1788. Following a suggestion made by the astronomer Abbé Francis Triesnecker of Vienna, Fixlmillner carefully studied the errors in Flamsteed’s mural quadrant in order to correct the transit time of the latter’s observation for instrumental effects. Fixlmillner discovered that calculations based on the corrected Flamsteed position and Mayer’s coordinates produced a slower mean motion for Uranus than those based on recent observations. In other words, his elements for the motion of Uranus could satisfy for an extended period either the prediscovery or the postdiscovery observations, but not both. In 1789, Fixlmillner calculated new elements for the motion of Uranus based solely on Mayer’s position, and the 1787 and 1788 opposition observations, and found residual errors no greater than 10s. Many astronomers, however, recognized Flamsteed as a very careful observer, and the problem of reconciling the position of his 1690 observation to theories of the planet’s motion would persist until 1846, when the optical discovery of the planet Neptune confirmed predictions by the French mathematician Urbain Le Verrier and the English mathematician John Adams that Uranus’s motion was being perturbed by a trans-Uranian planet.