PLACIDUS, a learned astronomer, was born at Achleuthen, near Linz, in Austria, on the 28th of May 1721. He received the rudiments of his education in the monastery of Kremsmunster, of which his uncle Alexander was abbot; afterwards went to Salzburg, where he studied a regular course of philosophy, but turned his attention particularly to the mathematics. In 1737, he was admitted as a novice into the Benedictine monastery, Kremsmunster, and the next year he took the solemn vow in presence of the abbot. After being two years in this monastery, during which time he devoted every leisure hour to the study of mathematics and philosophy, he returned to Salzburg to finish his studies in theology and jurisprudence, and at the same time acquired a competent knowledge of the oriental and modern languages, besides history and antiquities. In 1745, he obtained the degree of doctor in divinity, after which he received priest's orders in his own monastery, and was created professor of ecclesiastical law, which office he held during forty years, discharging its duties until within a few days of his death. He was also chosen as dean of the higher schools, and regent of the young nobility, offices which he retained during his life.

Fixlmillner wrote a commentary on the Jus Canonicum, but this work was never published. By the entreaties of his friends, he was induced to publish his Reipublicae sacrae Origines divinae, seu Ecclesiae Christi exterior junctura, imperium, et hierarchia, ex primigenia ejus institutione erecta et demonstrata. But it was his knowledge of astronomy which rendered him truly illustrious. His uncle Alexander fitted up an apartment for containing the instruments necessary for the pursuit of mathematical knowledge; and he also erected an observatory, which was begun in 1748, and completed in 1758, under the direction of Anselm Dering of Einsdorf, a celebrated architect. Whilst the observatory was building, Fixlmillner led a life of retirement and severe study, his favourite pursuit during this time being astronomy. When the edifice was finished, Dobler, a mathematician of some note, was appointed first astronomer; but the successors of his uncle the abbot having discovered Fixlmillner's extensive mathematical knowledge, made him an offer of the astronomical department, and the sole direction of the observatory; and this place he accepted in the year 1762, at the same time retaining his chair as professor of ecclesiastical law. He was not yet master of the learning which practical astronomy requires; but to remedy this defect he perused attentively Lalande's Exposition du Calcul Astronomique, soon after which he obtained the large astronomical work of the same author, and in 1766 published his Meridianus Speculae Astronomicae Cremifanensis, Steyer, 4to, by which he acquired considerable reputation. Ten years after this period he gave the world his Decennium Astronomicum, Steyer, 4to, containing many curious and important particulars respecting the theory and practice of astronomy. His Acta Astronomica Cremifanensia, Steyer, 1791, 4to, which did not appear till after his decease, still further increased his astronomical reputation; and he was, besides, a large contributor to many periodical publications in different countries.

He made and collected a great number of observations on the planet Mercury, the importance of which was publicly acknowledged by Lalande, as they greatly assisted him in constructing his tables of that planet. Fixlmillner was one of the first astronomers who calculated the orbit of the new planet Uranus (Georgium Sidus), his tables respecting which will be found in the Berlin Almanack for 1789. He also proved the truth of what had formerly been conjectured, that the thirty-fourth star of Taurus, which Flamsteed observed in 1690, was the new planet. It may be said of most philosophers, that they observe a great deal and calculate little; but the conduct of Fixlmiller was exactly the reverse. He turned his attention more than any of his predecessors to the solar spots, which he observed in the years 1767, 1776, 1777, 1778, and 1782, and from which he deduced important inferences respecting the revolution of the sun on his axis.

Fixlmiller had a genius singularly adapted for the study of mechanics, by which he was enabled to invent a new micrometer, and a machine for grinding concentric circles. As he resided in the country, he was in a great measure deprived of literary assistance; but his indefatigable zeal, diligence, and perseverance more than supplied the want of such aid. He was but little subject to the influence of the stronger passions. Like the laws of nature, which it was his chief delight to study, he was simple, uniform, and constant; and by the mildness and integrity of his character, he acquired the love and esteem of mankind. His high reputation never inspired him with vanity, and he rather wished to conceal than to propagate what had been written in his praise. His health having been very much impaired by his intense application, he terminated his earthly career on the 27th August 1791, in the seventy-first year of his age.