Placidus, a learned German astronomer, was born at Achleuthen, near Linz in Austria, in 1721. In 1737 he entered the Benedictine monastery at Kremsmünster, where a few years later he was appointed professor of ecclesiastical law, an office which he held for forty years. His fame, however, rests chiefly on his astronomical learning. The abbot had caused an observatory to be erected in the monastery, and Fixmillner was appointed chief astronomer. When he first entered upon the duties of this appointment his acquaintance with astronomy was very defective; but by severe study and unremitting application he not only overcame the deficiencies of his early training, but even earned a very respectable place in the second rank of astronomical observers. The value of his observations on the planet Mercury was publicly acknowledged by Lalande. Fixmillner was also one of the first to determine the orbit of the planet Uranus. His principal astronomical works are his Meridianus Specula Astronomicae Cremianensis, 1766; Decennium Astronomicum, He likewise contributed a large number of papers to many of the scientific journals of his own and other countries. He died in 1791.