Complex sentence: In 1875, after having finished his thesis, he work for three years at the Leiden Observatory, before becoming the first Professor of Astronomy and Theoretical Mechanics at the University of Groningen, which he remained until his retirement in 1921.
Split into simpler sentences: In 1875, after having finished his thesis, he work for three years at the Leiden Observatory. In 1878 he became the first Professor of Astronomy and Theoretical Mechanics at the University of Groningen and this until his retirement in 1921.