Complex sentence: Unusual events in the sky called into in question the standard model of the universe of that time, known sometimes as the Aristotelian model and sometimes as the Ptolmaic system which held that the stars were fixed in their positions and unchanging, while the planets moved around.
Split into simpler sentences: Unusual events in the sky called into in question the standard model of the universe of that time, known sometimes as the Aristotelian model and sometimes as the Ptolmaic system. This held that the stars were fixed in their positions and unchanging, while the planets moved around.