Simplify the following text for a Grade 9 native English-speaking person.
Original: Theon of Alexandria wrote a detailed treatise on the astrolabe , and argues that Ptolemy used an astrolabe to make the astronomical observations recorded in the Tetrabiblos .
Simplified: Theon of Alexandria wrote a complex book on the astrolabe . But says that Ptolemy used an astrolabe to make the astronomy observations recorded in the Tetrabiblos .