original_query: What evidence suggests that there were other documents on the theory of astrolabes circulating during the time al-Farghani wrote his treatise? What is the most relevant chunk to answer the query?
query: What evidence suggests that other documents on the theory of astrolabes circulated when al-Farghani wrote his treatise?
answer: Historical sources indicate that other documents on astrolabe theory, including one by al-Khwarizmi, existed around the time al-Farghani wrote his treatise.  However, al-Farghani stated in his treatise that he was unaware of such documents.