Amateur astronomers regularly serve as informal astronomy educators for their communities. This research inquires into the level of knowledge of basic astronomy concepts among amateur astronomers and examines factors related to amateur astronomy that affect that knowledge. Using the concept questions from the Astronomy Diagnostic Test Version 2, an online survey was developed as an assessment. In particular, astronomy club members with at least some college-level astronomy education score substantially higher on the assessment (mean score: 85) than do college undergraduates after taking their first astronomy course (mean score: 47). Astronomy club members scored up to 17% higher than unaffiliated amateurs, an indication that regular contact with like-minded hobbyists improves basic knowledge. Proportionally more astronomy club members report doing outreach than do unaffiliated amateurs (87% vs. 46%). It appears that those who are likely to be more knowledgeable are also those doing more outreach. (Contains 9 tables and 2 figures.)