A conventional constructivist approach to science teacher education calls for teachers and teacher educators to understand the prior knowledge their learners have when entering the learning environment. Teachers in many fields of science have lacked substantive opportunities to have formal college-level coursework in their assigned teaching domains. In the earth sciences, astronomy in particular, teachers have a generally acknowledged deficiency in formal college astronomy coursework. Consequently, teacher educators and professional development providers would benefit greatly from knowing the existing astronomy prior knowledge state of future and in- service teachers. This study uses a widely recognized conceptual assessment inventory to examine the existing knowledge state of teachers tasked with teaching basic astronomy concepts dictated by national science education reform documents. The Test Of Astronomy STandards (TOAST) assessment instrument is a 27 item multiple-choice survey tightly aligned to the consensus learning goals articulated by the American Astronomical Society, Achieve, Inc.’s Next Generation Science Standards (NGSS), the American Association for the Advancement of Science’s (AAAS) Project 2061 Benchmarks, and the National Research Council’s National Science Education Standards (NSES). In addition to documenting deficits in teachers’ understanding of astronomy concepts related to sky motions, solar system dynamics, and size and structure of the universe, this study provides a detailed item by item distractor analysis to determine the sensitivity and effectiveness of each item and compares those results to the existing literature on the teaching and learning of astronomy. The results of this study of more than 500 teachers provides a contemporary evaluation of K-12 teachers’ overall understanding of astronomy concepts outlined in modern science education reform and policy documents in the United States.