Astronomy concept is regarded as one difficult topic in both teachers’ and students’ perspective even though it strongly appeals to the human mind. This concept requires imagination and the ability to use various skills and knowledge, for example, actual motion, relative position, and coordination of views from several points on the Earth to generate an explanation.  As mentioned in the literature, the teacher plays a vital role in the teaching and learning process. Therefore, this study aimed to investigate 45 in-service science teachers’ understanding of astronomy concepts in a professional development program and to diagnose the misconceptions regarding astronomy concepts. A two-tier test, open-ended questions, and a semi-structured interview were used to gather information on the understanding and misconceptions, particularly on celestial motion concepts. The data were analyzed both qualitatively and quantitatively. The results pointed out that after the four-day professional development program, in-service science teachers gained significantly higher two-tier test scores. The highest progression was in the topic of the Sun’s apparent motion. However, most in-service science teachers still held some misconceptions relating to the concept of the seasons. The obstacles in teaching astronomy were also discussed. The study’s findings lead to the improvement of the professional development program for in-service science teachers required to teach astronomy concepts nationwide.