This study aims to determine the alternative conceptions that middle school students may have about the structure of celestial bodies, celestial events and the motions of celestial bodies. Since qualitative and quantitative methods were used in the study, the study method is mixed. Since quantitative data were collected first and qualitative data were collected after the quantitative data analysis, the study was designed as an explanatory design. The study group of the research consists of 332 students studying in the 5th, 6th, 7th, and 8th grades of a secondary school in a province in Türkiye in the 2022-2023 academic year. The sample in the study was determined by the monographic sampling method, which is one of the non-probability-based methods. The Alternative Concept Identification Test (ACIT), the quantitative data collection tool of the study, was created to identify alternative concepts. The test development stages were followed for ACIT, and the KR-20 value was found to be 0.82 after the necessary validity and reliability studies were conducted. There are 20 items in ACIT, each consisting of three stages. The alternative concepts of the students to whom ACIT was applied were identified, and frequency tables were created; then, semi-structured interview forms were prepared for 16 students randomly selected among the students determined by the typical case sampling method, and interviews were conducted with the students. The necessary validity and reliability studies were conducted for the semi-structured interview forms, which were the qualitative data collection tools of the study. As a result of the study, it was determined that students had alternative concepts in the subjects of the structure and motions of celestial bodies.