This lecture explores the physical concepts used in astrophysics to interpret observations, focusing on the mechanisms behind the radiation received from celestial bodies, their motion, and modeling, emphasizing the connection between theoretical concepts and observations. Covering a wide range of astrophysical applications at various scales of time and space, the course takes a journey through the universe from our solar system to galaxies, using images, digital tablet demonstrations, and highlighting the theoretical-observational links. The instructor, Frédéric Courbin, a researcher at EPFL's astrophysics laboratory, offers this course to second-year bachelor physics students, providing a foundational understanding for advanced astrophysics courses, with minimal prerequisites equivalent to first-year university physics knowledge.