Depending on your mathematical background, different resources may be more suitable: Astronomy: A Free, Online Astro 101 Textbook
: Spherical astronomy, celestial mechanics (gravity and motion), and astronomical instruments like telescopes and spectrographs. Introductory Astronomy and Astrophysics
: Formation and dynamics of planets, moons, asteroids, and the Sun. Depending on your mathematical background
A standard introductory course typically begins with fundamental physics and Earth-based observations before moving to the broader cosmos: celestial mechanics (gravity and motion)
: Morphology and dynamics of the Milky Way, other galaxies, and active galactic nuclei (quasars).