Introductory Astronomy And Astrophysics Now

: Spherical astronomy, celestial mechanics (gravity and motion), and astronomical instruments like telescopes and spectrographs.

Depending on your mathematical background, different resources may be more suitable: Astronomy: A Free, Online Astro 101 Textbook Introductory Astronomy and Astrophysics

: Understanding the electromagnetic spectrum, photometry, and how light reveals the temperature, mass, and composition of stars. : Spherical astronomy

: Formation and dynamics of planets, moons, asteroids, and the Sun. celestial mechanics (gravity and motion)