An Introduction to Solar System Astronomy http://www.astronomy.ohio-state.edu/~pogge/Ast161/
Copernican Derivations https://astro.unl.edu/naap/ssm/ssm_advanced.html
Kepler’s Laws https://web.njit.edu/~gary/320/Lecture4.html
History of Astronomy https://faculty.humanities.uci.edu/bjbecker/ExploringtheCosmos/index.html
별빛 세상 http://astronote.org