Basic understanding of the cycles of the sky: daily rising/setting, moon phases, seasons, eclipses. Some evidence that led to adoption of heliocentric model: e. g. Newton"s idea of universal laws that apply both on earth and off. 1 au = average distance between earth and sun (150 million km/ 1. 5 x 108 km) 1 light year = distance light travels in one year (9. 46x1012 km) nearly 1013 au. 1 pc (parsec) = 3. 26 ly = 3. 09x1013km. 1mpc (mega parsec) = 1 million parsecs. Distance to nearest star (besides sun) = 4. 37 ly / 1. 34 pc. Nearest star system is alpha centauri, 44 trillion km away. Nearest star is proxima centauri, 4. 2ly away from us (textbook) Distance to nearest star is 4. 37ly (ppt) Sun > earth by about 110 times in diameter. Moon"s diameter of earth"s & mercury a bit larger than the moon. Distance from venus to sun is about 0. 7au. Mars, about 1. 5 au from the sun.