ASTR 101 Lecture 4: Astronomy Summary Notes Chapter 2.docx
Document Summary
Earth: center of the universe, does not rotate, fixed to the celestial sphere, celestial sphere revolves around earth east to west once every sidereal day (23. 93. ) with respect to the slowly than the stars do. stars. ) with respect to the stars. than the stars do. Earth was not considered to be a planet. They wander, which is what the word planet means. Consequently, these planets are always within certain angles of the sun, and never opposite of the sun in the sky: aristotle"s model described the motions of the planets qualitatively (descriptively), but was not very accurate quantitatively (numerically). Ptolemy (2nd century ad: wrote megale syntaxis tes astronomias (great syntaxes of astronomy), also known as. Earth: revolves (orbits) around the sun once every tropical year, rotates (spins) on axis once every sidereal day. Moon: revolves (orbits) around earth once every sidereal month.
