Extrasolar planet: planet orbits star other than our sun. Seti: search for extraterrestrial intelligence (listening for signals from them) Astrobiology: study of life in the universe (earth life origin, habitable planets, lbe) 2300 years ago: ancient greece debate lbe. Sky for time tracking, seasons, agriculture & religion. Thales (624-546 bc) -> plato (428-348 bc) -> his student aristotle (384-322 bc) Celestial sphere: anaximander (thales student) diurnal motion - east->west motion completing a sphere or cylinder. Aristotle: saw earth"s shape shadow on moon during lunar eclipse. Meridian: line passing through zenith connecting north & south. Angles: 360 in circle of sky -> 60 arc-minutes -> 60 arc-seconds. Apparent retrograde motion: planet traces loop as it moves east mimicking west moving, before it ends the loops and heads east again. Ptolemaic model: ptolemy (100-170 ad) planets move around earth in small circles that turn around larger circles.