3 LECTURE (1/22/2008) 1 year = 365 solar days = 366 sidereal days Each day Sun appears to shift EAST relative to background stars by 1 degree. Seasons Stars that rise and set do so at the same sidereal time every day, all year long. Therefore, since the solar day begins 4 minutes later every day, these stars must ( on a solar day clock) appear to rise and set 4 minutes earlier every day. As the year goes by, the interval between sun rise and set will change relative to that of stars some stars come out at night which did not at other part of year etc. Ecliptic- apparent motion of Sun eastward by 1 degree per day (sidereal) relative to fixed star background traces path in sky Angle between plane of equator and plane of ecliptic- 23 degrees Equinox- day when sun's motion on celestial sphere the crosses celestial equator daynight-12hours e.g. March 21 (Vernal) Sun is moving from South Cel. To North Cel. Hemisphere. Moon Synodic vs Sidereal Period 1 orbit = 27.3 days to orbit the Earth = Sidereal Period 1 Synodic Period = 29.5 days o time for MOON to complete cycle of phases o time for moon to reach same point relative to sun on celestial sphere Earth shows complementary phases to the moon's if seen from Moon Planets Not fixed like stars Inferior planets: Mercury Venus Superior planets: Mars, Jupiter, Saturn, Uranus, Pluto, Neptune Larger orbit slower motion and longer its year Elongation- angle between planet and sun on sky for inferior planets, elongation < 90 degrees (Mercury, Venus, elongation <28 degrees) Opposition: elongation of 180 degrees only superiors