ASTR 101 Lecture 3 Outline of Last Lecture I The Milky Way A What is it II Define Zenith Horizon and Angular Size III FACTS Outline of Current Lecture I Days and Seasons A Summer Solstice B Winter Solstice C Spring Equinox D Fall Equinox II Phases of the Moon III Eclipses Current Lecture Days and Seasons As earth orbits the sun the sun appears to move eastward At midnight the stars on our median are opposite the sun Solar day 24 hours Sidereal day earth s rotation period 23 hours and 56 minutes From earth 2000 stars and Milky Way can be seen We can specify positions by altitude above horizon and direction along horizon Stars rise and set because of earth s rotation Seasons depend on how earth s axis affects the directness of sunlight It depends on tilt not distance from the sun Axis tilt changes directness of sunlight over the year Suns altitude changes with seasons Higher altitude means more direct light summer solstice Earth s axis points the same way all year round its position relative to the sun changes Without the axis tilt there would be no seasons The larger the tilt the stronger the weather patterns are Variation of Earth Sun distance is small 3 so it s overshadowed by the tilt We define 4 special points 1 2 3 4 Summer solstice highest path rise and set at extreme north of due east Winter solstice lowest path rise and set at extreme south of due east Spring vernal equinox sun rises at exactly due east and sets at exactly due west Fall autumnal equinox Seasonal changes are more extreme at higher latitudes Although the axis seems fixed it actually progresses over about 26 000 years Polaris won t be the north star forever Positions of equinoxes shift around orbit the spring used to be Aries and now it is Pisces Phases of the Moon Half of the moon is illuminated by the sun and half of it is dark We see a changing combo of the bright and dark faces as the moon orbits the earth We only see one side of the moon o Synchronous rotation the moon rotates exactly once with each orbit Eclipses The earth and moon cast shadows When either passes through the other s shadow we have an eclipse o Lunar eclipse only on a full moon Can be penumbral partial or total The moon s orbit is tilted 5 degrees to the elliptic plane So we have about 2 eclipse seasons with a lunar at a full moon and a solar at a new moon
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