ASTR 101 1st Edition Lecture 4 Outline of Last Lecture I The Universe II Scale of the solar system III Cosmic calendar Outline of Current Lecture The Reason for Seasons I Clicker questions II What causes the seasons III 4 special points IV The orientation of Earth s axis changes over time V 2 3 The moon Current Lecture I Clicker questions a On the northern spring equinox the Sun is moving from the southern part of the celestial sphere to the northern part of the celestial sphere on the celestial sphere and on the ecliptic b The Sun is at its most Southern position on the celestial sphere on the northern hemisphere s winter solstice II What causes the seasons a Depends on how Earth s axis affects the directness of sunlight b North and South poles barely intercept sunlight that s why they re always cold c The Sun is at its highest point in the summer and at its lowest point in the winter d Axis tilt this is why the seasons change It s essential III 4 special points a Summer solstice happens in June b Winter solstice happens in December c 2 equinoxes one in March and the other in September d The Sun rises and falls at various places on the meridian depending on the time of year IV The orientation of Earth s axis changes over time a The axis isn t fixed at its current position but precesses changes axis very slowly 26 000 years b The North Pole happens to be pointing to Polaris now but it used to point to the star Vega It won t point to Polaris forever These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute V The moon a The orbital period of the moon is about 30 days exactly 29 5 as referenced to the Sun Different times are given when referenced to different objects 27 3 days when referenced to the Earth b Phases of the moon consequence of a 27 3 day orbit of Earth It reflects the sunlight in different ways appearing differently to the US i Waxing when the moon appears to be growing ii Waning when the moon appears to be shrinking c Why do we only see one side of the moon i The moon s rotation period and orbital period are the same so we are only ever able to see one side This is referred to as synchronous rotation d Eclipses when Sun Earth and or moon pass through another s line shadow i For an eclipse to occur 1 It has to be a new or full moon New moon leads to a solar eclipse and a full moon leads to a lunar eclipse 2 The moon must be near orbit where it crosses its ecliptic plane ii Because of the moon s shadow we cannot see an eclipse everywhere on Earth at the same time The moon s shadow doesn t cover the entire earth