Astro 10-Lecture 3: Seasons, Phases of the Moon and EclipsesThe Celestial SphereSlide 3Concept TestAnother line on the celestial sphereThe Earth’s pole is tilted with respect to the eclipticThe sun appears to travels along the ecliptic over the course of the yearThe orbits of the other planets are also in the ecliptic planeSlide 9PrecessionSlide 11Defining SeasonsModels of Possible CausesChoosing a ModelPreferring a ModelRuling out a ModelThe orbit of the EarthRuling out another ModelTilt ModelThe size of the EarthSlide 21Slide 22Slide 23Slide 24Slide 25Slide 26The Moon: The Earth’s CompanionSlide 28Phases of the MoonSlide 30Why do we see phases?Moon rotates to keep same face toward the EarthEclipsesEclipses: LunarSlide 35Slide 36Eclipses: Solar (Partial)Eclipses: Solar (Total)Slide 39Slide 40Eclipses: SolarEclipses: Solar (Annular)Slide 43Eclipses can only happen at certain phases of the moon.Then why don’t we have eclipses every month?Slide 46Slide 47Astro 10-Lecture 3:Seasons, Phases of the Moon and EclipsesDr. Eric [email protected]://setiathome.ssl.berkeley.edu/~korpela/(510) 643-6538The Celestial SphereThe Celestial SphereConcept TestAnother line on the celestial sphereThe Earth’s pole is tilted with respect to the eclipticThe sun appears to travels along the ecliptic over the course of the yearThe orbits of the other planets are also in the ecliptic planeThe orbits of the other planets are also in the ecliptic planePrecessionPrecessionDefining Seasons•Climate variations •Day length variations•Winter: short, cold days•Summer: long, warm daysModels of Possible Causes•Changing distance of Earth from Sun(closer=warmer)•Variation in the Sun’s energy(Sun changes)•Tilt of Earth’s spin axis relative to orbital axis(tilt)How do we choose between them?Choosing a Model•Remember that any model must be supported by evidence!•Each of the models mentioned above makes predictions that we can testPreferring a Model•Do they all explain the variations in day length?–NO! Variations in the Sun’s energy or changing distance from the Sun do not cause changes in the length of a day–BUT – Maybe the day length variations are just a coincidence, and the climate variations are caused by one of theseHow do we rule out a model FOR SURE?Ruling out a Model•If closer = warmer is the cause of the seasons, then this model predicts that the size of the Sun should vary over the course of a year–Observation: But it doesn’t!•Also, if seasons were caused by a changing Earth-Sun distance, summer here would occur at the same time as summer in Australia!–Observation: But it doesn’t!•Seasons caused by changing Earth-Sun distance?The orbit of the Earth•Which of these is the most correct depiction of the elliptical orbit of the Earth?Ruling out another Model•Sun changes model makes at least 2 predictions–Celestial objects that shine by reflected light should be brighter in summer–Summer should be at the same time all over the Earth•Seasons caused by variations in Sun’s energy output?Tilt Model•We’ve ruled out 2 of our 3 models!•How does the tilt model explain the seasons?–Distance to the sun is less for the hemisphere pointed towards the Sun? •NO! Remember the scale model!–Tilt causes change in day lengths => more hours of sunlight.• YES!–Tilt causes change in angle of sunlight => warmer days•YES!The size of the Earth•Which of these is the most accurate depiction of the relative sizes and distances of the Sun, Earth, and Moon?Tilt ModelTilt ModelIn the summer the sun is higher in the sky. It spends more time (>12 hours) above the horizon than below it.In the winter, the sun is low in the sky, so it spends less time above the horizon (< 12 hours).Tilt ModelAmount of light captured by a certain area of ground decreases with angleTilt ModelAmount of light captured by a certain area of ground decreases with angleTilt ModelSummer solstice, the sun reaches (90-latitude)+23.5 degrees above the horizon. (75.5 degrees for us)Winter solstice, the sun only reaches latitude -23.5 degrees above the horizon (28.5 degrees for us)Difference in illumination per square meter of ground:sin(28.5)/sin(75.5)=0.550% less sun per square meter at noon.Tilt ModelLength of the day also differs…For us, longest day (summer solstice) of the year is 15 hours.The shortest (winter solstice) is 9 hours 50 minutes. Sunlight difference9.8 hours/15 hours=0.6535% fewer daylight hoursThe Moon: The Earth’s Companion•The moon is about 1/6th the size of the Earth•Distance is many times the diameter of the earth.The Moon: The Earth’s CompanionPhases of the MoonPhases of the MoonWhy do we see phases?Moon rotates to keep same face toward the EarthEclipsesEclipses: LunarEclipses: LunarEclipses: LunarEclipses: Solar (Partial)Eclipses: Solar (Total)Eclipses: Solar (Total)Eclipses: Solar (Total)•A lucky accident. The Sun is 400x as big as the moon, but it’s also 400x as large. They appear almost the same size on the sky.Eclipses: SolarEclipses: Solar (Annular)•Happen when Earth is nearer the sun than average (sun appears bigger) or when the moon is farther from the earth than average (moon appears smaller).Eclipses: Solar (Annular)Eclipses can only happen at certain phases of the moon.•If the moon is between us and the sun, it must be a new moon.•If we are between the sun and the moon, the moon must be full.Then why don’t we have eclipses every month?Concept Test•Tonight the moon is at first quarter…–About what time did the moon rise?–About what time will it set?–Could there be an eclipse tonight?Then why don’t we have eclipses every
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