Chapter 2 Discovering the Universe for Yourself Copyright 2009 Pearson Education Inc Summary The Real Reason for Seasons Earth s axis points in the same direction to Polaris all year round so its orientation relative to the Sun changes as Earth orbits the Sun Summer occurs in your hemisphere when sunlight hits it more directly winter occurs when the sunlight is less direct AXIS TILT is the key to the seasons without it we would not have seasons on Earth Copyright 2009 Pearson Education Inc Why doesn t distance matter Variation of Earth Sun distance is small about 3 this small variation is overwhelmed by the effects of axis tilt Copyright 2009 Pearson Education Inc How do we mark the progression of the seasons We define four special points summer solstice winter solstice spring vernal equinox fall autumnal equinox Copyright 2009 Pearson Education Inc We can recognize solstices and equinoxes by the Sun s path across the sky Summer solstice Highest path rise and set at most extreme north of due east Winter solstice Lowest path rise and set at most extreme south of due east Equinoxes Sun rises precisely due east and sets precisely due west Copyright 2009 Pearson Education Inc Why doesn t distance matter Variation of Earth Sun distance is small about 3 this small variation is overwhelmed by the effects of axis tilt Copyright 2009 Pearson Education Inc How do we mark the progression of the seasons We define four special points summer solstice winter solstice spring vernal equinox fall autumnal equinox Copyright 2009 Pearson Education Inc We can recognize solstices and equinoxes by the Sun s path across the sky Summer solstice Highest path rise and set at most extreme north of due east Winter solstice Lowest path rise and set at most extreme south of due east Equinoxes Sun rises precisely due east and sets precisely due west Copyright 2009 Pearson Education Inc Seasonal changes are more extreme at high latitudes Insert ECP 5e Figure 2 16 Path of the Sun on the summer solstice at the Arctic Circle Copyright 2009 Pearson Education Inc How does the orientation of Earth s axis change with time Although the axis seems fixed on human time scales it actually precesses over about 26 000 years Polaris won t always be the North Star Positions of equinoxes shift around orbit for example the spring equinox once in Aries is now in Pisces Earth s axis precesses like the axis of a spinning top Copyright 2009 Pearson Education Inc What have we learned What causes the seasons The tilt of the Earth s axis causes sunlight to hit different parts of the Earth more directly during the summer and less directly during the winter We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon Copyright 2009 Pearson Education Inc What have we learned How do we mark the progression of the seasons The summer and winter solstices are when the Northern Hemisphere gets its most and least direct sunlight respectively The spring and fall equinoxes are when both hemispheres get equally direct sunlight How does the orientation of Earth s axis change with time The tilt remains about 23 5 so the season pattern is not affected but Earth has a 26 000year precession cycle that slowly and subtly changes the orientation of the Earth s axis Copyright 2009 Pearson Education Inc 2 3 The Moon Our Constant Companion Our goals for learning Why do we see phases of the Moon What causes eclipses Copyright 2009 Pearson Education Inc Why do we see phases of the Moon Lunar phases are a consequence of the Moon s 27 3 day orbit around Earth Copyright 2009 Pearson Education Inc Phases of the Moon Half of the Moon is illuminated by the Sun and half is dark We see a changing combination of the bright and dark faces as the Moon orbits Earth Copyright 2009 Pearson Education Inc Phases of the Moon Phases of the Moon Copyright 2009 Pearson Education Inc Moon Rise Set by Phase Time the Moon Rises and Sets for Different Phases Copyright 2009 Pearson Education Inc Phases of the Moon 29 5 day cycle new crescent first quarter gibbous full gibbous last quarter crescent Copyright 2009 Pearson Education Inc waxing Moon visible in afternoon evening Gets fuller and rises later each day waning Moon visible in late night morning Gets less and sets later each day Thought Question It s 9 A M You look up in the sky and see a moon with half its face bright and half dark What phase is it A B C D first quarter waxing gibbous third quarter half moon Copyright 2009 Pearson Education Inc Thought Question It s 9 A M You look up in the sky and see a moon with half its face bright and half dark What phase is it A B C D first quarter waxing gibbous third quarter half moon Copyright 2009 Pearson Education Inc We see only one side of the Moon Synchronous rotation The Moon rotates exactly once with each orbit This is why only one side is visible from Earth Copyright 2009 Pearson Education Inc What causes eclipses The Earth and Moon cast shadows When either passes through the other s shadow we have an eclipse Copyright 2009 Pearson Education Inc Lunar Eclipse Lunar Eclipse Copyright 2009 Pearson Education Inc When can eclipses occur Lunar eclipses can occur only at full moon Lunar eclipses can be penumbral partial or total Copyright 2009 Pearson Education Inc Solar Eclipse Evolution of a Total Solar Eclipse Copyright 2009 Pearson Education Inc When can eclipses occur Solar eclipses can occur only at new moon Solar eclipses can be partial total or annular Copyright 2009 Pearson Education Inc Why don t we have an eclipse at every new and full moon The Moon s orbit is tilted 5 to the ecliptic plane So we have about two eclipse seasons each year with a lunar eclipse at new moon and solar eclipse at full moon Insert ECP 5e Figure 2 20 Copyright 2009 Pearson Education Inc Summary Two conditions must be met to have an eclipse 1 It must be a full moon for a lunar eclipse or a new moon for a solar eclipse AND 2 The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane its nodes Copyright 2009 Pearson Education Inc Predicting Eclipses Eclipses recur with the 18 year 11 1 3 day saros cycle but type e g partial total and location may vary Copyright 2009 Pearson Education Inc What have we learned Why do we see phases of the Moon Half the Moon is lit by the Sun half is in shadow and its appearance to us is determined by the relative positions of the Sun Moon and Earth What causes
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