Chapter 2 Discovering the Universe for Yourself 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 eclipses Lunar eclipse Earth s shadow on the Moon Solar eclipse Moon s shadow on Earth Tilt of Moon s orbit means eclipses occur during two periods each year Copyright 2009 Pearson Education Inc 2 4 The Ancient Mystery of the Planets Our goals for learning What was once so mysterious about the movement of planets in our sky Why did the ancient Greeks reject the real explanation for planetary motion Copyright 2009 Pearson Education Inc Planets Known in Ancient Times Mercury bottom Difficult to see always close to Sun in sky Venus above Mercury Very bright when visible morning or evening star Mars middle Noticeably red Jupiter top Very bright Saturn above Mars Moderately bright Copyright 2009 Pearson Education Inc What was once so mysterious about the movement of planets in our sky Planets usually move slightly eastward from night to night relative to the stars But sometimes they go westward relative to the stars for a few weeks apparent retrograde motion Copyright 2009 Pearson Education Inc We see apparent retrograde motion when we pass by a planet in its orbit Copyright 2009 Pearson Education Inc Explaining Apparent Retrograde Motion Easy for us to explain this occurs when we lap another planet or when Mercury or Venus laps us But it is very difficult to explain if you think that Earth is the center of the universe In fact ancients considered but rejected the correct explanation Copyright 2009 Pearson Education Inc Why did the ancient Greeks reject the real explanation for planetary motion Their inability to observe stellar parallax was a major factor Copyright 2009 Pearson Education Inc The Greeks knew that the lack of observable parallax could mean one of two things 1 Stars are so far away that stellar parallax is too small to notice with the naked eye 2 Earth does not orbit the Sun it is the center of the universe With rare exceptions such as Aristarchus the Greeks rejected the correct explanation 1 because they did not think the stars could be that far away Thus the stage was set for the long historical showdown between Earth centered and Sun centered systems Copyright 2009 Pearson Education Inc What have we learned What was so mysterious about planetary motion in our sky Like the Sun and Moon planets usually drift eastward relative to the stars from night to night but sometimes for a few weeks or few months a planet turns westward in its apparent retrograde motion Why did the ancient Greeks reject the real explanation for planetary motion Most Greeks concluded that Earth must be stationary because they thought the stars could not be so far away as to make parallax undetectable Copyright 2009 Pearson Education Inc
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