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TAMU ASTR 101 - Lecture05

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Copyright © 2009 Pearson Education, Inc.TypicalastronomersCopyright © 2009 Pearson Education, Inc.Chapter 2Discovering the Universe for YourselfCopyright © 2009 Pearson Education, Inc.2.3 The Moon,Our Constant Companion• Why do we see phases of the Moon?• What causes eclipses?Our goals for learning:Copyright © 2009 Pearson Education, Inc.Why do we see phases of the Moon?• Lunar phases are aconsequence of theMoon’s 27.3-dayorbit around Earth.Copyright © 2009 Pearson Education, Inc.Phases of the Moon• Half of the Moonis illuminated bythe Sun and halfis dark.• We see achangingcombination ofthe bright anddark faces as theMoon orbitsEarth.Copyright © 2009 Pearson Education, Inc.Phases of the MoonPhases of the MoonCopyright © 2009 Pearson Education, Inc.Moon Rise/Set by PhaseTime the Moon Rises and Sets for Different PhasesCopyright © 2009 Pearson Education, Inc.Phases of the Moon: 29.5-day cyclenewcrescentfirst quartergibbousfullgibbouslast quartercrescentwaxing• Moon visible in afternoon/evening• Gets “fuller” and rises later each daywaning• Moon visible in late night/morning• Gets “less” and sets later each day}}Copyright © 2009 Pearson Education, Inc.Thought QuestionA. first quarterB. waxing gibbousC. third quarterD. half moonIt’s 9 A.M. You look up in the sky and see a moonwith half its face bright and half dark. What phaseis it?Copyright © 2009 Pearson Education, Inc.A. first quarterB. waxing gibbousC. third quarterD. half moonIt’s 9 A.M. You look up in the sky and see a moonwith half its face bright and half dark. What phaseis it?Thought QuestionCopyright © 2009 Pearson Education, Inc.We see only one side of the Moon.Synchronous rotation:The Moon rotates exactlyonce with each orbit.This is why only one sideis 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, wehave an eclipse.Copyright © 2009 Pearson Education, Inc.Lunar EclipseLunar EclipseCopyright © 2009 Pearson Education, Inc.When can eclipses occur?• Lunar eclipses canoccur only at fullmoon.• Lunar eclipses canbe penumbral,partial, or total.Copyright © 2009 Pearson Education, Inc.Solar EclipseEvolution of a Total Solar EclipseCopyright © 2009 Pearson Education, Inc.When can eclipses occur?• Solar eclipses canoccur only at newmoon.• Solar eclipses can bepartial, total, orannular.Copyright © 2009 Pearson Education, Inc.Why don’t we have an eclipse at every new and fullmoon?—The Moon’s orbit is tilted 5° to the ecliptic plane.—So we have about two eclipse seasons each year, with a lunareclipse at new moon and solar eclipse at full moon.Insert ECP 5e Figure 2.20Copyright © 2009 Pearson Education, Inc.Summary: Two conditions must be metto have an eclipse1. It must be a full moon (for a lunar eclipse) or anew moon (for a solar eclipse). AND2. The Moon must be at or near one of the twopoints in its orbit where it crosses the eclipticplane (its nodes).Copyright © 2009 Pearson Education, Inc.Predicting Eclipses• Eclipses recur with the 18-year, 11 1/3-daysaros cycle, but type (e.g., partial, total) andlocation 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 inshadow, and its appearance to us isdetermined by the relative positions of theSun, 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 occurduring two periods each year.Copyright © 2009 Pearson Education, Inc.2.4 The Ancient Mystery of the Planets• What was once so mysterious about themovement of planets in our sky?• Why did the ancient Greeks reject the realexplanation for planetary motion?Our goals for learning:Copyright © 2009 Pearson Education, Inc.Planets Known in Ancient Times• Mercury (bottom)—Difficult to see; alwaysclose 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 brightCopyright © 2009 Pearson Education, Inc.What was once so mysteriousabout the movement of planets in our sky?• Planets usually move slightly eastward from night tonight relative to the stars.• But sometimes they go westward relative to the starsfor a few weeks: apparent retrograde motion.Copyright © 2009 Pearson Education, Inc.We see apparent retrograde motion whenwe 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 orVenus laps us).• But it is very difficult to explain if you thinkthat Earth is the center of the universe!• In fact, ancients considered but rejected thecorrect explanation.Copyright © 2009 Pearson Education, Inc.Why did the ancient Greeks reject the realexplanation 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 observableparallax could mean one of two things:1. Stars are so far away that stellar parallax is toosmall to notice with the naked eye.2. Earth does not orbit the Sun; it is the center of theuniverse.With rare exceptions, such as Aristarchus, the Greeksrejected the correct explanation (1) because theydid not think the stars could be that far away.Thus the stage was set for the long, historical showdownbetween Earth-centered and Sun-centered systems.Copyright © 2009 Pearson Education, Inc.What have we learned?• What was so mysterious about planetary motion in oursky?—Like the Sun and Moon, planets usually drifteastward relative to the stars from night to night; butsometimes, for a few weeks or few months, a planetturns westward in its apparent retrograde motion.• Why did the ancient Greeks reject the real explanationfor planetary motion?—Most Greeks concluded that Earth must bestationary, because they thought the stars could notbe so far away as to make parallax


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TAMU ASTR 101 - Lecture05

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