Chapter 2 Discovering the Universe for Yourself

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Chapter 2Discovering the Universe for Yourself

• Chapter Outline
• Patterns in the Night Sky
• The Reason for Seasons
• The Moon, Our Constant Companion
• The Ancient Mystery of the Planets

2/7/2005 255 PM
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2.1 Patterns in the Night Sky

• What are constellations?
• How do we locate objects in the sky?
• Why do stars rise and set?
• Why dont we see the same constellations
  throughout the year?

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The Celestial Sphere(Two-sphere Geometry)
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Why do we have constellations in the night sky?

• Constellations are not the result of an exercise
  in imagination.
• Constellations serve as an aid to time keeping,
  i.e., a primitive calendar.
• Constellations make nighttime navigation
  possible.
• For modern astronomers, constellations are a
  first level of organization of the sky with
  definite north-south and east-west boundaries.

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Constellation of Orion
Red lines mark constellation boundaries
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Definitions in Your Local Sky
(altitude 90o)
(altitude 0o)
(azimuth 90o)
(azimuth 0o)
(azimuth 180o)
(azimuth 270o)
Big Dipper
Angular measure is in degrees, minutes, and
seconds of arc 1 degree 60 arcminutes 1
arcminute 60 arcseconds
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Common Cyclic Phenomena

• Diurnal motion daily rising and setting
• Solar annual apparent motion yearly motion of
  Sun from west to east relative to the background
  stars
• Lunar phases monthly cycle of illumination for
  the Moon
• Solar and lunar eclipses shadow effect
• Planetary configurations changing position of
  the planets relative to the background stars

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Daily Rising and Setting
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Rising and Setting
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altitude of the celestial pole your latitude
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View from Earth in Our Galaxy
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2.2 The Reason for Seasons

• What causes the seasons?
• How do we mark the progression of the seasons?
• Does the orientation of the Earths axis change
  with time?

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The Suns Annual Motion

• As the Earth orbits the Sun, the Sun appears to
  move eastward along the ecliptic.
• At midnight, the stars on our meridian are
  opposite the Sun the in the sky.

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Seasonal Changes
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Precession of Earths Axis of Rotation

• Although the axis seems fixed on human time
  scales, it actually precesses over about 26,000
  years.
• Polaris wont always be the North Star.
• Positions of equinoxes shift around orbit e.g.,
  spring equinox, once in Aries, is now in Pisces!

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2.3 The Moon, Our Constant Companion

• Why do we see phases of the Moon?
• What causes eclipses?

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Moons Orbit
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Phases of the Moon
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Phases of the Moon
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Moon Rise/Set by Phase
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Why are eclipses important to astronomy?

• If one can predict an eclipse then we have an
  accurate time keeping system
• They can help in the study of the Sun (solar
  eclipse)

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Conditions for Solar and Lunar Eclipse
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Inclination of Lunar Orbital Plane to Ecliptic
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Solar Eclipses

• Solar eclipses can occur only at or near new
  moon.
• Solar eclipses can be partial, total, or annular.

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Lunar Eclipses

• Lunar eclipses can occur only at or near full
  moon.
• Lunar eclipses can be penumbral, partial, or
  total.

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Solar Eclipses
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2.4 The Ancient Mystery of the Planets

• 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?

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What observations did ancients make of planetary
motion?

• They did recognize the five naked-eye planets as
  moving relative to background stars
• They observed planetary configurations and
  measured synodic periods
• They observed retrograde motions of planets, such
  as Mars

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Inferior Planet Configurations
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Conjunction
Superior Planet Configurations
Sun
Western Quadrature
Eastern Quadrature
Earth
Opposition
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Synodic and Sidereal Month

• Sidereal month - 360o revolution measured
  relative background stars
• Approximately 27.3d
• Synodic month - 386o revolution measured relative
  to Earth/Sun line
• New moon to new moon
• Approximately 29.5d

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Synodic and Sidereal Periods

• Synodic period time interval between successive
  occurrences of the same planetary configuration
• Sidereal period time interval for 360o
  revolution of planet about the Sun measured
  relative to the stars

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Understanding Retrograde Motion
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Mars Retrograde Motion in 2003
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Retrograde Motion for Mars
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The Big Picture

• You can enhance your enjoyment of learning
  astronomy by observing the sky. The more you
  learn about the appearance and apparent motions
  of the sky, the more you will appreciate the
  universe of which you are a part.
• From Earth, it is convenient imagines that we are
  at the center of a great celestial sphere-even
  though we really are on a planet orbiting a star
  in a vast universe. We can understand what we
  see in the local sky by thinking about how the
  celestial sphere appears.
• Most of the phenomena of the sky are relatively
  easy to observe and understand. The more complex
  phenomena, particularly eclipses and apparent
  planetary motion, challenged our ancestors for
  thousands of years and helped drive the
  development of science and technology.