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Chapter 2 Discovering 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 2:55 PM. 2.1 Patterns in the Night Sky. What are constellations?

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chapter 2 discovering the universe for yourself

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 2:55 PM

2 1 patterns in the night sky

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 don’t we see the same constellations throughout the year?

why do we have constellations in the night sky
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.
constellation of orion
Constellation of Orion

Red lines mark constellation boundaries

definitions in your local sky

(altitude = 90o)

Definitions in Your Local Sky

(altitude = 0o)

(azimuth = 0o)

(azimuth = 90o)

(azimuth = 270o)

(azimuth = 180o)

Big Dipper

Angular measure is in degrees, minutes, and seconds of arc

1 degree = 60 arcminutes

1 arcminute = 60 arcseconds

common cyclic phenomena
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

Daily Rising and Setting


Equinox position

Summer solstice

Winter solstice


2 2 the reason for seasons

2.2 The Reason for Seasons

What causes the seasons?

How do we mark the progression of the seasons?

Does the orientation of the Earth’s axis change with time?

the sun s annual motion
The Sun’s 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.
precession of earth s axis of rotation
Precession of Earth’s Axis of Rotation
  • 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; e.g., spring equinox, once in Aries, is now in Pisces!
2 3 the moon our constant companion

2.3 The Moon, Our Constant Companion

Why do we see phases of the Moon?

What causes eclipses?

why are eclipses important to astronomy
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)
solar eclipses
Solar Eclipses
  • Solar eclipses can occur only at or near new moon.
  • Solar eclipses can be partial, total, orannular.

Lunar Eclipses

  • Lunar eclipses can occur only at or near full moon.
  • Lunar eclipses can be penumbral, partial, or total.
2 4 the ancient mystery of the planets

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?

what observations did ancients make of planetary motion
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
inferior planet configurations

Superior Conjunction


Greatest Western Elongation

Greatest Eastern Elongation

Inferior Conjunction


Inferior Planet Configurations

Superior Planet Configurations



Eastern Quadrature

Western Quadrature



synodic and sidereal month

Earth’s Orbit

 26o

One Month Later


 26o

Month 1

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
synodic and sidereal periods
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
the big picture
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.