Title: Chapter 2 Discovering the Universe for Yourself
1Chapter 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
22.1 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?
3The Celestial Sphere(Two-sphere Geometry)
4Why 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.
5Constellation of Orion
Red lines mark constellation boundaries
6Definitions 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
7Common 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
8Daily Rising and Setting
9Rising and Setting
10altitude of the celestial pole your latitude
11View from Earth in Our Galaxy
122.2 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?
13The 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.
14Seasonal Changes
15Precession 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!
162.3 The Moon, Our Constant Companion
- Why do we see phases of the Moon?
- What causes eclipses?
17Moons Orbit
18Phases of the Moon
19Phases of the Moon
20Moon Rise/Set by Phase
21Why 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)
22Conditions for Solar and Lunar Eclipse
23Inclination of Lunar Orbital Plane to Ecliptic
24Solar Eclipses
- Solar eclipses can occur only at or near new
moon. - Solar eclipses can be partial, total, or annular.
25Lunar Eclipses
- Lunar eclipses can occur only at or near full
moon. - Lunar eclipses can be penumbral, partial, or
total.
26Solar Eclipses
272.4 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?
28What 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
29Inferior Planet Configurations
30Conjunction
Superior Planet Configurations
Sun
Western Quadrature
Eastern Quadrature
Earth
Opposition
31Synodic 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
32Synodic 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
33Understanding Retrograde Motion
34Mars Retrograde Motion in 2003
35Retrograde Motion for Mars
36The 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.