Astronomy 10: Discussion 1

Discussion 1

Reading Assignment: Arny pp. 1-40, Cosmos Chpt. 1


I. The World Around You (continued)

  1. The Horizon
    1. Where Earth meets Sky
    2. How far to it?
    3. How much sky can you see at different altitudes?
  2. Local Coordinates
    1. Zenith - point directly above your head
    2. Nadir - point opposite in the celestial sphere from the Zenith
    3. Meridian - line joining North and South through the Zenith
    4. Azimuth - angle along the horizon starting from North.
    5. Altitude - anlgle above the horizon

II. The Shape and Size of Earth

  1. What is the Shape of the Earth?
    1. Looks Flat
      - As we saw in the horizon discussion the closer to Earth's Surface you get the more and more you cannot discern the curvature and it looks flat.
    2. Evidence for Curvature
      - As ships head out to sea they don't disappear all at once, but rather their masts disappear last.
      - The Sun's position in the sky is different from different locations on the same day.
      - As you move north or south you can see different stars that were previously hidden by the horizon. Not possible on a flat Earth.
    3. Evidence for Spherical
      - The Moon and the Sun are circular in projected appearance (the Moon's Phases give away its spherical shape), Why not Earth?
      - Earth's shadow on the Moon during a lunar eclipse is circular. Eclipses happen at all altitudes so Earth must be Spherical (Aristotle ~300 B.C.)
  2. What is the size of Earth?
    1. Eratosthenes figures it out.

III. Motions of the Stars, Sun, and Planets

  1. Daily or Diurnal Motion
    1. Everything in the sky rises in the East makes a great arc across the sky and sets in the West
    2. This Motion is Due to the rotation of Earth
    3. The stars are all so far away that we have no sense of their distance from us. So they appear fixed on a sphere, named the Celestial Sphere.
    4. Earth rotates West to East (San Francisco heads toward Washington D.C.). If we imagine ourselves as motionless then the Celestial Sphere appears to rotate East to West.
    5. The angle that the paths of the stars make with the horizon is equal to the latitude of the observer.
    6. Imagine what the star paths looks like from the extremes
      1. The Equator
      2. The Poles
      3. At 45° latitude

  2. Annual Motion
    1. On any given night at any given time you can see thousands of stars (or maybe only hundreds to dozens if you are looking from a bright city location), their collective patterns (constellations) stay the same throughout the night and from night to night all year long.
    2. However, as the year passes different constellations appear in the sky.
    3. This changing of constellations throughout the year is caused by Earth's orbit around the Sun.
    4. These sky rhythms changed with the seasons and helped ancient people predict the changing of the seasons.

  3. The Ecliptic
    1. The Earth orbits the Sun in a plane, and this plane is called the Ecliptic
    2. The ecliptic is titled with respect to the Celestial Equator. This is because Earth's rotation axis is tilted by an angle of 23.5° with respect to its orbital plane.
    3. This tilt causes the Sun's position in the sky at any given time to change throughout the year.
    4. The tilt causes the length of the days to change and the location of the rising and setting Sun to change also.
    5. The extreme rising and setting places occur on the Solstices.

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