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Introduction and The Celestial Sphere

1.1 Our Place in Space

  • Earth is average; it doesn't hold a special place in the universe.
  • The universe encompasses all space, time, matter, and energy.
  • Astronomy is the study of the universe.
  • The scales of the universe are very large.
  • Light-years are used to measure these vast distances.
    • One light-year is the distance light travels in a year (about 6 trillion miles).

1.2 Scientific Theory and the Scientific Method

  • A theory is a framework of ideas and assumptions used to explain observations and make predictions about the real world.
  • Theories must be continuously tested using theoretical models of physical objects or phenomena.
  • These models account for the physical properties seen.
  • Scientific theories must be testable.
  • Scientists use the scientific method to test theories.
  • The scientific method involves:
    • Observation/question
    • Research topic area
    • Hypothesis
    • Test with experiment
    • Analyze data
    • Report conclusions

1.3 The “Obvious” View

  • Simplest observation: looking at the night sky.
  • Approximately 3000 stars are visible at any one time.
  • They appear randomly distributed, but the human brain tends to find patterns.
  • Stars are grouped into constellations, which are figures having meaning to those doing the grouping.
  • There are 88 recognized constellations.
  • Polaris, also known as the North Star, is directly over Earth’s north rotational pole.
  • Stars that appear close in the sky may not actually be close in space.
  • The celestial sphere is the concept that stars seem to be on the inner surface of a sphere surrounding the Earth.
  • Two-dimensional spherical coordinates (similar to latitude and longitude) can be used to locate sky objects.
  • The Earth rotates in a counterclockwise direction, so objects on the celestial sphere appear to rise in the east and set in the west.
  • When an object transits, it reaches its highest point in the sky.
  • During transit, it is found on the meridian.
  • Zenith is the point directly overhead.

1.4 Earth’s Orbital Motion

  • The twelve constellations the Sun moves through during the year are called the zodiac constellations.
  • The path that the Sun travels along the celestial sphere throughout the year is called the ecliptic, and the zodiac constellations are found along this path.
  • Seasonal changes to the night sky are due to Earth's motion around the Sun.
  • Ecliptic: the apparent annual path of the Sun across the celestial sphere.