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.