Celestial Sphere

Sphere of Stars

• The night sky looks like an upside down bowl set on the horizon, but as it turns around during the night it is easy to think of it as a giant sphere. To think of the stars as lying on the inside surface of a giant celestial sphere which rotates around us once a day explains the appearances of diurnal motion with simplicity and elegance. With good reason this explanatory scheme was adopted by ancient Greek astronomers, beginning with the 6th century B.C. Pythagoreans, and it is remains the most convenient way to learn observational astronomy today.
• Any rotating sphere has two poles at each end of the axis of rotation, and an equator which bisects the sphere in a plane that is perpendicular to the axis of rotation. Use a celestial globe model to identify the north and south celestial poles and the celestial equator.
• Become familiar with a model celestial globe of the sort used in the planetarium labs. Note that the constellations depicted on these models appear reversed, since you're on the "outside looking in." Look through and across a model celestial sphere to inspect the constellations as they appear from the earth.
• The celestial sphere concept facilitates the use of coordinate systems using imaginary lines inscribed on the celestial sphere. These lines rotate with the celestial sphere, and therefore do not depend on the observer's location, time of observation, or horizon.

Coordinate Systems

• Purpose: Any coordinate system provides a grid of perpendicular lines by which it is possible to specify the unique location of any single point on the celestial sphere.
• Base: Any spherical coordinate system is based on a great circle, which bisects the celestial sphere into two equal hemispheres.
  • Example: Is the horizon a great circle?
• Step 1: Each coordinate system begins with a measurement made along the great circle.
  • Example: A measurement along the horizon is called the azimuth.
• Step 2: Each coordinate system also involves measurements above or below the great circle.
  • Example: A measurement above or below the horizon is called the altitude.
• How: Measurements in angular degrees are made with a quadrant.
• A familiar coordinate system is based on the Earth's equator (terrestrial longitude and latitude).
• Three great circles are used as the basis of three different celestial coordinate systems:
  • Horizon (altitude and azimuth)
  • Celestial Equator (right ascension and declination)
  • Ecliptic (celestial latitude and celestial longitude)

Coordinate Systems review

1. Which celestial coordinate corresponds to a projection of earthly latitude?
2. Which celestial coordinate corresponds to a projection of earthly longitude?
3. What is the declination of the summer solstice?
4. What is the declination of the March equinox?
5. Celestial longitude is measured along the... (check one)
   • ecliptic
   • celestial equator