Celestial Sphere

• An imaginary, hollow dome surrounding the Earth
  • It's essentially the sky we see when we look up
  • Earth's coordinate system has been extended to the sphere
• It's what you see when you look up at the sky; you are at the center
• The path the Sun takes on the celestial sphere is called the ecliptic
  • Apparent yearly path of the Sun
  • Projection of the Earth's orbit onto the sky
  • Tilted by 23.5 degrees with respect to the celestial equator

Coordinate System

• Declination
  • Vertical position of an object
  • Celestial equivalent of latitude
  • Degrees, arcmin, arcsec
• Right ascension
  • Horizontal position of an object
  • Celestial equivalent of longitude
  • Expressed as either degrees or units of time
    • Celestial sphere turns around the Earth once a day
      • As Earth turns on its axis
      • 360˚ can easily just be 24 hours
      • 15˚ arc is equal to 1 hour
    • Hours, mins, sec
  • Starts getting counted at the vernal equinox
• Altitude
  • Vertical position of an object
  • Angle from the horizon to a point on the sphere
  • Zenith is 90˚
  • Altitude of Polaris with give you the latitude of where you are on Earth
• Azimuth
  • Horizontal position of an object
  • Angle measured clockwise around observer's horizon from north
  • Object due north has azimuth of 0˚

Relative to Viewer

• Zenith
  • Point directly above your head
• Horizon
  • Where the dome meets the Earth

Space Astronomy