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Celestial Sphere

The 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

Celestial tilt

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