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LESSON 15: Celestial Coordinate Systems. Learning Objectives Know the ultimate goal of celestial navigation. Know the definitions of terms and components associated with the terrestrial, celestial , and horizon coordinate systems.

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lesson 15 celestial coordinate systems
LESSON 15:Celestial Coordinate Systems
  • Learning Objectives
    • Know the ultimate goal of celestial navigation.
    • Know the definitions of terms and components associated with the terrestrial, celestial, and horizon coordinate systems.
    • Know the relationship between the terrestrial, celestial, and horizon coordinate systems.
    • Apply correct procedures to describe the location of a celestial body in reference to the celestial and horizon coordinate systems.
the goal of celestial navigation
The Goal of Celestial Navigation
  • The solution of spherical triangles of sides based on the observed positions of celestial bodies, in order to determine the position of a vessel.
  • 100 years ago, this involved some complicated spherical trigonometry.
  • Today, it requires the use of tables or a navigational calculator (HP makes one).
basic assumptions
Basic Assumptions
  • First, assume the earth does not move, but instead the celestial bodies rotate about it in a predictable manner.
  • The celestial sphere: celestial bodies are assumed to be on the inner surface of a vast, hollow sphere of infinite radius, which has the earth at its center.
2 celestial coordinate system
2. Celestial Coordinate System
  • The celestial coordinate system is best understood as a projection of the terrestrial coordinate system, outward into space onto the celestial sphere.
celestial coordinate system
Celestial Coordinate System
  • Celestial Equator (Equinoctial)
  • Celestial Meridians
  • Hour Circles
  • Hour Circle of Aries
  • Declination
    • celestial equivalent of latitude
  • Hour Angles
    • celestial equivalent of longitude
hour angles
Hour Angles
  • Three different references are used:
  • Sidereal Hour Angle (SHA)
    • Hour Circle of a celestial body, as referenced from the hour circle of Aries (measured westerly)
  • Greenwich Hour Angle (GHA)
    • Hour circle of a celestial body, as measured relative to Greenwich Celestial Meridian
  • Local Hour Angle (LHA)
    • Hour circle of a celestial body, as measured relative to the local celestial meridian
hour angles9
Hour Angles

GHA = GHA(Aries) + SHA

3 horizon coordinate system
3. Horizon Coordinate System
  • In the terrestrial and celestial coordinate systems, the basic references are the poles and the equator.
  • The horizon coordinate system, however, is based on the observer’s position.
  • This system is necessary because stars are sighted with respect to the observer’s position.
horizon coordinate system
Horizon Coordinate System
  • Zenith and Nadir
    • analogous to north and south poles
  • Vertical Circle
    • analogous to meridian (terrestrial system) or hour circle (celestial system)
  • Prime Vertical (east and west)
  • Principal Vertical (north and south)
horizon coordinate system13
Horizon Coordinate System
  • Celestial Horizon
    • analogous to the equator
  • True Azimuth (Zn)
    • analogous to longitude
  • Altitude
    • analogous to latitude
  • Sextant Altitude (hs)
    • Measured altitudes of celestial bodies
  • Visible or Sea Horizon
    • Used as a reference for celestial body altitude measurements
  • Observed Altitude (Ho)
    • Conversion of Sextant Altitude, since sea horizon is not at celestial horizon.
the celestial triangle
The Celestial Triangle
  • These three coordinate systems (TERRESTRIAL, CELESTIAL, AND HORIZON) are then combined to form the celestial triangle, which is used to determine our position.
  • One leg from each triangle forms the new triangle on the celestial sphere.
the celestial triangle16
The Celestial Triangle
  • Azimuth Angle (Z)
  • Meridian angle (t)