LESSON 15: Celestial Coordinate Systems

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# LESSON 15: Celestial Coordinate Systems - PowerPoint PPT Presentation

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

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Presentation Transcript
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 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
• 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
• 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 Equator (Equinoctial)
• Celestial Meridians
• Hour Circles
• Hour Circle of Aries
• Declination
• celestial equivalent of latitude
• Hour Angles
• celestial equivalent of longitude
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 Angles

GHA = GHA(Aries) + SHA

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
• 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 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
• 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 Triangle
• Azimuth Angle (Z)
• Meridian angle (t)