Theme: Geodesy

Observational concepts

Here we define some basic observational concepts, like angles and coordinates, defined in geodesy (and astronomy as well), mostly from the viewpoint of the local observer.

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  Plumbline or vertical: the direction of local gravity, or the line that results by following it.
  
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  Zenith: the point on the celestial sphere where the direction of the gravity vector in a point, extended upwards, intersects it. It is more correct to call it adirection rather than a point.
  
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  Nadir: the opposite point—or rather, direction—where the direction of gravity extended downward intersects the (obscured) celestial sphere.
  
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  Celestial horizon: a plane perpendicular to a point's gravity vector.
  
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  Azimuth: the direction angle within the plane of the horizon, typically counted clockwise from the north (in geodesy and astronomy) or the south (in France).
  
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  Elevation: the angular height of an object above the horizon, Alternatively zenith distance, being equal to 90 degrees minus elevation.
  
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  Local topocentric coordinates: azimuth (direction angle within the plane of the horizon), elevation angle (or zenith angle), distance.
  
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  North celestial pole: the extension of Earth's (precessing and nutating) instantaneous spin axis extended northward to intersect the celestial sphere. (Similarly for the south celestial pole.)
  
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  Celestial equator: the (instantaneous) intersection of Earth's equatorial plane with the celestial sphere.
  
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  Meridian plane: any plane perpendicular to the celestial equator and containing the celestial poles.
  
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  Local meridian: the plane containing the direction to the zenith and the direction to the celestial pole.