|| Starpath online classroom || Celestial Navigation Glossary || Glossary Index || Home ||

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z

Zenith Distance The angular distance between the point directly overhead (the zenith) and the celestial body sighted. It is the complement of the observed height (Ho) figured from: z = 90° - Ho. Zenith distance is a key concept in celestial navigation since it can be shown to be numerically equivalent to the distance between the observer and the geographical position (GP) of the sighted body. If a star is 70° above the horizon, its zenith distance is 20°, and at that moment you are located precisely 20° x (60 miles/1°), or 1,200 miles from the GP. The Nautical Almanac tells precisely where the GP is located at all times, so this observation alone has established a giant circle of position. The rest of celestial navigation covered in a sight reduction is just a trick to get a segment of this circle onto a plotting sheet, so we can call it a line of position. See Geographical Position, Observed Height, Sight Reduction, Circle of Position, and Nautical Mile. Abbreviation:  z

[close window]