Polar distance (astronomy)
In the celestial equatorial coordinate system
Definition[edit]
Polar distance in celestial navigation is the angle between the pole and the Position of body on its Declination.
Referring to diagram:
P- Pole , WQE- Equator , Z - Zenith of observer ,
Y- Lower meridian passage of body
X- Upper meridian passage of body
Here body will be on declination circle ( XY). The distance between PY or PX will be the Polar distance of the body.
NP=ZQ=Latitude of observer
NY and NX will be the True altitude of body at that instant.
Polar distance (PD) = 90° ±
Polar distances are expressed in degrees and cannot exceed 180° in magnitude. An object on the celestial equator has a PD of 90°.
Polar distance is affected by the precession of the equinoxes.
If the polar distance of the Sun is equal to the observer's latitude, the shadow path of a gnomon's tip on a sundial will be a parabola; at higher latitudes it will be an ellipse and lower, a hyperbola.