Polar distance (astronomy)
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Polar distance (PD) is an astronomical term associated with the celestial equatorial coordinate system Σ(α, δ) and it is an angular distance of a celestial object on its meridian measured from the celestial pole, similar as declination (dec, δ) is measured from the celestial equator:
Polar distance (PD) = 90° ± δ
Polar distances are expressed in degrees and cannot exceed 90° in magnitude. An object on the celestial equator has a PD of 90°.
Polar distance is not 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.