Terminology on the celestial sphere — which term is NOT the angular distance from the pole? On the observer’s meridian, the angular distance of a celestial object measured from the pole is called what? Identify the incorrect term among the following options.

Introduction / Context:
Precise terminology on the celestial sphere avoids confusion when reducing astronomical observations. Several “co-” quantities are defined as complements of primary angles. Only some of these relate directly to the angular distance of a celestial body from a pole along the meridian (i.e., the polar distance).

Given Data / Assumptions:

• We consider a celestial object lying on the observer’s meridian.
• Angular distance from the pole along the meridian is the focus.
• Standard celestial definitions apply.

Concept / Approach:
Polar distance (PD) is the angular distance of a body from the celestial pole: PD = 90° − δ (with sign conventions handled appropriately). Co-declination is defined as 90° − δ, which numerically equals the polar distance; hence, co-declination and polar distance are equivalent terms. Co-latitude equals 90° − φ and relates to the observer’s position, not directly to the body’s position but is sometimes involved in formulas. Co-altitude equals 90° − h and is measured from the zenith to the body, not from the pole. Therefore, among the listed terms, “co-altitude” is not the angular distance from the pole; it is the complement of altitude with respect to the zenith.

Step-by-Step Solution:

Verification / Alternative check:
Standard spherical astronomy texts equate PD with 90° − δ; co-altitude is used in the astronomical triangle but is referenced to the zenith side, confirming the distinction.

Why Other Options Are Wrong:

• Co-declination: correct synonym for polar distance.
• Polar distance: the exact definition of the distance from the pole.
• Co-latitude: not the body’s pole-distance, but not the most incorrect; still, it does not match the definition.

Common Pitfalls:
Mixing up complements referenced to different vertices of the astronomical triangle (pole vs. zenith vs. equator).

Final Answer:
Co-altitude