Si la declinación de un astro es igual que la latitud de un observador en latitud N:If the declination of a celestial body equals the latitude of an observer in North latitude:

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Si la declinación de un astro es igual que la latitud de un observador en latitud N:If the declination of a celestial body equals the latitude of an observer in North latitude:

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Correct answer: d) None of the answers is correct.

Why it is correct

If δ = φ, the body passes through the zenith (Ho = 90°) only at the instant of upper meridian transit (when LHA = 0°); throughout the rest of the day its hour angle runs from 0° to 360° and its altitude is less than 90°. Moreover, it is only circumpolar if φ ≥ 45° N.

Why the others are not

a) Local hour angle changes continuously with Earth’s rotation (about 15° per hour).
b) True altitude is 90° only at the instant of upper meridian transit.
c) Being circumpolar requires δ ≥ 90° − φ, which for δ = φ only holds if φ ≥ 45°.

How to work it out in the exam

If δ = φ: passes through the zenith only at upper transit (not always at 90° altitude or circumpolar).

Where this question comes from

Junta de Andalucía · 2016 · 1ª convocatoria · CY Módulo de navegación · question 5 Official paper · Official answer key

Try it: Sight reduction

Pending instructor review. Machine translation, not reviewed.