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Día 21 de Abril de 2021. Hora civil de Greenwich (HcG)= 02:26, se encuentran en situación estimada= 31º 32,8´N – 065º 53,3´W, continúan navegando a todo trapo por rumbo ortodrómico con viento y marejada del WNW y en ese instante el Capitán obtiene nuevo horario en Greenwich de Aries (HGY)= 245º 52,8´y calcula el horario en Greenwich de la estrella “Arcturus” (HG*), sabiendo que el ángulo sidéreo de esta estrella (A.S.) es= 145º 50,4´On 21 April 2021 at HcG = 02:26, in DR position 31º 32.8' N, 065º 53.3' W, given the Greenwich hour angle of Aries (HGY) = 245º 52.8' and the sidereal hour angle (A.S.) of Arcturus = 145º 50.4', calculate the Greenwich hour angle of Arcturus (HG*):
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Correct answer: d) HG* = 031º 43.2'
Why it is correct
A star's Greenwich hour angle is the sum of the GHA of Aries and its sidereal hour angle: HG* = 245º 52.8′ + 145º 50.4′ = 391º 43.2′. Subtracting a full circle (360º) gives HG* = 031º 43.2′.
Why the others are not
- a) Subtracts the sidereal hour angle from GHA Aries instead of adding it.
- b) Mixes the DR longitude into a calculation that only asks for Greenwich hour angle.
- c) Subtracts longitude from the result instead of giving HG*.
How to work it out in the exam
- GHA* = GHA Aries + SHA* (if over 360º, subtract 360º).
Where this question comes from
- Región de Murcia · 2021 · 1ª convocatoria · CY Tipo 1 · question 19 Official paper · Official answer key
Pending instructor review. Machine translation, not reviewed.