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 P: 49 Hi, When you have to calculate the rising or setting time of a celestial body, you have to handle with hour angle and sidereal time. Sidereal time for the rising is given by T = alpha - H and by T = alpha + H for the setting (alpha = right ascension). Why - H in one hand, and + H on the other hand ? Maybe because : 1/ hour angle H is computed from east toward west, so H=0 when crossing meridian, in south direction. Before crossing meridian, H = - $\left|H \right|$ so it's negative and it is positive after meridian is crossed H = $\left|H \right|$ 2/ we can say that rising and setting are symmetric with respect to meridian transit, which occurs when T = alpha (i.e H=0)... so -H for rising and +H for setting Which explanation is better ? If none of them, could you explain the right way ? Thnaks