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 = - [itex]\left|H \right|[/itex] so it's negative and it is positive after meridian is crossed H = [itex]\left|H \right|[/itex]
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 ?