Hi all, According to the sunrise equation, the hour angle of the sun at sunset is: cos H = -tan(a)tan(d) where H = the hour angle, a = latitude and d = solar declination angle. This equation says that H at sunset = -H at sunrise. Now, I have a few questions concerning that: 1) I was wondering how you could calculate the change of the hour angle in between...? So, dH/dt = d(H at sunset - H at sunrise)/(time of sunset - time or sunrise)...? 2) I was calculating some values and found that, for example, when the sun rises with at an hour angle of -1.641 radians, it must go down at the angle of +1.641 rad, but if you calculate the difference between both it is not equal to pi (or 180 degrees)? How does that come? Thanks already!