from the well known
senH = senD*senL + cosD*cosL*cosT
at sunrise and/or sunset (with no semi-diam... , refrac...etc...)
H = 0  --> senH = 0
0 = senD*senL + cosD*cosL*cosT
 - senD*senL = cosD*cosL*cosT
 - (senD*senL)/(cosD*cosL) = cosT
 - (senD/cosD)*(senL/cosL) = cosT
 - tgD*tgL = cosT
 
jorge

Reply via email to