The astronomical Almanac takes account of refraction and solar radius : rise time and set time must be calculated for the solar limb , not for the center.
Best regards Alberto Nicelli Italy (45,5 N : 7,50 E) > ---------- > From: reto ambrosini[SMTP:[EMAIL PROTECTED] > Sent: venerdì 20 marzo 1998 20.01 > To: [email protected] > Subject: cos(T)=-tan(lat)*tan(decl) > > Hello All ! > > I tried to calculate the sunrise time with the formula > > cos(T)=-tan(lat)*tan(decl) > > I took the datas from the astronomical Almanac, here is an > example for the 3. of september 1998 at latitude +46 > > Sun declination : 7 42 13.5 = 7.704 deg > Rise time : 5h23m > Set time : 18h35m > Day length : 13h12m > > T=arccos(-tan(46)*tan(7.704))=98.05 deg =6.537 hours > > Calculated rise time : 12-6.537 = 5.463h = 5h27m47s > Calculated set time : 12+6.537= 18.54h = 18h32m13s > Calculated day length : 6.537*2 = 13.07 h = 13h04m25s > > As you see there is a difference between the calculated datas > and the datas of the nautical almanac, the difference in the day > length is about 8 minutes (2 degrees!) that's quite a lot. > > Why do I have such a difference? > Did I do some wrong calculation or is this formula only > approximative? > > Thanks > > Reto Ambrosini > > [EMAIL PROTECTED] > > > >
