1. The problem statement, all variables and given/known data I need to calculate the length of daylight and all of my answers are about 15 minutess too long according to actual data on the web. Can someone give me insight as to why the equation is off by this much? 2. Relevant equations Daylight=2[cos-1(-tan(latitude degree)tan(declination angle degree))] 3. The attempt at a solution For example, the length of day at latitude 80 degrees on February 1st, which should be zero, works out to 0.244 hours or 15 minutes. Declination angle is calculated to be -17.9 degrees. Maybe that's the problem? Thanks for any help.