@submariner- Stated simply, solar declination is the angle that the North Pole tips toward or away from the sun (Tilt of the earth's north and south axis). This is essentially the seasonal solar effect. The declination of the earth on the summer solstice is 23.45 degrees and during the winter solstice, it is -23.45 degrees. To find the declination for any given day you can use the following equation: 23.45 x sin[360/365(n+284)] where n is the day of the year.
Declination is important because it will tell you what angle solar panels should be set at during different times of the year. Panels should be set at the latitude of the location when solar radiation falls on earth at a perpendicular angle (zero degrees declination). The angle should be higher than the latitude in the winter and lower than the latitude in the summer. The panels should face south in the northern hemisphere, and north in the southern hemisphere.