Daylight Hours Length of daylight (in addition to solar intensity) is another principal factor that determines solar radiation receipt on earth. Circle of illumination – any given time half of the earth is illuminated by the sun. Daylight length changes daily throughout the year for every location not on the equator So, solar radiation exposure time, like solar intensity, is not distributed uniformly on earth. Except on the equinoxes (everywhere receives 12 hrs of days and 12 hrs of night).
Solstices Earth – Sun Geometry at 40 o N: Solstices Sun has it’s longest arc… Rises North of East Sets North of West
Solstices Earth – Sun Geometry at 40 o N: Solstices Sun has it’s shortest arc… Rises South of East Sets South of West
Equinoxes Earth – Sun Geometry at 40 o N: Equinoxes Equinoxes: –Midway between solstices Autumnal equinox – September 22 or 23 Vernal equinox – March 21 or 22 Sun arc is “normal”… Rises in the East Sets in the West