It derives its name from the fact that solar radiation is focused on a line instead of on a point. Below is an illustration from the Solar Radiation Data Manual for Flat Plate and Concentrating Collectors . The time of day based on the longitude of the zone meridian associated with a locality. The East-West angular distance of a locality from the Prime Meridian . The Prime Meridian is the location of the Greenwich Observatory in England and all points North and South of it.
Longwave radiation is associated with heat energy.