# Solar geometry

Solar geometry is the measurement of the angle of the sun to the earth and the corresponding amount of solar energy hitting a given object or surface. It is helpful to consider the position of the sun when deciding the placement of a structure’s windows and the amount of sunlight entering the building. Faces of the building receive differing amounts and quality of light depending on their orientation, time of year and time of day. The position of the sun is furthermore vital to solar energy and sustainable building designs. The position of the sun can be described by two different angles. The first angle is the solar azimuth (denoted by α, alpha), which is defined as the clockwise angle between the sun and the direction of true north. It is measured up to the horizontal projection of the sun’s position onto the Earth’s surface. The second angle is the solar altitude or elevation (denoted by Φ, phi), indicating the angle of the sun’s position from the horizontal. The angle of incidence is not a measure of the sun’s position, but rather a measure of the amount of radiation incident on a vertical surface. Together, the two angles provide useful information about the orientation of incoming sunlight on an object or structure. Understanding sun angles is imperative in designing for sun access and penetration into interior spaces.

The Finnish architect Alvar Aalto (1898-1976) used solar geometry diagrams in the design of the Piamio Sanatorium, drawing different angles of sunlight to ascertain how light would enter patient rooms, enhancing their therapeutic treatment.