SOLAR TODAY - May 2012

scheMatics: allisoN Gray

In all but the simplest situations, knowl- edge of solar geometry is vital for creating successful solar-responsive designs. Even though we have known since Copernicus’ time that the Earth revolves around the sun, for our present purposes we will assume that the sun revolves around the building site. Not only is that the only way to understand solar geometry in relation to building design, but it is also what we see every day: the sun rises in the eastern sky and sets in the western sky.

Let’s also assume that a very large dome covers the building site, a dome that is opaque except for the part where the sun shines directly through during the year. This transparent part of the sky dome is called the solar window, and it is defined by the paths of the sun on the summer solstice (June 21) at the top and on the winter solstice (Dec. 21) on the bottom (see figure 1, at right). The other curves shown in figure 1 represent the 21st day of the other months. Although the solar window is essentially the same shape everywhere on the globe, its position on the sky dome changes with latitude. At the equator, its center is on the very top of the dome, and its position moves down as one moves away from the equator. Figure 1 shows the solar window for 32 degrees north latitude.

In temperate climates, the solar window has two parts. The upper part is called the “summer solar window,” which extends from the summer solstice (June 21) down to the part of the dome corresponding with the month when summer ends in that climate. It is shown in figure 2 (at right) in various shades of red to reflect the typical temperatures in each month (the darker the red, the hotter). The lower part of the solar window is called the “winter solar window,” which extends up from the winter solstice (Dec. 21) to the part of the dome corresponding with the month when winter ends in that climate. It is shown in figure 2 in shades of blue, again reflecting the typical temperatures in each month (the darker the blue, the colder). Although the location of the solar window is a function of latitude, the size of both the summer and winter solar windows is a function of the severity of the climate at the building site (i.e., how hot it is in the summer and how cold in the winter). One major goal of solar-responsive design is to collect the sun shining through the winter solar window while rejecting the sun when it shines through the summer solar window.

Also shown on the sky dome in figure 1 are reference lines to define the altitude (vertical angle) and azimuth (horizontal angle) of a sun ray passing through the solar window on its way to the building beneath the center of the

Designing to respond to
the thermal, Not solar, year

Sky domes are usually shown as horizontal projections, which are available at intervals of 4 degrees latitude (figure 3, page 30). Such projections are similar to what one would see looking directly down on the sky dome from an infinite distance. These horizontal projections

Figure 1. Solar geometry is best understood by means of an imaginary sky dome over the building site. The part of the sky dome through which the sun shines during the year is called the solar window. The curves represent the sun’s paths on the 21st day of each month. The compound angle of a sun ray can be broken into a vertical component (altitude) and a horizontal component ( azimuth). The skydome has a grid to help us measure these components. The parallel circles define the altitude angle, and the radial lines define the azimuth angle measured from south.

sky dome. The parallel circles define the altitude angle of the sun ray, with the base circle representing 0 degrees altitude and the highest point 90 degrees altitude. The radial lines define how many degrees the sun ray is from a north-south line. The radial line from the highest point to the south mark on the base represents 0 degrees azimuth. A sun ray from due east would have an azimuth of 90 degrees east of south, and, similarly, a sun ray from due west would have an azimuth of 90 degrees west of south.

Figure 2. During the summer, the sun shines through the upper part of the solar window (red), and during the winter, it shines through the lower portion (blue).

solartoday.org SOLAR TODA Y May 2012 29 are called sun path diagrams, because the curves represent the sun paths on the 21st day of each month. Only seven sun paths are needed to represent all 12 months because of the annual solar symmetry. The sun makes the same path across the sky dome on the 21st days of January and November, of February and October, of March and September, of April and August, and of May and July. Only the June 21 and Dec. 21 sun paths represent one day each. The altitude and azimuth lines are also projected as concentric circles and radial lines respectively.

To make use of sun path diagrams in designing buildings, it is critical to understand that the solar year is out of phase with the thermal year. Although the Earth receives the most solar