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ABOVE: A basic illustration that shows the ability of a properly oriented building ability to collect, store and release solar energy over the course of a solar day.

ABOVE: Example of a house with a clear story. A series of windows that are designed to allow light and (during winter) solar radiation into the building.

ABOVE: Artist illustration showing the three primary sun positions over a 365 day period. Winter Solstice, Equinox and Summer Solstice. BELOW: Artist illustration showing the effects of solar radiation penetration into a passive solar designed and orientated building. Notice the overhang and how is helps to control the direct radiation from the summer sun, but the winter sun is allowed to heat the interior space naturally.

Passive solar buildings aim to maintain interior thermal comfort throughout the sun's daily and annual cycles whilst reducing the requirement for active heating and cooling systems. Passive solar building design is one part of green building design, and does not include active systems such as mechanical ventilation or photovoltaic’s.

The scientific basis for passive solar building design has been developed from a combination of climatology, thermodynamics (particularly heat transfer), and human thermal comfort (for buildings to be inhabited by humans). Specific attention is directed to the site and location of the dwelling, the prevailing climate, design and construction, solar orientation, placement of glazing-and-shading elements, and incorporation of thermal mass. While these considerations may be directed to any building, achieving an ideal solution requires careful integration of these principles. Modern refinements through computer modeling and application of other technology can achieve significant energy savings without necessarily sacrificing functionality or creative aesthetics In fact it is for this reason that this newly coined term, known as Architectural Science or Architectural Technology, has become an upcoming subject area in most schools of Architecture worldwide.

he ability to achieve these goals simultaneously is fundamentally dependent on the seasonal variations in the sun's path throughout the day

This occurs as a result of the inclination of the earth's axis of rotation in relation to its orbit. The sun path is unique for any given latitude. Generally the sun will appear to rise in the east and set in the west.

In Northern Hemisphere non-tropical latitudes farther than 23.5 degrees from the equator:

· The sun will reach its highest point toward the South (in the direction of the equator)

· As winter solstice approaches, the angle at which the sun rises and sets progressively moves further toward the South and the daylight hours will become shorter

· The opposite is noted in summer where the sun will rise and set further toward the North and the daylight hours will lengthen

The converse is observed in the Southern Hemisphere, but the sun rises to the east and sets toward the west regardless of which hemisphere you are in.

In equatorial regions at less than 23.5 degrees, the position of the sun at solar noon will oscillate from north to south and back again during the year.

In regions closer than 23.5 degrees from either north-or-south pole, during summer the sun will trace a complete circle in the sky without setting whilst it will never appear above the horizon six months later, during the height of winter.

The 47-degree difference in the altitude of the sun at solar noon between winter and summer forms the basis of passive solar design. This information is combined with local climatic data (degree day) heating and cooling requirements to determine at what time of the year solar gain will be beneficial for thermal comfort, and when it should be blocked with shading. By strategic placement of items such as glazing and shading devices, the percent of solar gain entering a building can be controlled throughout the year.

One passive solar sun path design problem is that although the sun is in the same relative position six weeks before, and six weeks after, the solstice, due to "thermal lag" from the thermal mass of the Earth, the temperature and solar gain requirements are quite different before and after the summer or winter solstice. Movable shutters, shades, shade screens, or window quilts can accommodate day-to-day and hour-to-hour solar gain and insulation requirements.

Careful arrangement of rooms completes the passive solar design. A common recommendation for residential dwellings is to place living areas facing solar noon and sleeping quarters on the opposite side. A heliodon is a traditional movable light device used by architects and designers to help model sun path effects. In modern times, 3D computer graphics can visually simulate this data, and calculate performance predictions.

ABOVE: While these structures were built primarily for defense, they also take advantage of passive solar heating.