Passive solar design is based on utilizing the sun’s heat energy and its predictable movements through the seasons.
As the Earth rotates around the sun on its annual cycle, it is tilted at an angle on its vertical axis. This impacts how the sun’s rays strike various locations on Earth. The Earth is its most extreme tilt at the winter and summer solstices.
The sun appears to rise in the east and it sets in the west. In actuality, the Earth is rotating on its axis and around the sun.
Here is a diagram that compare the sun’s path on the winter and summer solstices.
This affects how low or high the sun appears in relation to the horizon.
In the winter, the sun is relatively low in the sky with its lowest arc through the sky on the winter solstice, on December 21st.
In the summer, the sun travels a high path through the sky and is at its highest angle on the summer solstice, on June 21st.
The equinox falls on the point between the solstices and indicates the arrival of spring or fall.
This picture shows the sun’s path throughout the year. The highest arc represents the sun’s path on the summer solstice, while the shortest, lowest arc is the sun’s path on the winter solstice.
On each equinox, the sun travels a path that is right in the middle of the path that it travels on the solstices.
March 21st – Vernal Equinox
June 21st – Summer Solstice (highest path)
Sept 21st – Autumnal Equinox
Dec 21st – Winter Solstice (lowest path)
Passive solar design uses the predictable movements of the sun to best utilize its energy within the building’s overall design both for heating and cooling purposes. Many passive solar buildings also include active solar aspects, such as photovoltaic panels, as shown on the roof in the first diagram located at the top of this post.