Sun path

Sun path refers to the apparent significant seasonal-and-hourly positional changes of the sun (and length of daylight) as the Earth rotates, and orbits around the sun. The relative position of the sun is a major factor in the heat gain of buildings and in the performance of solar energy systems. Accurate location-specific knowledge of sun path and climatic conditions is essential for economic decisions about solar collector area, orientation, landscaping, summer shading, and the cost-effective use of solar trackers.

Sun paths at any latitude and any time of the year can be determined from basic geometry. The Earth's axis of rotation tilts about 23.5 degrees, relative to the plane of Earth's solar system orbit around the Sun. As the Earth orbits the Sun, this creates the 47-degree peak-to-peak solar altitude angle difference, and the hemisphere-specific difference between summer and winter.

In the Northern Hemisphere, the winter sun (December, January, February) rises in the southeast, peaks out at a low angle in the south (more than halfway up from the horizon in the tropics), and then sets in the southwest. It is on the south (equator) side of the house all day long. Vertical south-facing (equator side) window is excellent for capturing solar thermal energy. For comparison, the winter sun in the Southern Hemisphere (June, July, August) rises in the northeast, peaks out at a low angle in the north (more than halfway up from the horizon in the tropics), and then sets in the northwest. There, the north-facing window would let in plenty of solar thermal energy to the house.

In the Northern Hemisphere in summer (June, July, August), the Sun rises in the northeast, peaks out slightly south of overhead point (lower in the south at higher latitude), and then sets in the northwest, whereas in the Southern Hemisphere in summer (December, January, February), the Sun rises in the southeast, peaks out slightly north of overhead point (lower in the north at higher latitude), and then sets in the southwest. A simple latitude-dependent equator-side overhang can easily be designed to block 100% of the direct solar gain from entering vertical equator-facing windows on the hottest days of the year. Roll-down exterior shade screens, interior translucent-or-opaque window quilts, drapes, shutters, movable trellises, etc. can be used for hourly, daily or seasonal sun and heat transfer control (without any active electrical air conditioning).

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