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.[1] 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.[2]
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To gather solar energy effectively, a solar collector (glass, solar panel, etc.) should be within about twenty degrees either side of perpendicular to the sun. Also, shades need to be placed, so that the building does not warm up too much in summer and then thus requires cooling. The farther from perpendicular, the lower the solar gain. More than thirty-five degrees from perpendicular results in a significant portion of sunlight being reflected off the solar collector surface.
An effective solar energy system (passive solar, active solar, building, equipment, etc.), takes into account the significant seasonal 47-degree solar elevation angle difference above the horizon, and the sunrise/sunset solar azimuth angle from summer to winter.
Precise knowledge of the path of the sun is essential to accurately model, and mathematically predict, annualized solar system performance - To explain, for example, why vertical equator-facing glass is cost-effective, the benefit of solar energy reflectivity off winter snow when the sun is low, and why roof-angled glass (in greenhouses, skylights and conservatories) can be a solar furnace during the summer, (when the sun is nearly perpendicular to the glass), and then lose more energy in the winter than it collects, (when the sun is 47-degrees lower on the horizon, and warm interior air rises and transfers heat out of the building on cold winter nights).[1]
Earth's rotation tilts about 23.5 degrees on its pole-to-pole axis, relative to the plane of Earth's solar system orbit around our sun. As the Earth orbits the sun, this creates the 47-degree peak solar altitude angle difference, and the hemisphere-specific difference between summer and winter.
In the northern hemisphere, the winter sun rises in the southeast, peaks out at a low angle above the southern horizon, and then sets in the southwest. It is on the south (equator) side of the house all day long. Vertical south-facing (equator side) glass is excellent for capturing solar thermal energy.
In the northern hemisphere in summer (June, July, August), the sun rises in the northeast, peaks out nearly straight overhead (depending on latitude), and then sets in the northwest. 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).
The latitude (and hemisphere)-specific solar path differences are critical to effective passive solar building design. They are essential data for optimal window and overhang seasonal design. Solar designers must know the precise solar path angles for each location they design for, and how they compare to place-based seasonal heating and cooling requirements.
In the U.S., the precise location-specific altitude-and-azimuth seasonal solar path numbers are available from NOAA - The "equator side" of a building is south in the northern hemisphere, and north in the Southern hemisphere, where the peak summer solstice solar altitude occurs on December 21. The sun rises roughly in the east and sets in the west everywhere on Earth, except in high latitudes in summer- and winter-time.
On the Equator, the sun will be straight overhead and a vertical stick will cast no shadow at noon (solar time) on March 21 and September 23, the equinox. 23.5 degrees north of the equator on the Tropic of Cancer, a vertical stick will cast no shadow on June 21, the summer solstice for the northern hemisphere. The rest of the year, the noon shadow will point to the North pole. 23.5 degrees south of the equator on the Tropic of Capricorn, a vertical stick will cast no shadow on December 21, the summer solstice for the southern hemisphere, and the rest of the year its noon shadow will point to the South pole. North of the Tropic of Cancer, the noon shadow will always point north, and conversely, south of the Tropic of Capricorn, the noon shadow will always point south. North of the Arctic circle, and south of the Antarctic circle there will be at least one day a year when the sun is not above the horizon for 24 hours, and at least one day (six months later) when the sun is above the horizon for 24 hours.
In the moderate latitudes (between the circles and tropics, where most humans live), the length of the day, solar altitude and azimuth vary from one day to the next, and from season to season. The difference between the length of a long summer day, versus a short winter day increases as you move farther away from the equator.[2]
Before the days of modern, inexpensive, 3D computer graphics, a heliodon (precisely-movable light source) was used to show the angle of the sun on a physical model of a proposed building. Today, mathematical computer models calculate location-specific solar gain (shading) and seasonal thermal performance, with the ability to rotate and animate a 3D color graphic model of a proposed building design.
Passive solar building design heating and cooling issues can be counterintuitive (like roof-angled glass). Precise performance calculations and simulations are essential to help avoid reinventing the wheel and duplicating previously-made expensive experimental construction errors (like a summer solar furnace).