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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.
Collecting solar energy
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).
Effect of the Earth's axis tilt
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 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 solar noon on the equinoxes. On the vernal equinox, north of the subsolar point (on the equator) the vertical stick's shadow will point a little westwards of true north (NNW) reading 336.5° from true north and little eastwards of true south (SSE) reading 156.5° from true north. On the autumnal equinox, north of the subsolar point (on the equator), the shadow will point a little eastwards of true north (NNE) reading 23.5° from true north (and south of the subsolar, the shadow will point a little westward of true south (SSW) reading 203.5° from true north).
The same stick will cast no shadow on the summer solstice in the Northern Hemisphere when the subsolar point is on the Tropic of Cancer 23.5° north of equator. Although north of 23.5°N the shadow will point towards true north and south of 23.5°N the shadow will point towards true south. The reverse occurs on the winter solstice in the Northern Hemisphere when the subsolar point will be on the Tropic of Capricorn 23.5°S and a vertical stick will cast no shadow along that point. But north of the Tropic of Capricorn solar noon shadows will point towards true north and south of the Tropic of Capricorn shadows will point towards true south.
The solar noon shadows of objects on points beyond and below subsolar points will point towards true north and true south respectively only when the solar declination has its maximum positive (δ☉ = +23°26') or maximum negative (δ☉ = −23°26') value. On the other hand, on the equinoxes when the sun is neither declined north nor south (δ☉ = 0°) and solar time noon shadows point NNW north of the equator and SSE south of the equator on the vernal equinox (and point NNE north of the equator and SSW south of the equator on the autumnal equinox).
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 during the winter solstice, and at least one day when the sun is above the horizon for 24 hours during the summer solstice.
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 and a short winter day increases as one moves farther away from the equator.
Building design simulation
|This section does not cite any references or sources. (September 2013)|
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.
Heating and cooling issues in passive solar building design can be counterintuitive. Precise performance calculations and simulations are essential to avoid reinventing the wheel and duplicating expensive experimental design errors, such as skylights that turn a building into a solar furnace in summer (see Daylighting#Sawtooth_roof).
- Passive solar design
- Solar access
- Effect of sun angle on climate
- Position of the Sun
- "Solar Resource Information". National Renewable Energy Laboratory. Retrieved 2009-03-28.
- Khavrus, V.; Shelevytsky, I. (2010). "Introduction to solar motion geometry on the basis of a simple model". Physics Education 45 (6): 641. doi:10.1088/0031-9120/45/6/010.
- Khavrus, V.; Shelevytsky, I. (2012). "Geometry and the physics of seasons". Physics Education 47 (6): 680. doi:10.1088/0031-9120/47/6/680.
- Librorum, Helluo (2012). "Notes from Noosphere: The simple geometry of sun, moon, and star paths". notesfromnoosphere.blogspot.com. Retrieved September 19, 2013.
- U.S. Naval Observatory Sun or Moon Altitude/Azimuth Table
- The simple geometry of sun, moon, and star paths
- Sun path in augmented reality
- Sun path by location and date
- Sun positions, diagram and paths around the world by location and date
- Seasonal and Hourly Sun Path Design Issue Tutorial
- "Three Decades of Passive Solar Heating and Cooling Lessons Learned"
- Sun path on map, charts and table, sun position for every location and date