ARCH 348 SOLAR CONTROL IN ARCHITECTURE

3. Solar Geometry

3.1. The Motion of the Earth Around the Sun

        Before entering into the description of the solar control, it is necessary to give a short summary of the astronomical relations of sun and the earth.

        The Earth travels in an almost circular orbit around the sun. It is rotating about its own axis (N,S) at the rate of one revolution per day, and it is moving at the same time in the closed path in a year. Its axis (N, S) is not perpendicular to the plane of its orbit ,it is tilted 23 ° 27’ (assume the 23.5 
° for practical purpose) with respect to the plane of the earth’s orbit around the sun. This plane, geometrically described by the Sun-Earth line, is called by astronomers the solar “ecliptic”. It is useful to visualize the Sun-Earth line as a cluster parallel light beams. On two days of the year, March 21 and September 21 (Spring and Fall Equinox), the sun’s light beams are parallel to the earth’s equatorial plane. From the earth’s point of view on these two dates, the sun rises and sets due east and west, respectively. From the sun’s position, an observer would see the earth’s tilt at 23.5° with respect to the ecliptic. The angle measured at any point on the earth’s surface between the sun-earth line and the plane defined by the earth’s equator is the “solar declination”. Because the two are parallel at the spring and fall Equinox, the declination angle is 0° on these two dates. The declination angle varies between +23.5° on June 21 and –23.5° on December 21.To understand sun angles for purpose of building design, the earth’s orbit can be considered circular with the sun at its center. In fact, the orbit is an ellipse with the sun off-center, with resulting changes in orbital speed, slowing as the earth moves closer to the sun and accelerating as it moves away. The eccentricity and obliquity of the earth’s orbit result in differences between Solar Time, measured by the sun’s position in the sky, and Mean Time, measured by our clocks running at constant speed. The Equation of Time is a formula by which to correct the discrepancies between Solar Time and Mean Time.

        The earth’s position, tilted with respect to its orbital plane around the sun, provides the geometric basis for the annual variation in solar energy received on the earth’s surface.

3.2. Graphical Methods to Determine Sun’s Path Equidistant Sun Path Diagram

        For an efficient solar control in architecture the relationship between the sun and the building should be understood in order to achieve a successful design.

        The solar-control means; to elininate or minimize the sun's effect on the building when we do not want more heat and glare during overheated period of the year. To get maximum solar radiation when we need heat during the under-heated period.

        For solar control first of all, the position of he sun in relation to the building elevation on a specific date should be determined.

3.2.1. Solar Angles

        The position of the sun in the sky hemisphere can be determined by two angles;

        a.    Solar Altitude Angle ( θ ) is the vertical angle at the point of observation, between the horizon plane and the line connecting the sun to the 
observer.

        b.    Solar Azimuth Angle ( α ) is the angle at the pint of observation measured at the horizontal plane between the north direction and the vertical plane containing the sun.

        Azimuth angle is measured clockwise from North towards East. Thus;

                North Direction  : (α ) = 0˚ or 360˚ 
                East Direction    : (α ) = 90˚
                South Direction  : (α ) = 180˚
                West Direction   : (α ) = 270˚

        These two angles (altitude and Azimuth) can be read directly for any date of the year and any hour of the day from SOLAR CHARTS or SUN PATH DIAGRAMS prepared for a specific latitude.

3.2.2. Sun Path Diagrams

        There are several methods of projections to present the apparent movement of the sun on the sky hemisphere. By using any of these projection methods, the apparent three-dimensional movement of the sun can be represented on a two dimensional chart which is called SOLAR CHARTS or SUN PATH DIAGRAM. The most commonly used projections are EQUIDISTANT and STEREOGRAPHIC Projection Methods.

3.2.3. Equidistant Sun Path Diagram

        Sun path diagram prepared by the aid of the equidistant projection gives easy and direct reading to the users. On this diagram;

        a)    The circle with 90 mm radius represents the sky hemisphere on the horizontal plane.
        b)    The center of the circle represents observation point.
        c)    The perimeter scale gives the azimuth angles (α).
        d)    Concentric circles give the altitude angles (θ).
        e)    Group of curves extending from East to West show the sun path at various dates. The two extreme curves show sun path in two solstices., June 21 and December 21. Sun paths for other days lie between these extremes.
        f)     Vertical radius represent solar noon. Group of curved lines on both sides of the vertical radius represent solar hours between sunrise and sunset. 

        Sunrise and sunset times can be read from the intersection of sun path curve and the peripheral circle.

        In equinox days, March 21 and September 21, the sun rises at 6:00 am and sets at 6:00 pm. In summer the sun rises earlier and sets later, in winter it rises late and sets earlier.

3.2.4. Definition of the Sun`s Position ( Azimuth and Altitude Angles)

        For a certain location, for a certain day and hour, azimuth and altitude angles may be defined by the following procedure. For this purpose the sun path diagram prepared for this location should be used.

Example : Define the position of the sun in Gazimagusa at 9:00 am of December 21.