Solar Position

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Solar Position The geometric position of the sun as seen from a particular place on the surface of the earth varies from day to day and hour to hour. At any given instance, the sun’s position can be fixed by two angles, altitude and azimuth (Figure 10-2). A ltitude is the angle between the sun and the horizon. When the sun is on the horizon (at sunrise and sunset), this angle is zero. Solar altitude is at the maximum at solar noon. The complement of solar altitude angle, or the angle of the sun from a vertical line directly overhead, is called the zenith angle. A zimuth is the angle between a north-south line on the earth’s surface and the horizontal projection of the sun’s rays; it is measured from true south. By convention, solar azimuth is negative before noon and positive after noon. To calculate the sun’s angle we need to know not only the relative position of the sun, but also the geographical location of the observer on earth. Two angles define the position of any point on the surface of the earth. L atitude is the angular distance measured along a meridian north or south of the equator. All points on the equator have a latitude of zero; the north and south poles have latitudes of +90o (90o N) and –90º (90o S). Los Angeles is located at a latitude of 33.9º N. L ongitude is the angular distance measured east or west from the Prime (International) Meridian -- an imaginary circle passing through Greenwich, England. Since the period of rotation of earth about its own axis is 24 hours, each hour covers 360/24 = 15º longitude. Los Angeles has a longitude of 118° W. Calculating the relative position of the sun in the sky from earth at different locations and times is outside the scope of this book, but readers are encouraged to refer to more advanced texts.2


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