# Definitions

There are a few basic terms in common use to describe the attributes of direct solar radiation. These are defined below, without lengthy explanation. The full implications of their meaning will become clear later in the text.

Altitude of the sun (a). This is the angle a direct ray from the sun makes with the horizontal at a particular place on the surface of the earth. It is illustrated in Figure 7.2(a). For a given date and time, the sun’s altitude is different at different places over the world.

Azimuth of the sun (z). This is the angle the horizontal component of a direct ray from the sun makes with the true north-south axis. It may be expressed in degrees west or east of south (in the northern hemisphere), as illustrated in Figure 1.2(b), but is more commonly given as an angular displacement through 360° from true north.

Wall-solar azimuth (n). This is the angle the horizontal component of the sun’s ray makes with a direction normal to a particular wall. It is illustrated in Figure 1.2(c), where it can be seen that this angle, referring as it does to direct radiation, can have a value between 0° and 90° only. When the wall is in shadow (n > 90°) the value of n has no meaning.

Declination (d). This is the angular displacement of the sun from the plane of the earth’s equator. The value of the declination will vary throughout the year between +23 and -23 V20 because the axis of the earth is tilted at an angle of about 23 j ° to the axis of the plane in which it orbits the sun. Declination is expressed in degrees north or south (of the equator). The geometry of declination is illustrated in Figure 7.3.

 (a) (b)  North Fig. 7.2 (a) Definition of solar altitude; (b) Definition of solar azimuth; (c) Definition of Wall-solar azimuth. (c) Latitude (L). The latitude of a place on the surface of the earth is its angular displacement above or below the plane of the equator, measured from the centre of the earth. It is illustrated in Figure 7.4. The latitude at London is about 51.5° North of the equator.

Longitude. This is the angle which the semi-plane through the poles, and a particular place on the surface, makes with a similar semi-plane through Greenwich. The semi-plane N Fig. 7.4 The latitude of a place on the surface of the earth.

Through Greenwich is an arbitrary zero, and the line it makes in cutting the earth’s surface is termed the Greenwich meridian. Longitude is measured east or west of Greenwich and so its value lies between 0° and 180°. Latitude and longitude together are co-ordinates which locate any point on the surface of the earth.

Sun time (7). This is the time in hours, before or after noon, noon being defined as the time when the sun is highest in the sky. It is sometimes called local apparent time.

On the Greenwich meridian, sun time does not always exactly equal Greenwich Mean Time (or the local mean time for other longitudes). This is because the orbital velocity of the earth in its path about the sun is not a constant, since the orbit is elliptical, not circular. The variation, according to the ASHRAE Handbook (1997a), is the amount by which the sun time is different from the local mean time (for example from GMT on longitude 0°) and varies from nearly -14 minutes to just over +15 minutes, depending on the month. A good approximation is 4 minutes of time for each degree of longitude, as implied by the hour angle.

Hour angle (h). This is the angular displacement of the sun from noon. Thus,

Or, put another way 1 hour corresponds to 15° of angular displacement.

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