The geometrical position of the sun relative to a location is specified by an altitude angle a_s, and an azimuth angle g_s of a coordinate centered at the point of observation on earth. Figure 1.1.1 illustrates the geometrical configuration. It is customary to set the reference coordinate in the cardinal directions.

The solar altitude angle is the angle measured between the line drawn towards the sun from a point on earth and the horizontal surface. The solar azimuth angle is the angle the projection of the line to the sun makes with the southern direction. The zenith angle of the sun is the angle between the line to the sun makes with the zenith or the direction perpendicular to the earth. The sum of altitude angle and zenith angle equals 90 º .

At any given moment, the positions of the sun as seen from different locations on earth appear differently. At noon on 21 March, an observer on the equator will see the sun directly over his head ( a_s= 90 ° , g_s= 0 ° ). But an observer at latitude 23.5 ° on the tropic of cancer will see the sun 23.5 ° due south from the zenith ( a_s= 90 ° - 23.5 ° = 66.5 ° , g_s= 0 ° ).

Bangkok is situated at latitude 13.7 ° north, and longitude 100.5 ° East. For Bangkok , the sun is due north at noon on June 21 st . On December 21 st , the sun is due south. Table 1.1.1 shows the altitude and azimuth angles of the sun for daytime on the 4 reference days.

Figure 1.1.2 gives graphical plots of the altitude and azimuth angles of sun, using the values in Table 1.1.1. The traverses of the sun in the sky drawn from the information given in Table 1.1.1 gives a clearer graphical visualization, as in Figure 1.1.3.

For most times of the year, the sun rises in the southeast and sets in the southwest directions in Bangkok . This is true also for other locations in Thailand .

Since Southeast Asia is located within latitude 23.45 ° N and 23.45 ° S, any location in the region will experience the same phenomenon of the sun raising in the north-eastern direction in summer and in south-eastern direction in winter.

In Thailand , the 105 ° E longitude is taken as the reference longitude. The standard time for Thailand at 105 ° E is ahead of the Greenwich Mean Time (GMT) by 7 hours.

But Bangkok is at the longitude 100.5 ° E, when the sun is overhead at noon at the longitude 105 ° E, it appears to be due East as seen from Bangkok . It takes another [(105 ° - 100.5 ° ) ´ 4] = 18 minutes for the sun to appear overhead in Bangkok . It takes the sun 4 minutes to traverse one degree of longitude, so that it completes 360 ° in 24 hours. For a location at 105 ° E, the solar time and the local time are virtually identical. The solar time is the time coincident with the position of the sun.

The sun-path diagram is a plot of the angular position of the sun as it traverses the sky on a given day. In such diagram, the horizontal axis shows the azimuth angle, and the vertical axis shows the altitude angle. Figure 1.1.4 shows a sun-path diagram for Bangkok for the 4 reference days.