An Algorithm for Estimating the Length and Direction of Shadows with Reference to the Shadows of Shelter Belts

Abstract

In studies of the effect of light on crop production, the amount of light reaching a forest floor, the shadow cast by tall buildings, the siting of glasshouses, or of the light climate in the vicinity of shelter belts, reliance is placed upon reading the sun's declination and the equation of time (or the local hour angle) from tables. However, if simulation models, which require computer evaluation, are to be used, it becomes impracticable either to look up a large number of values in tables or to waste the computer's store by housing the tables of an Almanac within the machine. Thus, a mathematical representation of the equation of time and of the sun's declination is the most convenient for computational techniques. The equations given by Walsh (1961, pp. 50-3) can then be applied to calculate the sun's altitude and azimuth. The characteristic of this work is that a high degree of accuracy is not required, but rather computational speed. The accuracy of the algorithm given here is approximately ?0 5 min in estimating the declination, and approximately ?0 75 sec in the equation of time.