THE CHARACTER OF SUNLIGHT WITH CLEAR SKY When the sky is clear the actinicity of sunlight depends almost wholly on the sun's position with regard to the horizon. Many exposure tables give the exposures for certain latitudes and the different months and hours of the day, but the combination of these three conditions is solely for the purpose of conforming to known heights of the sun above the horizon. As this last may be seen at a glance it seems unneces sary to refer to the non-essentials of hour, month and latitude. The sun must always be at the zenith from some point or other on the earth's surface and if from this point as a center a circle be imagined to be drawn on the earth say from it then from every point on that circle the sun would rest at 30° from the horizon and the sun's light to reach any point on that circle must pass through the same dis tance of the earth's atmosphere. The same would of course be true of any other distance from that point at which the sun is at the zenith. The problem is really very simple and consists in knowing the maximum intensity of the sunlight when it shines through the earth's layer of at mosphere at right angles to it and then to know how much that intensity is reduced as the sun shines from directions approaching the horizon. It is a matter of fact, as has already been ex plained and used in this book, that when the sun is at the zenith a least visible tint is made by its light assisted by that of the sky on gelatin printing out paper in L of a second.
These facts suggest that in clear weather the sky may conveniently be divided into in tensity zones, the intensity for each zone being the p.o.p. time when the sun is in that zone. The position of the sun by this method is always found by making a comparison between it and that part of the horizon immediately beneath it. When the lower rim of the sun rests on the horizon the p.o.p. time is usually 8 seconds and the light is therefore only as intense as it is with the sun at the zenith. The diameter of the sun is a little more than half a degree and if its lower edge is less than one of the sun's diameters above the horizon the sun may be said to be in the one degree zone. It is to be
understood that the 16 degree zone includes the belt of sky from that height down half way to the horizon or to 8 degrees; the 8 degree zone from 8 to 4 degrees, etc. The zone of which the zenith is the center, that from upward, should be known as the "maximum" zone. These zones may also be spoken of simply by their p.o.p. time, the maximum zone being that of i second. The table of zones and the p.o.p. time of each follows.
Intensity zones in which The p.o.p. time of sun the sun rests light in these zones 1° zone 1° and lower 8 seconds 2° " 1° to 2° 4 If 4° " 2° to 4° 2 " 8° " 4° to 8° 1 ,g 16° " 8° to 16° 1/2 " 40° " 16° to 40° 1/4 " Maximum " 40° to 90° 1/8 " The weaker values, when the sun is near the horizon, are extremely variable by reason of temporary weather conditions, and the after noon light is also frequently slower than the morning light. It must therefore be under stood that the above table is only for perfectly clear weather conditions. Figure 11 illustrates these zones. The sun is shown in the maxi mum ors second zone. Figure 11.
Wherever in the sky the sun may be, the p.o.p. time of its light may be known at a glance by memorizing the following two methods of ob servation: The former to be used when the sun is 8° or less from the horizon and the latter when the sun is above 8°. When the sun is low, this method, obviously, is effective only where the horizon is of the same altitude as the observer, as on the sea or on level ground. Among hills and mountains the light should be measured by one of the methods given.
The zone in which the sun rests when rather low may be seen by the number of its diameters it is above the horizon. To observe the sun one may look through a piece of smoked glass or a piece of old negative, which need not be more than a half inch square and which may be cut from a spot which on trial proves the correct density to allow the sun to be looked at con veniently through it.