Fig. 23. Determination of Net Trend Density.
Some of the symbols and factors, appearing here for the first time, require brief explanation. D stands for Sector Density. The constant, 220, is the reciprocal of the quotient of the angular diameter of the moon divided by 2. T is Time In, arrived at by subtracting the total number of minutes of time out, as noted for each hour on the original data sheets, from 60. "No. of Birds" is the number for the sector and hour in question as just determined on the plot form. The symbol
represents the angle between the mid-line of the sector and the azimuth line of the moon. The quantity is found by the equation:
| α = 180° - η + ψ0 | (3) |
The symbol
η here represents the position of the mid-line of the sector expressed in terms of its 360° compass reading. This equation is illustrated in [Figure 21]. The values of
η for various zones are given in the upper right-hand corner of the form ([Figure 20]). The subsequent reductions of the equations, as they appear in the figure for four zones, are self-explanatory. The end result, representing the sector density, is entered in the rectangular box provided.