Figure 2-26. Atmospheric refraction bends sun rays.

TM 9-258

Figure 2-27. Sun below horizon seen by refracted light.

c. The bending of the paths of rays of light in both

observations on different days and under different

conditions. In the aiming of guns over ]wide expanses of

vertical and a horizontal plane is due to variation of the

water as must be done by coast artillery, a special

refractive index of air due to moisture and temperature

instrument (the depression position finder) takes

variations. This variation is generally so small that it can

atmospheric refraction as well as other conditions into

be neglected in many types of observation made with

consideration.

fire-control instruments. In very careful surveys of

large areas, this effect is eliminated by repeating the

2-19