TOPIC:

ID Code: N1-31
Purpose: Show how a "sun dog" is formed.
Description: A prism is mounted below a fast rotator, approximating the effect of hexagonal ice crystals. Rotation of the crystal has the same effect as scattering off many flat hexagonal ice crystals aligned along the horizontal plane but rotated randomly in that plane (See demonstration N1-32 ICE CRYSTALS - PAPER MODELS). The "sun dog" consists of a relatively sharp cutoff at 22 degrees, the minimum angle of dispersion for ice crystals, with a long tail of decreasing intensity. For the prism the deflection angle is greater than 22 degrees because the index of refraction of glass is greater than that of ice. The outer thirds of each face of the equilateral prism is covered by tape, making it appear to the incoming light more like a hexagonal ice crystal.
Availability: Available

A recent photo of a Moon Dog: apod.nasa.gov/apod/ap160321.html