758 Marvels of the Untverse 
THE MOON’S HALO 
“Last night the moon had a golden ring,” sang the poet, and he was no doubt alluding to the 
phenomenon which we are about to discuss. It is of sufficiently rare occurrence to require a descrip- 
tion, and, though as a spectacle it is more particularly associated with the moon, it can, never- 
theless, be observed in the sun. The reason for its greater apparent frequency round the lesser 
luminary is that its effect is usually overwhelmed by the strength of the solar rays. This 
“golden ring,” in a complete state, is a circular band of light concentric to the moon’s disc, and 
is distant forty-five times the moon’s breadth from its edge. It has all the colouring, though in 
a fainter degree, of the rainbow, with the red tint at the edge nearest to the moon and a blue or 
purple one at the opposite margin—a particular characteristic which distinguishes the halo from the 
lunar or solar “ corona.’ In the ‘‘corona” the ring comes much nearer the luminous orb—about 
eleven times nearer—and has the prismatic colours ranged in reverse order to those of the halo, 
that is to say, the blue is nearest to the moon (or sun), and the red forms the outer margin. It 
is an effect of light seen through an atmosphere surcharged with aqueous vapour, and is more 
frequently seen than the true halo. 
This halo is due to entirely different causes, of which the chief factor is the refraction of 
light. At the time of its appearance the higher regions of the air are charged with minute prism- 
shaped crystals of ice. A prism catches the light, breaks it up into rainbow colours and refracts or 
bends the ray at a given angle, the angle varying according to the position of the prism. But there 
is one position at which the prism can sustain a longer phase of turning without causing any per- 
ceptible variation in the angle of the refraction of transmitted light, and it is when myriads of the 
ice-prisms are placed in this position, between the light and the eye of the observer, and throwing 
the rays directly towards the eye, that the phenomenon is apparent. The position of the crystal 
CRYSTALS. 
These six examples of crystals are typical of the six systems of Crystallography. Reading from left to right, those in the 
upper row are: Cubic, tetragonal, hexagonal; and in the lower, rhombic, monoclinic ard anorthic. Th: bases of the crystals will 
be the same as those of the models. 
