Weather forecasts in ancient times were often based on the appearance of the moon, and observers would take particular note if they observed it to be surrounded by a ring of light. Readers of yesterday's Weather Eye will be experts on coronae, one possible kind of ring, but the bright circle might also be a halo - and the difference is profound:
If the moon, she shows a silver shield,
Be not afraid to reap your field;
But if she rises haloed round
We soon will tread on deluged ground.
The former, the "silver shield", is the corona, a bright disc about two or three "moons" in diameter. The halo is a much larger ring, and much less common. It has an angular radius of 22s0]. To get this in perspective it is helpful to remember that this means an angular diameter of 44s0]. If the moon is positioned so that the bottom of the halo appears just to touch the horizon, the top of the ring will be close to half-way between the horizon and the zenith. In other word, a halo is a large phenomenon; it occupies a significant proportion of the sky.
A lunar halo is a consequence of the rays of sunlight reflected from the moon reaching our eyes by a somewhat roundabout route. It occurs when the moon is partially obscured by a thin layer of very high cloud, located 20,000 ft or more above the ground where the temperature is well below zero. Naturally such cloud - which meteorologists call cirrostratus - consists not of drops of water but of ice crystals.
We are quite familiar with ice crystals in their conglomerate form as snowflakes, but a single ice crystal has a much simpler shape: it is commonly a hexagonal cylinder - rather like a short pencil, but very much smaller at less than tenth of a millimetre in diameter.
Now a ray of light impinging on one of the sides of a hexagonal prism of this kind is refracted, or bent, as it passes through by an amount that depends upon its angle of approach; indeed the geometry is such that the vast majority of incident rays are deflected from their original path by an amount very close to 22s0]. If, therefore, a thin curtain of millions of these tiny ice crystals lies between the moon and you, you will see, in addition to the moon itself, deflected moonbeams anywhere in the sky where you look away from the moon by an amount corresponding to 22or in effect, the bright ring we call the halo.