january: moon halo

One of the things visitors to Inconstant Moon most frequently ask for is an explanation of the phenomenon seen here: a halo of light around the Moon.

The halo always has a diameter of 22°, which gives a clue to its cause. High in the atmosphere, under certain conditions, ice crystals form - millions of them, all with the same shape: an elongated hexagon. As the light of the Moon passes through these crystals it is refracted by 22°, hence the size of the halo.

The same thing can happen during the day with light from the Sun, though because of the bright sky it is less noticeable.

Image: Juan Carlos Casado.