1 Occasional appearance of the Moon or Sun when partly obscured by large particles in the atmosphere, as in dust storms, or following forest fires or great volcanic explosions. When the Moon or Sun is viewed through dust or smoke trails it usually appears to be very white, but when the suspended particles are predominantly of one size it sometimes appears blue, at other times green or orange. The phenomenon is attributed to diffraction, although no full explanation seems to be known. The smaller the particles, the more the colour of the Sun or Moon tends to the blue end of the spectrum. The phenomenon is believed to be more common in China than elsewhere. (In the neighbourhood of the 1883 Krakatoa volcanic explosion the Sun was seen as an azure blue sphere.)
2 In 1946 ‘blue Moon’ was defined in the magazine SkyandTelescope as the third full Moon in a calendar season in which there are four.