Tuesday, 29 December 2009 17:12
• Daily Leader staff report
A moon dog or moondog (scientific name paraselene, plural paraselenae, i.e. “beside the moon”) is a relatively rare bright circular spot on a lunar halo caused by the refraction of moonlight by hexagonal-plate-shaped ice crystals in cirrus or cirrostratus clouds. Moondogs appear to the left and right of the moon 22 degrees or more distant. They are exactly analogous to sun dogs, but are rarer because to be produced the moon must be bright and therefore full or nearly full. Moondogs show little color to the unaided eye because their light is not bright enough to activate the cone cells of humans’ eyes.
Moon ring weather folklore
Folklore has it that a ring around the moon signifies bad weather is coming, and in many cases this may be true. So how can rings around the moon be a predictor of weather to come?
The ice crystals that cover the halo signify high altitude, thin cirrus clouds that normally precede a warm front by one or two days. Typically, a warm front will be associated with a low pressure system which is commonly referred to as a storm.
It is believed that the number of stars within a moon halo indicate the number days before bad weather will arrive. Give it a try the next time when observing a moon halo.