Magi's Homeland: The Moon

Blue moons are not just the stuff of imagination and fantasy - they are real. They occur when dust particles and pollution in the earth's atmosphere filter more light at the red end of the colour spectrum than at the blue end. The longer wavelengths, including red and yellow, scatter, while the shorter wavelengths, including blue or green, intensify. This gives to moon a blue or green appearance against the night sky, especially if it is low on the horizon, where its light is subject to the greatest refraction by the atmosphere. Dust from Canadian forest fires caused a famous blue moon on 29 September 1950. Similarly, the moon turned blue following the huge volcanic eruption of Krakatoa in in 1883.

Full moons that occur twice in the same calendar month are also called blue moons. Because the lunar cycle is 29/5 days, the phases of the moon gradually slip backwards through the progression of the calendar until a full moon appears on or near the beginning of the month and again before the end of the month. These blue moons occur roughly every 2.5 years, and are most likely to occur in the months with 31 days.