Once in a blue moon -- a rare event. The full moon on this Friday will be the second full moon of July and is astronomically referred to as “Blue Moon”. This rare phenomenon happens every two-and-a-half years, on an average.
“Generally in a calendar year at least one full moon is visible in the sky every month. However, sometimes two full moons appear in the same month. Hence, in the modern western folklore, metaphorically, to describe the rarity of this occurrence of second full moon in a month is popularly referred to as Blue Moon,” N. Raghunandan, director and founder secretary, Planetary Society, India, told HT.
Full moons are separated by 29 days while most months are 30 or 31 days long, so it is possible to fit two full moons in a single month. The first full moon of the month was witnessed on July 2. The moon on July 31 will be the usual pearly-gray.
The last time such an event occurred was on August 31, 2012, and it is anticipated that the next time it will occur in 2018.
On July 31, the moon will rise at 6.43 pm towards the east, although technically it will reach its fully illuminated total phase at 4:13pm itself, Raghunandan said. According to a Nasa release, “On rare occasions the Moon can turn blue."
A truly-blue Moon usually requires a volcanic eruption. Back in 1883, for example, people saw blue moons almost every night after the Indonesian volcano Krakatoa exploded with the force of a 100-megaton nuclear bomb. Plumes of ash rose to the very top of Earth’s atmosphere, and the Moon turned blue”.