There will be a ‘Blue Moon’ on October 31. There are two full moons this October, said Arvind Paranjpye, director of Nehru Planetarium. The term Blue Moon is used to indicate a second full moon in a month and has nothing to do with the colour of the moon.
The first full moon this month occurred on October 2 at 2.35 am. The second will occur on October 31 at 8.19 pm. There were two Blue Moons in 2018, the first on January 31 and the second on March 31.
According to Paranjpye, a lunar month comprises 29 days, 12 hours, 44 minutes and 38 seconds. To have two full moons in a month, the first one should take place on the first or second day of the month, Paranjpye said. In other words, when a Blue Moon occurs in a year, that calendar year will have 13 full moons instead of the normal 12, Paranjpye said.
The last Blue Moon in a month with 30 days occurred on June 30, 2007 and the next one will be on September 30, 2050.
The phrase ‘once in a blue moon’, used to describe a rare occurrence, was first used in the early 16th century to indicate something absurd. The earliest reference to a Blue Moon as ‘the third full moon in a season of four full moons’ was found in the Maine Farmers’ Almanac, published in the late 1930s.
Colours of the Moon
The colour of the rising Moon appears reddish but when the Moon is well above the horizon, it loses that red hue and looks white with many shades of grey. According to experts, this is due to the scattering of light by the Earth’s atmosphere, which is made up of nitrogen (78%), oxygen (21%) and argon (0.9%). The remaining 0.1% of the atmosphere consists of carbon dioxide, methane, water vapour and dust particles. When a ray of light enters the atmosphere, it is scattered in different directions by air molecules. The blue part of the light is scattered the most and the red the least. Closer to the horizon, when the Moon is rising or setting, this light has to pass through a thicker layer of atmosphere than when it is well above the horizon. By the time the ray of light reaches us, most of the blue component is scattered away and only red light remains. So, the rising and setting Moon appears reddish. This phenomenon is also applicable to sunrise and sunset. The scattered blue component of sunlight that is reflected in all directions makes the sky look blue. When the Moon is seen through smoke and dust caused by a forest fire or volcanic eruption, most of the red component is blocked and only the blue rays reach us, giving the Moon a bluish hue.
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