Why do we say once in a blue moon?
A sixteenth-century rhyme by an anonymous author counsels us that, ‘If they say the Moon is blue, We must believe that it is true.’
The statements of poets and song writers about the Moon are rightly open to question. But in Edinburgh on September 26, 1950, Scottish astronomer Charles Wilson focused his telescope on the Moon and measured its spectrum its breakdown of colours. He proved that the Moon on that night was blue. Later, Wilson deduced that the Moon’s blueness was caused by oily particles floating high in the atmosphere. The debris came from a forest fire across the Atlantic in Canada, of such intensity that smoke and dust were swept thousands of metres upwards on convection currents.
What we see as light is a combination of colours, from red at one end of the spectrum to blue, indigo and violet at the other. Atmospheric dust tends to give light from the Sun or the Moon a reddish hue. This is because fine dust breaks up the light’s combination of colours, scattering the blue and leaving the red to come through more strongly.
That is why we often get a red sky at night or in the morning. The Sun is low, and its light passes through more dust particles than when it is high in the sky. At the other extreme, large particles of water vapour scatter sunlight’s basic colours almost equally, making the clouds look white (or grey and foggy).
Medium sized particles, such as those from the forest fire, scatter red light more effectively than blue light. The red light is filtered out, vanishing against the blackness of the night, and we see that phenomenon a blue Moon.
Why are blue Moons so rare that we have a saying about them? It’s unusual for mediumsized particles of dust or other matter to be swept into the atmosphere. Fine dust is easily blown high into the sky.
Water vapour, too, rises readily, and cold air condenses it into large particles which, when heavy enough, will fall as rain or hail. But, once in a blue Moon, a huge fire or a volcanic eruption will send up particles of just the right size to give our poets and astronomers something to sing about.
The statements of poets and song writers about the Moon are rightly open to question. But in Edinburgh on September 26, 1950, Scottish astronomer Charles Wilson focused his telescope on the Moon and measured its spectrum its breakdown of colours. He proved that the Moon on that night was blue. Later, Wilson deduced that the Moon’s blueness was caused by oily particles floating high in the atmosphere. The debris came from a forest fire across the Atlantic in Canada, of such intensity that smoke and dust were swept thousands of metres upwards on convection currents.
What we see as light is a combination of colours, from red at one end of the spectrum to blue, indigo and violet at the other. Atmospheric dust tends to give light from the Sun or the Moon a reddish hue. This is because fine dust breaks up the light’s combination of colours, scattering the blue and leaving the red to come through more strongly.
That is why we often get a red sky at night or in the morning. The Sun is low, and its light passes through more dust particles than when it is high in the sky. At the other extreme, large particles of water vapour scatter sunlight’s basic colours almost equally, making the clouds look white (or grey and foggy).
Medium sized particles, such as those from the forest fire, scatter red light more effectively than blue light. The red light is filtered out, vanishing against the blackness of the night, and we see that phenomenon a blue Moon.
Why are blue Moons so rare that we have a saying about them? It’s unusual for mediumsized particles of dust or other matter to be swept into the atmosphere. Fine dust is easily blown high into the sky.
Water vapour, too, rises readily, and cold air condenses it into large particles which, when heavy enough, will fall as rain or hail. But, once in a blue Moon, a huge fire or a volcanic eruption will send up particles of just the right size to give our poets and astronomers something to sing about.
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