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Moon blues
THE LAST WORD Moon blues Last year I was watching the sun set in Crete, as it went through all shades of red in the final minutes. Then I glanced at the moon and saw it had turned blue. How was this possible, and is it the basis of the saying “once in a blue moon”?
n The chances are that this blue moon was an illusion – the afterimage of the sun created by your eyes and brain. By staring at the bright red disc of the setting sun for a significant amount of time, say more than 20 seconds, the red photoreceptors in your eyes become desensitised. When you switch your gaze to a white object like the moon, the after-image of the sun is seen in its complimentary colour – cyan or light blue. This is because the response of the red photoreceptors has been reduced, leading to the perception that all or part of the red light has
“Genuine blue moons can appear if volcanic eruptions inject particles into the atmosphere” been removed; and white light minus red gives cyan. The more yellow that was mixed into the sunset, the bluer the moon would have looked. This illusion is all the more convincing because the discs of the moon and sun we see are the same size so fill the same amount of space in our vision. Genuine blue moons can appear if volcanic eruptions or
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
Last words past and present, plus questions, at last-word.com
throughout the year. fires inject particles with a fairly The 12 full moons we see each uniform diameter of around a micrometre into the atmosphere. year are named according to their relationship with the equinoxes This diameter is just bigger than the wavelength of red light, which and solstices. The names vary in different regions, but well-known is around 650 nanometres. For examples are the harvest moon, example, the particles released which is the first full moon after by the 1883 eruption of Krakatoa the autumnal equinox, and the caused the moon to appear blue hunter’s moon, which is the for nearly two years. second full moon after the Muskeg, or peat bog, fires have autumnal equinox. Similarly the the same effect. Fires that had Lenten moon, the last full moon been smouldering for several of winter, is always in Lent, and years in Alberta, Canada, flared up on 23 September 1950. This “The term ‘blue moon’ produced oily droplets about a comes from the traditional micrometre in diameter, which agricultural naming of the scattered light at the red end of year’s full moons” the visible spectrum. With red light scattered out of the line of the egg moon (or the Easter moon, sight, the discs of both the sun and the moon looked blue, at least or paschal moon), which is the first full moon of spring, is always when the smoke cleared so that in the week before Easter. they could be seen. By this system there are usually Of course, it might be three full moons between an tempting to ascribe the blue equinox and a solstice, or vice moon in Crete to the forest fires versa. However, because the lunar that plague the Mediterranean. cycle is slightly too short for there Mike Follows to always be three full moons in Willenhall, West Midlands this stretch of time, occasionally n You can replicate this effect there are four full moons. When at home. Stare at a computer this happens, to ensure that the screen showing a bright red image full moons continue to be named for a couple of minutes and then correctly with respect to the immediately glance at a white solstices and equinoxes, the third sheet of paper – it will appear to of the four full moons is called a be bright blue for 30 seconds or blue moon. so before your eyes readjust. There are seven blue moons Mike Sparks in every 19 year period. The last By email, no address supplied blue moon was on 21 November 2010, and the next will be on n The term “blue moon” comes 21 August 2013. from the traditional agricultural Aidan Copeland naming of the full moons Chester, UK
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
This week’s questions Urine and out
Does cranberry juice cure cystitis? If it does, how does it work – surely urine is just urine by the time it is excreted from the body? If it doesn’t, how did the myth arise? Pauline Greenshiels Liverpool, UK Trees of green
On my teacher training course we are handed stacks and stacks of papers. However, I am aware of the need for environmental sustainability. So I would like to know how many A4 sheets are in an average tree used for paper making. Joanna Fernandes Plymouth, Devon, UK Hang on lads…
At the end of the original 1969 movie of The Italian Job, the thieves are in a bus hanging over the edge of a cliff, with a stack of gold bars about to slide out of the back doors. All the protagonists crowd to the front of the bus to balance out the weight of the gold. Their problem is they can’t get at the gold, because every time they try to edge down the bus to grab it, the bus tips again and the gold slides nearer the doors. The movie ends before the dilemma is resolved. Assuming no outside action is permissible, how could the thieves save the gold? Helen Morten Birmingham, UK
Why can’t elephants jump? The latest collection: witty, brilliant, intelligent and packed with insight Available from booksellers and at www.newscientist.com/elephants
n The chances are that this blue moon was an illusion – the afterimage of the sun created by your eyes and brain. By staring at the bright red disc of the setting sun for a significant amount of time, say more than 20 seconds, the red photoreceptors in your eyes become desensitised. When you switch your gaze to a white object like the moon, the after-image of the sun is seen in its complimentary colour – cyan or light blue. This is because the response of the red photoreceptors has been reduced, leading to the perception that all or part of the red light has
“Genuine blue moons can appear if volcanic eruptions inject particles into the atmosphere” been removed; and white light minus red gives cyan. The more yellow that was mixed into the sunset, the bluer the moon would have looked. This illusion is all the more convincing because the discs of the moon and sun we see are the same size so fill the same amount of space in our vision. Genuine blue moons can appear if volcanic eruptions or
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
Last words past and present, plus questions, at last-word.com
throughout the year. fires inject particles with a fairly The 12 full moons we see each uniform diameter of around a micrometre into the atmosphere. year are named according to their relationship with the equinoxes This diameter is just bigger than the wavelength of red light, which and solstices. The names vary in different regions, but well-known is around 650 nanometres. For examples are the harvest moon, example, the particles released which is the first full moon after by the 1883 eruption of Krakatoa the autumnal equinox, and the caused the moon to appear blue hunter’s moon, which is the for nearly two years. second full moon after the Muskeg, or peat bog, fires have autumnal equinox. Similarly the the same effect. Fires that had Lenten moon, the last full moon been smouldering for several of winter, is always in Lent, and years in Alberta, Canada, flared up on 23 September 1950. This “The term ‘blue moon’ produced oily droplets about a comes from the traditional micrometre in diameter, which agricultural naming of the scattered light at the red end of year’s full moons” the visible spectrum. With red light scattered out of the line of the egg moon (or the Easter moon, sight, the discs of both the sun and the moon looked blue, at least or paschal moon), which is the first full moon of spring, is always when the smoke cleared so that in the week before Easter. they could be seen. By this system there are usually Of course, it might be three full moons between an tempting to ascribe the blue equinox and a solstice, or vice moon in Crete to the forest fires versa. However, because the lunar that plague the Mediterranean. cycle is slightly too short for there Mike Follows to always be three full moons in Willenhall, West Midlands this stretch of time, occasionally n You can replicate this effect there are four full moons. When at home. Stare at a computer this happens, to ensure that the screen showing a bright red image full moons continue to be named for a couple of minutes and then correctly with respect to the immediately glance at a white solstices and equinoxes, the third sheet of paper – it will appear to of the four full moons is called a be bright blue for 30 seconds or blue moon. so before your eyes readjust. There are seven blue moons Mike Sparks in every 19 year period. The last By email, no address supplied blue moon was on 21 November 2010, and the next will be on n The term “blue moon” comes 21 August 2013. from the traditional agricultural Aidan Copeland naming of the full moons Chester, UK
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
This week’s questions Urine and out
Does cranberry juice cure cystitis? If it does, how does it work – surely urine is just urine by the time it is excreted from the body? If it doesn’t, how did the myth arise? Pauline Greenshiels Liverpool, UK Trees of green
On my teacher training course we are handed stacks and stacks of papers. However, I am aware of the need for environmental sustainability. So I would like to know how many A4 sheets are in an average tree used for paper making. Joanna Fernandes Plymouth, Devon, UK Hang on lads…
At the end of the original 1969 movie of The Italian Job, the thieves are in a bus hanging over the edge of a cliff, with a stack of gold bars about to slide out of the back doors. All the protagonists crowd to the front of the bus to balance out the weight of the gold. Their problem is they can’t get at the gold, because every time they try to edge down the bus to grab it, the bus tips again and the gold slides nearer the doors. The movie ends before the dilemma is resolved. Assuming no outside action is permissible, how could the thieves save the gold? Helen Morten Birmingham, UK
Why can’t elephants jump? The latest collection: witty, brilliant, intelligent and packed with insight Available from booksellers and at www.newscientist.com/elephants