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 鈥渙nce in a blue moon鈥�?
鈥� The chances are that this blue moon was an illusion 鈥� the after-image 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 鈥� cyan or light blue.
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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 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 fires inject particles with a fairly uniform diameter of around a micrometre into the atmosphere. This diameter is just bigger than the wavelength of red light, which is around 650 nanometres. For example, the particles released by the 1883 eruption of caused the moon to appear blue for nearly two years.
鈥淕enuine blue moons can appear if volcanic eruptions inject particles into the atmosphere鈥�
Muskeg, or peat bog, fires have the same effect. Fires that had been smouldering for several years in Alberta, Canada, flared up on 23 September 1950. This produced oily droplets about a micrometre in diameter, which scattered light at the red end of the visible spectrum. With red light scattered out of the line of sight, the discs of both the sun and the moon looked blue, at least when the smoke cleared so that they could be seen.
Of course, it might be tempting to ascribe the blue moon in Crete to the forest fires that plague the Mediterranean.
Mike Follows, Willenhall, West Midlands
鈥� You can replicate this effect at home. Stare at a computer screen showing a bright red image for a couple of minutes and then immediately glance at a white sheet of paper 鈥� it will appear to be bright blue for 30 seconds or so before your eyes readjust.
Mike Sparks
鈥� The term 鈥渂lue moon鈥� comes from the traditional agricultural naming of the full moons throughout the year.
鈥淭he term 鈥榖lue moon鈥� comes from the traditional agricultural naming of the year鈥檚 full moons鈥�
The 12 full moons we see each year are named according to their relationship with the equinoxes and solstices. The names vary in different regions, but well-known examples are the harvest moon, which is the first full moon after the autumnal equinox, and the hunter鈥檚 moon, which is the second full moon after the autumnal equinox. Similarly the Lenten moon, the last full moon of winter, is always in , and the egg moon (or the Easter moon, or paschal moon), which is the first full moon of spring, is always in the week before Easter.
By this system there are usually three full moons between an equinox and a solstice, or vice versa. However, because the lunar cycle is slightly too short for there to always be three full moons in this stretch of time, occasionally there are four full moons. When this happens, to ensure that the full moons continue to be named correctly with respect to the solstices and equinoxes, the third of the four full moons is called a blue moon.
There are seven blue moons in every 19 year period. The last blue moon was on 21 November 2010, and the next will be on 21 August 2013.
Aidan Copeland, Chester, UK