Friday's Blue Moon and lunar size-shifting explained

August 28, 2012
Friday's Blue Moon and lunar size-shifting explained
The moon on the Moreton Bay horizon at 5.10pm.

(—A lunar event that occurs once in a blue moon will happen this Friday night, August 31.

Look at the and you will see a full , the second this month ( the first was August 2). Two full moons in the same month occur just once every 2.5 years or so.

QUT Dr Stephen Hughes said we had a full moon on August 2 and the next on August 31, the blue moon, will make two "squeezed" into one month.

"Blue Moons occur only about seven times in 19 years—that's about once every two-and-a-half years," Dr Hughes said.

"A bit like the leap year, a blue moon happens as a calendar catch-up because it takes the moon about 27 and a half days to go round the earth and since this is less than the number of days in a calendar month, two full moons can sometimes occur in a single month.

"If you get paid fortnightly you might notice that you sometimes get paid three times in a month rather than the usual twice. These 'blue pays' occur about as frequently as blue moons."

Dr Hughes said a was not coloured blue although the moon could appear blue when there were smoke or volcanic suspended in the atmosphere.

"A blue-tinged moon is a rarity and so maybe that's where the saying came from. There are reports of blue moons after the Krakatoa eruption in Indonesia in 1883 in which was ejected 80km high, on the edge of the ," he said.

Friday's Blue Moon and lunar size-shifting explained
At its zenith on Moreton Bay at 9.34pm.

"When it is high in the sky, the moon shines with a white light reflected from the sun. At moonrise the moon can appear either yellow or orange but when there's a lot of smoke in the air, after a , for example the moon can appear red.

"The moon looks red because the blue component in moonlight is scattered in the atmosphere so the remaining moonlight contains more red."

Dr Hughes said while musing on the moon we should look at another mystifying phenomenon: when the moon rises or sets below the horizon it appears much larger than it does when it's higher in the sky.

Dr Hughes and QUT PhD student Adam Ellery used digital cameras to test whether the moon is actually larger close to the horizon or if it is an illusion.

"Adam took photos of the over the sea (Moreton Bay) and I took some photos of the moon inland: close to the horizon at Murrayville in south western Victoria and another at an elevation of about 60m in the Adelaide Hills.

"Our experiment showed that contrary to what our eyes tell us, the moon is actually smaller at the horizon."

The apparent increase in the size of the moon at the horizon is called the moon illusion. It is caused by refraction, a process whereby light rays take longer to pass through the middle of the lens because it is thicker there.

"This means the light rays exit the edges of the lens first and the central rays emerge later. The net effect is bending of the rays towards a point beyond the lens called the focus.

"The curved atmosphere of the earth acts like a lens, so when a ray of light from the moon passes into the atmosphere it is deflected.

"The rays from the bottom of the moon come into the atmosphere at a slightly shallower angle than the top of the moon and so get refracted more, resulting in the moon being scrunched in the vertical direction.

"When the moon is high in the sky, the rays strike the atmosphere vertically and are hardly deflected so the moon stays the same shape."

He said the apparent size of the moon does actually change slightly throughout the month because the distance between the centre of the earth and moon varies between 362,000 km and 405,000 km. However, this accounts for only about 10 per cent of the change in the diameter of the moon throughout the month.

Explore further: Rare New Year's Eve 'blue moon' to ring in 2010

More information: A free version of the journal paper on this experiment can be downloaded from .

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al germain
3 / 5 (2) Aug 28, 2012
How can this be only 7 times in 19 years - once every 32 months? Wouldn't it happen every time a full moon lands on the 1st or second day of the month (or third in months with 31 days. That would be about twice per year. What am I missing?
3 / 5 (2) Aug 29, 2012
Nothing, you're just confused or use a wildly different calendar to the rest of us.
not rated yet Aug 29, 2012
This is a very disappointing article. The explanation of the moon illusion is completely wrong. The illusion is that the moon appears larger when near the horizon. Refraction causes it to appear smaller, as the article says. So how can someone write that the illusion is caused by refraction?

(And what does the thickness of the middle of some lens have to do with atmospheric refraction?)

The actual cause of the moon illusion is psychological. People perceive the sky to be further away at the horizon than it is directly overhead (because that's true of clouds). Since the moon is approximately the same angular size in both cases, it appears larger near the horizon.
not rated yet Aug 29, 2012
Al: You were missing the fact that the lunar month is 29.5 days. (Not surprising, the article doesn't mention this.) It takes the moon about 27.5 days to complete an orbit as Hughes said, but that's not relevant. In that time the Earth has moved, so it takes two extra days to complete the lunar cycle from full moon to full moon.

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