The Eclipse

My 3-y old grandson Remy in 2007, when asked what an eclipse was, said "...the sun went behind the moon which is why the moon had clips on it". 

The below article was written in 2007. It is still relevant to tonight's event (11 December 2011).  From anywhere in NZ in the early hours you can see the eclipse from just before 3am tomorrow morning, until the moon sets. You will see it low in the night sky between west and northwest. It will be visible in Australia, NZ, Alaska, northern Canada and central and eastern Asia. For everyone in Hawaii and the bulk of North America the moon will set still in eclipse. Those along the west coast of the US and Canada will see the beginning of the totality just as the moon disappears below the western horizon, and the rest of the world will miss maximum eclipse.

A lunar eclipse occurred on 28 August 2007 for both hemispheres. Because the northern hemisphere are 12 hours behind the southern, viewers there had to wait till their 8pm rolled around before they saw what southerners had witnessed 12 hours earlier. Most NZers and Australians had a reasonable view. The following day the front page of the NZ Herald showed a photo of the eclipsing full moon with the caption "a break in the clouds meant most of NZ was treated to a rare red moon last night."  Really? The country is nearly two thousand kms long. That's some break!  Perhaps the Herald was trying to uphold and preserve its errant forecast of the previous day, which read "..unsettled weather is forecast for the total lunar eclipse"; implying that there was only a slim chance of anyone seeing the event. But the whole five-hour event could be seen from most parts of the country, as well as from most parts of Australia.

Meteorologists seem not to know that despite what happens during the day, the night sky is typically clear on the actual night of the full moon, which is why we see usually it in all its glory, causing poets and lovers to feel weak in the knees. The phenomenon has been noted widely, for instance "One thing is a meteorological certainty: the full moon very frequently clears the sky.." [1] .  "All the stars appear in the sky at the night of Full-Moon Day" -Myanmar lunar calendar [2] , and the common old nautical saying which was "the full moon eats clouds". For more on this, just Google 'full moon cloudless night' and note the huge number of references, observations and stories. For the sake of balance you could also google 'full moon covered with clouds'. But most listed there seem to be references to the nearly full moon, which is a big difference, and the clouds until the moon rose. I accept that there is also the possibility that people would feel more compelled to write stories when they see the full moon than when they can't but that notwithstanding, there is sound reasoning behind the full moon/clear sky theory. Here's why. 

The transiting moon daily increases the height of the sea – called the tide. As lunar gravitational forces go through the air to reach the sea, so, too, is the air subject to a tidal change. The result is an increase in air height when the moon is above the horizon, at times up to 20%. That weather balloons float higher on new or full moon days, indicating king tides in the air is well known to meteorologists. On a full moon night, moonrise is at sunset. The air warmed by the land which was in turn warmed by the sun throughout the day is lifted upwards by and towards the rising moon. This warmer air now aloft prevents the cooler night air from condensing the water vapor near the ground and forming clouds. The result is a perceived clearing of the night sky. In short, the full moon makes you see it, by lifting the air (warmed by the day's sun on the ground) that pushes back the colder air coming down from the edge of space that would otherwise cause the appearance of clouds, that would otherwise block out the sight of the moon. To be fair, there are some clouds occasionally in full moon nights, but if so they are usually very thin and quickly passing, and the full moon is typically still visible in the cloud breaks or through or beyond them. All over the world, deeply overcast skies on full moon nights are rare.

Usually the full moon passes above or below the earth’s shadow because the moon's orbit is tilted 5deg from the Earth's orbit around the Sun, called the ecliptic. A lunar eclipse occurs when the full moon is nodal, that is, it is sitting right on the ecliptic, like shooting at something at point blank range instead of standing off to the side.  That is why in an eclipse the earth blocks out the sunlight that usually shines through space past the earth and onto the moon, to be always seen (in full moon time) from the dark side of earth. As the moon moves eastwards in the sky around the earth at 13 degrees per day, completing a 360deg circuit in a month, the earth passing between sun and moon takes a day to complete its pass, which is why both hemispheres sometimes get time to see the eclipse event. When the full moon passes through the Earth's shadow sunlight that bends through the Earth's atmosphere gives an eclipsed moon a reddish look.

A solar eclipse occurs in the daytime, when the new moon passes in front of and blocks out the sun. Any eclipse can only be when the earth, sun and moon are in a line. An eclipse will never occur on a quarter moon. As well as total eclipses, lunar and solar, there are 'partial' , 'hybrid', annular' and 'penumbral' eclipses, depending on how much of the eclipse can be seen from different vantage posts on Earth. The eclipse of Aug 28th was total for us because the event was centred over the Pacific.

How often do eclipses occur?  At the rate of one every 173 days, there are two eclipse seasons per year. An eclipse season lasts for 37 days; and since the Moon only takes 29 days to complete an orbit, we're guaranteed one solar and one lunar eclipse (which may or may not be total) every eclipse season. With an eclipse season happening a little more often than every 6 months, every year sees at least 4 eclipses (2 solar and 2 lunar), somewhere on the Earth. Lunar eclipses occur twice per year. Lunar eclipses are not always total. Two total lunar eclipses occur in a single year about once every 3.5 years, and 3 in a year about every 200 years. (updated) A total lunar eclipse occurred on Feb 21, 2008, but was not seen from our shores. The next occurred in the early evening of Dec 21, 2010 and again in the early morning of June 16, 2011, and because of lack of darkness both were not seen easily. But the one on Dec 10/11, 2011 will be in the wee small hours after midnight and will be perfect.

Does an eclipse affect weather? In the middle of the August 28, 2007 eclipse the winds above Auckland went suddenly dead and the clouds stopped moving. This lasted for about twenty minutes. An analogy is the way the winds blow strongly with an incoming or outgoing tide but suddenly go still on the turn. Full moon, perigee, nodal crossing and lunar equinox all affect weather. When these factors bunch up more turbulent weather can be the result. Eclipses were feared in olden times for threatened reasons peculiar to certain religions, like dragons/serpents/bears eating the moon or sun because humans had misbehaved. The sight of the moon disappearing for a couple of hours every year without warning would have been scary enough.

But in the extreme distant past the moon was closer, by a few hundred miles when ancient villages 700,000 and more years ago stood on islands that have long disappeared beneath oceans that have risen since the last ice age. A recent fossil discovery in Pakefield, Suffolk, was identified and nicknamed pioneer man, and was dated at 700,000 years old. It pushed back the first evidence of humans by some 200,000 years. Pioneer man was known to have lived in southern Europe 800,000 years ago and probably made the journey to Britain via a connecting land bridge [3] . There is no reason to close the door on that age, as there is evidence for humans in Europe 1.2m years ago, and humans of one species or another have existed for more than 2m years [4] . Egypt alone has been settled for at least 600,000 years, and archaeologists are always looking for older villages. Ancient Hindu texts describe an age called Manvantara, which is 852,000 years, and they say we are in the 7th Manvantara now, or 5.9m years [1] . If mankind has existed for that long, then it is tempting to think that a then closer moon, by over a thousand miles, would have brought more weather extremes at times of these astronomical conjunctions than today, and so for the humans around then, an eclipse would indeed have been very good reason to be frightened.

Perhaps that is why Stonehenge and other stone monuments are set up to calculate eclipses. It takes 56 years for the moon to complete one full eclipse cycle and return to the same highest point around midnight above the same horizon. Stonehenge has a circle of 56 "Aubrey Holes" named after the man who discovered them. The cycle of solar and lunar eclipses around the earth repeats itself each 223 lunar months, and 223 full Moons must be counted before the Earth, Moon and Sun are again in the same positions in the sky. These were counted around the Aubrey Holes using marker stones that were moved. Aubrey Holes 51, 56 and 5 were fixed markers. Eclipses of the Moon in summer or winter took place when any marker stone arrived at AH56 or AH28, the two holes that lie on the main axis of Stonehenge. When a white marker reached AH5 or AH51, equinox eclipses would occur [5] . 

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At 19-yrs the weather repeats, because the moon and sun calendars coincide. Around the outer bank of Stonehenge, one foot represents one week exactly of a giant 19-yr cycle. 7 x 19 years brings a larger cycle. In 1875, just one year less than 7 x 19, Britain experienced huge floods and that was also the only year when two floods are recorded (July and October). The equivalent is this year and because of this we can expect another round of floods in the UK at the end of October. Parts of NZ, too, may experience flooding then. According to Albert Park records, on Oct 31st, 1875, Auckland received 56mm in one day.

Eclipses remind us of the magnificence of solar and lunar conjunctions, and we recall the importance ancient peoples placed on these events. Bringing the old science back to predict future extreme weather events is possibly where climate research should be concentrated. To do that meteorologists and climatologists must become astronomers, archaeologists and historians. Until then, how much they understand the larger picture will perhaps be on a par with my then three year old grandson.

Refs