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by Graham Farr
Melbourne, Australia (SPX) Jan 08, 2013
The end of the world - mis-predicted from the Mayan calendar for December 2012 - failed to live up to expectations, but the year was still a memorable one for celestial alignments.
In this one year we had the last Transit of Venus most of us will ever see, a total solar eclipse, an annular solar eclipse, and two lunar eclipses. All this action on the line between the earth and the sun made for spectacular observing experiences and close encounters with history. We will not experience such a collection of alignments again in our lifetimes, so it is worth savouring them.
They are all special cases of syzygy, when Earth, Sun and another body - the Moon or another planet in our solar system - form a nearly straight line in space. This word is worth knowing: for a start, if you exclude plurals, it's an equal-longest word in English without vowels. (Puzzle for Scrabble players: how many points would you get for the word `syzygy'?)
The first of these alignments was the annular solar eclipse of 20/21 May. Media coverage focused on observations from the western USA, Tokyo or Taipei. But the path of annularity started in mainland China, and included several large cities near the south-east coast. It was the monsoon season there, which means lots of cloud, and as expected, many millions of people within this path missed out. Even the `China Daily', in reviewing the eclipse the next day, made no mention of observations from its own mainland.
But some observers in China were lucky, and were among the first to see this remarkable spectacle. This seemed fitting, as it was a Chinese scholar, Liu Xiang, who gave the first recorded correct explanation of solar eclipses, writing in about AD 20 that "when the sun is eclipsed, it is because the moon hides him as she moves on her way".
I was visiting Xiamen, an island-cum-city linked by bridges to the coast of Fujian province on the shore of the Taiwan Strait, and had the privilege of seeing this eclipse. It started as a red shark-fin shape emerging from cloud on the horizon. By the start of annularity about forty minutes later, the sun had entered another cloud. Watching through eclipse glasses, I saw a shifting series of views in which different parts of the ring were visible at different times. Could I really say that I had seen the annular eclipse, if this was as good as it got?
Fortunately, cloud and sun began to go their separate ways, and the complete ring became visible in a spectacular view that lasted for a couple of minutes or so. Even this thin circular sliver of sun was too brilliant to look at without eye protection.
The next solar eclipse of 2012 was even better: a total eclipse, which we observed on 14 November from Palm Cove on the eastern coast of far north Queensland in Australia. This was an early morning eclipse, with a high probability of cloud blocking the view, especially as it was so low in the sky.
One unusual feature of this eclipse for us was that we saw not one, but two solar coronas! We saw a solar corona as well as the solar corona! Let me explain.
During the first partial phase of the eclipse (between first and second contacts), we only had brief views of it. At one point, beautiful crepuscular rays could be seen both above and below a large thick cloud, giving converging lines that pinpointed the hidden sun's position. The rays above the cloud held an unexpected and subtle phenomenon: faint circular arcs of colour, a little like a rainbow fragment but with thicker colour bands and only three of them: two red bands with a green band between them. Geometrically, these arcs appeared to be roughly a sixth of a circle, centred on the sun and hence perpendicular to its rays.
This was a solar corona, not of the kind you see only during a total solar eclipse, but an optical phenomenon in the earth's atmosphere, formed by diffraction of light by water droplets or ice crystals. Given the occasion, it was as if nature was having a play on words! You can see such coronas, at times, around either sun or moon, though the former kind is less often noticed because of the sun's brilliance. A good picture of a lunar corona, taken by Keith Edwards of Dundee, can be seen here. What we saw was very like a fragment of the outer three bands of that one. It was a delicate and delightful sight, and a morale-booster while sun and cloud continued their struggle.
Fortunately, the sun cleared the clouds before totality. Once the crescent sun had shrunk to a short fragment of its former self, it underwent a beautiful transition: shrinking further, then the corona just beginning to be visible while the disk within it remained, for an instant, the same colour as the sky around it, then the diamond ring, then totality, all in a second or two.
Totality finished much sooner than I expected: somehow time seems to pass differently in the moon's shadow! If this event had been organised by people, I would have objected to such a premature conclusion. But in this case it was difficult to identify an authority who might be sympathetic to such a complaint.
We remained in a state of post-eclipse euphoria long after it ended. Indeed, so remarkable was the experience that it seemed strange to see the world and its people carrying on as usual over the rest of the day. One after-effect reported by one of our group was "eclipse glasses sunburn": is this a first?!
Each of these two solar eclipses was a wonderful show on several levels. There was the mathematical beauty of the moon's circle slowly moving across the sun's, according to the precise laws of celestial mechanics and following the script exactly. Even the irregularity of the partially eclipsed sun's shape - not a simple circle, but now with two sharp points where the moon's circle cuts it off - was but one manifestation of a deeper regularity of the motions of the heavenly bodies.
There was the acting out of mythological roles, as the queen of the night was revealed during her normally-secret journey past the sun; she brought a fragment of night to day, acknowledging the sun by blocking its light rather than reflecting it. There was the slow build-up to an exciting climax, with real uncertainty as to how it would turn out, and relief at the happy resolution. It was a one-off live performance, and one which many observers were to miss out on, so we had been granted a rare privilege. Observing with other people increased the excitement and the sense of occasion. The audience was enlightened and uplifted.
Total solar eclipses still play a role in astronomical research, even in this age of space-based instruments. They allow observations which would be much more costly using a satellite. Some measurements, such as of the sun's inner corona, are still best done during a total eclipse, when the moon serves as a more effective barrier to the sun's radiation than an artificial obstruction in a satellite-mounted instrument.
The first of the two lunar eclipses was on 4 June, just a fortnight after the annular solar eclipse. Happily, this served as a prelude to the rarest of these five alignments, two days later.
I had been advertising the Transit of Venus of 6 June 2012 for about eight years - indeed, ever since observing the previous one on 8 June 2004. Mostly I did this while moonlighting as a Night Sky Guide at Melbourne Observatory. One of the instruments there, still in use, is an 8" refractor made by Troughton and Simms in London in the early 1870s and installed in time to observe the transit of 1874.
Whenever we bring people to this instrument, the story of Transits of Venus is told, looking back to Captain Cook's observation at Tahiti in 1769 and forward (until now) to 6 June 2012. I impressed the date on the visitors: 6/6/12, with the mnemonic 6+6=12. So I was delighted to be rostered on for the last evening tour before the transit, 4 June, which was also the evening of the partial lunar eclipse!
But that curtain-raiser was a fizzer. It must have been about the worst weather I have experienced when on duty at this observatory. Our preparations included mopping up rainwater that had got in through one of the domes. During the tour, we showed our visitors the observatory's historic instruments, including the 8" refractor and a 4" photoheliograph (also installed for the 1874 transit). We explained how they worked and talked about the long and fascinating history of the observatory, as some compensation for missing the partial lunar eclipse.
Fortunately, the weather on the morning of the transit was much better. We observed it from Jells Park in eastern suburban Melbourne, which gave good views to the east, where the transit began at 8:16am. In a magnified, projected image, the edge of the sun was shimmering, but soon we found the little circular-arc dent in the sun's limb made by Venus as it began to pass in front. Over eighteen minutes, Venus crossed the edge of the sun, breaking free of it at 8:34am, with no teardrop effect visible to us. We then enjoyed over an hour of unimpeded observation of the transit, as Venus crawled inwards from the sun's limb.
Cloud then covered the sun for several hours, but fortunately we had clear views of the later stages of the transit too, including the last two contacts (2:26pm and 2:44pm). As Venus crossed the Sun's limb on its way out (between the last two contacts), my son Richard remarked that it looked like part of a jigsaw piece: an edge with a curved notch in it, seemingly made for another piece to fit into.
Transits of Venus were originally important because of their role in determining the astronomical unit, the distance from the Earth to the Sun. This distance is now known very precisely, and other methods for determining it have superseded the use of transits. But repeating the transit-based determination remains an excellent student exercise, and has been used as such for the last two transits. And transits are not just of educational value. The transits of 2004 and 2012 have been used to study the atmosphere of Venus, with measurements from the 2012 transit complementing those obtained from the space probe Venus Express.
For this study, the crucial time of the transit is the few minutes approaching second contact, or just after third contact, during which the disc of Venus is crossing the sun's limb. At this time, sunlight refracted by Venus's atmosphere appears as a thin arc (the aureole), outlining the part of the planet that still lies outside the sun's disc. This light, on reaching instruments on Earth, is analysed for clues about the Venusian atmosphere.
Transits of Venus are a "metronome of history", with a different and more complex beat than Halley's Comet, to which that phrase has also been applied. This was the last beat of the metronome for virtually everyone alive today. We are now in the pause between beats - longer than a human lifespan, but a tiny blink in a cosmic timescale, a reminder of the huge size of astronomical timescales and of the comparative transience of human affairs.
The last lunar eclipse was a penumbral one, on 28 November, a fortnight after the total solar eclipse. Such lunar eclipses are unspectacular and often overlooked, and this one was not widely reported.
It is tempting to think (particularly in the midst of apocalyptic hype!) that it might be especially unusual to have a transit of Venus in the same year as total and annular solar eclipses and two lunar eclipses. But this combination is not much rarer than transits of Venus themselves. Solar eclipses occur at least twice a year.
While it is rare to have two total eclipses in one year - the last time was 1889, and the next two are 2057 and 2252 - it is common to have an annular eclipse and a total eclipse in the same year (although they won't be visible from the same place). So it is not surprising for this to happen in the year of a transit of Venus. It did not happen for the last transit of Venus - there were just two partial solar eclipses in 2004 - but it will happen for each of the next nine such transits.
The next transit of Venus in a year without two annular/total solar eclipses is 2611. The occurrence of the two lunar eclipses is also unsurprising: the same close alignment of earth, moon and sun which produces a solar eclipse (at new moon) is usually close enough, two weeks before or after, to produce a lunar eclipse at the previous or next full moon.
Transient alignments of heavenly bodies, when they occur, are a routine by-product of orbital mechanics, and nature does not favour them over other configurations. But these alignments have yielded remarkable progress in science, as well as having great influence on human affairs. Some of these influences have waned, now that their underlying mechanism is understood and other techniques have overtaken them in astronomical research. But their capacity to educate, entertain and inspire is undiminished.
+ Scientific research using solar eclipses + Scientific research using the transit of Venus: + Student exercise on the transit of Venus Parts of the above article appear in the Astronomical Society of Victoria's newsletter, Crux. Graham Farr can be contacted via gefarr at gmail com.
NASA eclipse and transit site
Eclipse and transit photos
Solar and Lunar Eclipses at Skynightly