41 P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_11, © Springer Science+Business Media, LLC 2010 In the Sky at Night, I have covered several total eclipses, and the first of these way back in 1961, was a pioneering effort. In 2006, the sky was clear, and in our television programme all went well. Chapter 11 Turkish Delight Turkish eclipse (Credit: Pete Lawrence) 42 11 Turkish Delight There are many glorious sights in Nature, but to me, as I have often said there is nothing even remotely comparable with a total eclipse of the Sun. For the few moments, when the brilliant solar disk is fully covered by the Moon, everything seems to be put into a state of suspended animation; only the sky changes. I am not surprised that ancient peoples were frightened and in backward countries this is still true. An annular eclipse is fascinating, but no more, and it is hard to become really enthusiastic about one that is partial. I am lucky that I have seen seven totalities, and have presented Sky at Night programmes for most of them; the first, way back in 1961, was a real pioneering effort. But the eclipse of 23 March 2006 was one that I knew I would have to miss; I simply wasn’t fit to travel to the track of totality, which ran from Brazil across to Africa, cutting through Turkey and grazing Egypt before going on to end in Mongolia. John Mason and his team from the South Downs Planetarium opted for Egypt; the Sky at Night contingent, with Chris Lintott and including Pete Lawrence and Bruce Kingsley, preferred the coast of Turkey. For once in a way, politics did not interfere. I was envious. From Selsey there was a small, partial eclipse – <20%, but as a matter of principle I decided to photograph it. Joined by Alan Schultz and Tim Wright, I watched it from my garden outside the observatory, talking to Chris enjoying the pleasant Turkish heat. I had a mediocre view through scattered cloud In Turkey, the sky was crystal-clear and conditions could not have been better. Totality was due just before 2 o’clock in the afternoon Turkish time, and the expedi- tion members began to make their final preparations. No two totalities are alike. Sometimes, the sky becomes really dark; generally, the light-level at mid-totality is about equal to full moonlight, but one never knows quite what to expect. The shape of the corona is more predictable because it is linked with the solar cycle; at sunspot maximum it is fairly symmetrical, while at spot-minimum it is “spiky”, with streamers stretching out in all directions. In March 2006 we had just passed the low point of the cycle, so that the corona should have been of the spot-minimum type, but Pete Lawrence’s H-alpha telescope showed several prominences, so that the Sun was not entirely quiet even though no spot- groups could be seen on the disk. Quite apart from the corona, there are various phenomena to be observed. Before totality there are shadow bands, wavy lines on the Earth’s surface seen when the disk has been reduced to a narrow crescent. Then, there are Baily’s Beads, when the sunlight streams through low-lying areas of the Moon’s limb; they were seen very clearly at the last annular eclipse. The Moon’s shadow rushes across the landscape at almost 200 mph, both before and after totality. I remember seeing that, at the Cornish eclipse of 11 August 1999, though from our site the eclipse itself was clouded out (a gentle rain fell throughout totality), and we sheltered under umbrellas, muttering words such as “Tut, tut!” and “Most annoying!”. And just as totality ends there is the wonderful “Diamond Ring”, as the first segment of the photosphere pokes out from behind its temporary screen. All these marvels were seen by the observers in Turkey, and they were suitably impressed; “awesome” and “emotive” were two of the adjectives used by our 4311 Turkish Delight commentators. The temperature dropped sharply by 5 or 6°, and just before the corona shone out there was what was described as “a strange twilight”. The corona itself was of typically minimum variety, with long streamers, and there were two naked-eye prominences. This time the sky remained bright; Venus was prominent, and Mercury could be glimpsed, but Chris reported that he could not see any of the stars of Orion. Not that there was much time to look around; totality lasted for a mere three and a half minutes – and as I know from experience, the time seems to flash by. One of the most interesting experiments was undertaken by Pete Lawrence. During totality, he photographed the Moon, which was of course directly in front of the Sun and so was illuminated solely by Earthlight. The pictures he obtained were very good indeed; the outlines of the main maria were unmistakable, together with some of the craters. Totality is the only time to see the completely New Moon. Yes, Turkey was a great success, both as a television programme and, far more importantly, to give dedicated observers a chance to carry out useful work. Nobody who took part in the expedition is ever likely to forget it. Moreover, eclipse chasing is addictive, and all the team members who went to Turkey began to make plans for the next totality, on 1 August 2008, even though it did mean the slight inconve- nience of travelling to North Greenland. 45 P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_12, © Springer Science+Business Media, LLC 2010 In the spring and summer of 2006, Saturn was well-placed, with the ring system still widely open. For this programme, I was joined by John Zarnecki, Carl Murray, and photographers Pete Lawrence, Damian Peach and Dave Tyler. Indications of liquid areas on Titan had been found, but true revelations about the “Lake District” came later – see The Lakes of Titan. What is the most beautiful object in the sky? Many people will favour a spiral galaxy, such as the Whirlpool; others will opt for a lunar crater, or a great comet with a glowing head and long, gently-curved tail – but my vote goes unhesitatingly Chapter 12 Ringed World Saturn from Cassini (Credit: NASA) 46 12 Ringed World to Saturn, the planet with the rings. There are plenty of spirals, many lunar craters and the occasional spectacular comet, but there is only one Saturn. It is a giant world, almost 75,000 miles across; it has a gaseous surface, and though there is a hot, presumably silicate core, the overall density of the globe is less than that of water. It is often said that if you could drop Saturn into an ocean, it would float – but finding an ocean of sufficient size would be quite a problem! Saturn’s mean distance from the Sun is 886 million miles, but even from this range it still outshines most of the stars; when it is at its best, only Sirius and Canopus outrank it. Its slow movement and its dull, yellowish glare led to its being named after the God of Time – Jupiter’s father, and his predecessor as ruler of Olympus. Saturn takes 29½ years to compete one journey round the Sun, but its day is only 10¼ days long, shorter than for any other planet apart from Jupiter; the quick spin means that the equator bulges out, and any small telescope will be good enough to show that the disk is markedly flattened. The upper atmosphere, rich in hydrogen together with some helium, is very cold, at a temperature of about −180°C. Eight satellites can be seen with a good, modern amateur-owned telescope, but of these only one (Titan) is bigger than our Moon. There are over 30 much smaller satellites, some of which have retrograde motion and are almost certainly captured asteroids. In some ways, Saturn is not unlike Jupiter. It also has cloud belts and brighter zones, but the surface is less obviously varied than Jupiter’s, and neither are there any vivid colours, so that the disk seems comparatively bland, and there has never been anything to rival the Jovian Great Red Spot. However, there are strong winds and storms, and between latitudes 35 and 36 we find a turbulent region nicknamed “Storm Alley”. It has to be admitted that for really detailed views we have to rely on space probes, notably the Voyagers of a quarter of a century ago and now the Cassini mission, which was launched in 1997 and was still sending back invaluable data over 10 years later (of course, it carried the Huygens lander which made a gentle touch-down on Titan). But we must not forget the Hubble Space Telescope, which has monitored Saturn and has sent back remarkable pictures of aurorae there. Like ours, the aurorae are caused by particles of the solar wind which are trapped by Saturn’s strong magnetic field and plunged down into the upper atmosphere, making it glow. Saturnian aurorae are brightest in high latitudes north and south because the rotational axis and the magnetic axis virtually coincide. White spots sometimes appear on the disk. The brightest of modern times have been those of 1933 (discovered by W.T. Hay, better remembered by most people as Will Hay, the stage and screen comedian) and 1990, but there was a reasonably noticeable white spot in 1996. When a new white spot appears, as no doubt will happen before too long, there is a good chance that it will be first seen by an amateur. When I began the Sky at Night series, in 1957, there was no telescope anywhere which could be used to match these – but we have entered the Electronic Era, and it has to be said that photography now looks decidedly old-fashioned. Cassini, now happily moving round the planet, is the first Saturn orbiter; Pioneer 11 and the Voyagers were fly-by missions because the Pioneer encounter was really an afterthought on the part of NASA and the Voyagers were on their way to the outer Solar System. Cassini began its main work immediately after arriving, and 4712 Ringed World one picture was particularly spectacular; viewed from the space-craft, the Sun passed directly behind Saturn, so that Cassini lay in the planet’s shadow and the rings were brilliantly back-lit. This lasted for twelve hours, and the NASA planners wasted no time. An entirely new ring was discovered, engulfing the midget satel- lites Janus and Epimetheus, and this was something of a surprise. It had been sug- gested that meteoritic impacts on both these satellites might send a certain amount of fine material into orbit, but nobody had really expected a complete ring, albeit a very tenuous one. (En passant, I could have been the discoverer of Janus. In 1966, I was using the 10-in. refractor at Armagh Observatory, in Northern Ireland, and made three observations of the then-unknown Janus, but as I did not recognise it as being new I can claim absolutely no credit!) Early images from Cassini also showed the diffuse, extensive E-ring, with one of the familiar satellites, Enceladus, sweep- ing through its outer part. Cassini also took a picture of our Earth, the first time our world had been imaged from a range of over 900 million miles. It appears as a tiny, featureless dot. The F ring, outside the main system, is both “clumpy” and variable. The particles are kept in their orbits by two small shepherd satellites, Prometheus and Pandora. Cassini showed that Prometheus, a mere 63 miles in diameter, is interacting with the ring and actually pulling particles off it: Pandora is rather smaller, but no doubt acts in the same way. Enceladus, discovered by William Herschel as long ago as 1787, proved to be an amazing world even though it is so small (300 miles in diameter). There is an exces- sively thin atmosphere; the surface is icy, and reflects almost 100% of the sunlight falling upon it, so that the albedo is higher than for any other body in the Solar System. The main surprise was the discovery of geysers in the South Polar Region, sending water-ice particles high above the surface, so that there must be a heat- source below – just about the last thing that anyone had expected. Earlier, it had been found that Enceladus causes disturbances in Saturn’s magnetic field as it moves along in its orbit, and this indicated the presence of a conducting medium (water?) below the ice-sheet. Very probably, there really is an underground sea of ordinary water, though it would be premature to speculate about Enceladan life-forms. Hyperion and Iapetus, the two outer satellites of the “original eight”, have also perplexed us. Hyperion is cratered, and does not have captured or synchronous rota- tion; it takes 21½ days to complete one orbit, but is “tumbling along”, and at the moment it spins round in a mere two days. This is not the main puzzle. Hyperion is not regular in shape; it measures 255 × 162 × 137 miles, but because its density is 1½ times that of water it ought to have become a sphere. Why hasn’t it? And if it is the broken-off half of a larger body, where’s the other half? There is no sign of it Iapetus is larger, almost 900 miles across, and has an orbital period of 79 days; this is the same as the axial rotation period. The distance from Saturn is 2,200,000 miles. Parts of its surface are bright and icy, while other parts are as black as a black- board. Cassini results indicated that the blackness is due to a surface deposit rather than material welling up from below. (Why Cassini? Because the Italian observer G.D. Cassini paid close attention to Saturn during the seventeenth century; he discovered Iapetus, Rhea, Dione and Tethys, plus the main division in the ring 48 12 Ringed World system, now named after him. Rather confusingly, perhaps, the main dark area on Iapetus has been christened Cassini Regio.) Iapetus is curiously-shaped; 928 × 930 × 891 miles, making it seem slightly squashed. The most peculiar feature is the equatorial ridge running over 800 miles through the middle of the Cassini Region; it follows the equator almost perfectly, but does not extend on to the bright areas. It is around 12 miles wide, and in places rises to 12 miles above the ground, making it much higher than Everest and comparable with anything on Mars. All sorts of theories have been put forward to account for it. One involves a collision between two smaller bodies which merged, while according to another idea both the ridge and the dark patches were created when Iapetus grazed the outer edge of the ring system long ago. Or could Iapetus itself have had a ring? We have to admit that we simply do not know. Titan is the giant of Saturn’s family, and is unique among planetary satellites inasmuch as it has a thick atmosphere, denser than ours. The Huygens lander, carried most of its way by Cassini, made a controlled touch-down upon “spongy” ground, and sent back excellent images; obviously, it could not transmit for long, but it exceeded all expectations. Since then Cassini has made regular passes, and radar has shown beyond reasonable doubt that there are extensive seas, not of water but of a mixture of ethane and methane. Saturn and its satellites have already given us plenty of surprises, and no doubt more are in store. Which intrigues you most? The new ring, the chemical seas of Titan, the polar aurorae, the towering equatorial ridge of Iapetus or the spongy tumbling Hyperion It is not easy to decide, but all in all I would choose the fountains of Enceladus. When William Herschel first glimpsed the satellite over 200 years ago, he surely could not have expected that the tiny speck seen in his home-made telescope would prove to be an active world, with geysers hurling ice-crystals high into space. 49 P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_13, © Springer Science+Business Media, LLC 2010 Of all the problems faced by a modern astronomer, that of “dark matter” is one of the most baffling. In the Sky at Night we return to it periodically, and for this programme I was joined by Professors Gerry Gilmore and Bob Nichol. Look up into the sky, and you will see bodies of all kinds – planets, stars, galaxies. The Universe seems to be a crowded place. Yet, we now know that most of it is invisible. We can make out less than 10% of it. The rest cannot be seen at all. The man who first realised this, during the second half of the twentieth century, was Fritz Zwicky, who was Swiss by blood, born in Bulgaria, and spent most of his Chapter 13 Matter We Cannot See Fritz Zwicky (Caltech) 50 13 Matter We Cannot See working life in America. It is fair to say that he was one of the most eccentric astronomers of his (or any other) time, but of his brilliance there was no doubt at all. He examined the cluster of galaxies in the constellation Coma, measured their motions, and realised that they were moving around so quickly that they should fly apart. Yet they didn’t. Something was “glueing” them together; the cluster must contain a vast amount of invisible material. Next, the stars in rotating galaxies were not moving as they ought theoretically to do, because they did not obey Kepler’s laws. In the Solar System the centre of motion is the Sun, which also contains over 99% of the mass. Kepler’s Laws state that planets closest to the Sun must move the fastest, and the furthest must be the slowest, which is exactly what we find; Mercury is the quickest (which is why it was named after the scurrying Messenger of the Gods) and remote Neptune is the most leisurely. Now consider a spiral galaxy, such as M.31 in Andromeda or, for that matter, our own Milky Way Galaxy. The stars are rotating round the nucleus of the system, and Kepler’s Laws should apply. The Sun is about 25,000 light-years from the galactic centre, and takes 225 million years to complete one orbit, a period often referred to as the “cosmic year”. Stars further out should move more slowly, but this is not true. The rotation is more like that of a solid, spinning cartwheel. How can this be so? Again Zwicky had the answer. In a galaxy, the mass is not concentrated at the centre, but is spread through the entire system. This explains why the stars behave in the way that they do, but what precisely is the unseen material – Zwicky’s “missing mass”? He did not know, and neither do we, well over 50 years later. All kinds of suggestions have been made. Among these are vast numbers of low-mass stars, too dim to be detected; material locked up in Black Holes and therefore cut off from the rest of the universe; ordinary matter, but so tenuous that it evades us; neutrinos, with a certain amount of “rest mass” – all these were inves- tigated, and found to be wholly inadequate. More popular today are “WIMPs” – Weakly Interacting Massive Particles, which are not the same as the matter we know, and are beyond the reach of our equipment. This may sound plausible, but it is really fudge, and an admission that we simply do not know. The one certain fact is that unless all our measurements are wrong, dark matter definitely exists. Back to Zwicky. In our Galaxy we have occasional stellar explosions which are far more violent than ordinary “new stars”, or novae, which are not really new at all; what happens is that the white dwarf component of a binary system suffers an outburst which makes it flare temporarily up to many times its normal bril- liancy before subsiding back to its former state. The more cataclysmic outbursts are different; for them, Zwicky coined the term “supernovae”. They are of several different kinds, but a Type 1a supernova involves the total destruction of a white dwarf, which literally blows itself to pieces. All supernovae of this kind reach about the same luminosity, which means that we can find their distances – and since a 1a can become as powerful as all the stars in a galaxy combined, it can be seen across vast stretches of the universe. During the past 1,000 years only three supernovae have been seen in our Galaxy, the stars of 1006, 1054, 1572 and 1604 (the most celebrated of these was that of 1054, which was not a 1a; we see its remnant now as the Crab Nebula). Zwicky [...]... best-known, though not the largest, is Pluto But things have not always been as straightforward as this We have a good idea of the age of the Solar System, because we are confident that the Earth is about four and a half thousand million years old The planets were formed in a rotating disk of material round the youthful Sun, which was not then as P Moore, The Sky at Night, DOI 10.1007/978-1 -44 19- 640 9-0_15, ©... identified in other stars, of which the faint variable T Tauri was the first Most of the light gas (hydrogen, with some helium) was blown away, leaving only the rocky materials This is why the Earth is comparatively deficient in hydrogen, which is the most plentiful element in the universe as a whole (atoms of hydrogen far outnumber the atoms of all the other elements put together) There was a period... this disk led to the gradual formation of “cores” containing both ice and water These cores built up to bodies around 15 times the mass of the Earth, a process which took at least a million years Their gravitational pulls were now great enough to collect more material from the solar nebula, and the cores or “protoplanets” making up our Jupiter and Saturn were particularly massive The growth must have... very plausible Moreover, the Uranian satellites share the same inclination, and an impact could hardly have taken them along It is much more likely that interactions with the other giants and with the debris caused a slow, steady tip-over In the inner part of the forming Solar System, another important factor had to be taken into account The Sun went through a period of great activity, sending out... Interactions between the ice-giants and the scattered material meant that there may have been what is usually termed planetary migration, which obviously took a long time; Uranus and Neptune were driven outward, possibly exchanging places, though Jupiter and Saturn were less affected because they had built up to bodies of much greater mass By now the migrations have stopped, and the Solar System has... supernovae show that they are further away than they ought to be if the rate of expansion is constant The recessional velocities of far-away systems ought to slacken off, because of the effects of gravity Instead, the velocities are increasing We live in an “accelerating universe” Albert Einstein once introduced a force which he called the cosmological constant – the opposite of gravitation He subsequently... faint that its discovery will be largely a matter of luck There are “irregular” objects, often with highly eccentric and inclined orbits, such as Sedna, which has an orbital period of over 10,000 years, and whose path it takes from rather beyond the Kuiper Belt out to the region of the much more distant Oort Cloud, far beyond the reach of even the Hubble Space Telescope Some satellites of the giant... stop, its matter swirls downwards towards the core, and the infall results in a pair of jets emerging from the rotational poles of the doomed star; the 14 Gamma-Ray Bursters 55 shock waves break into space, and their immense energy is released in the form of gamma-rays A short burst is more probably due to a collision between two neutron stars; which hit each other and fuse to form a black hole The whole... so, the cores of our Uranus and Neptune did not become gravitationally powerful until much of the nebular material had been dispersed These two are composed largely of ices, and each may have no more than two Earth masses of gas There was also the “left-over” material, with a total mass which was far from negligible; at least 35 times that of the Earth, and this had a very marked influence upon the. .. when debris bombarded all the newly formed planets; we still see the effects in as much as solid planets and satellites are crater-scarred, though in some cases (notably Earth) most of the craters have been eroded away, while in others (Venus, and Jupiter’s satellite Io) the craters have been removed because volcanic activity has provided total re-surfacing long after the Great Bombardment ended, 3.9 . 41 P. Moore, The Sky at Night, DOI 10.1007/978-1 -44 19- 640 9-0_11, © Springer Science+Business Media, LLC 2010 In the Sky at Night, I have covered several total eclipses, and the first of these. willing to come to a Sky at Night studio, and talk to whoever has succeeded me as presenter of the programme! 53 P. Moore, The Sky at Night, DOI 10.1007/978-1 -44 19- 640 9-0_ 14, © Springer Science+Business. any other planet apart from Jupiter; the quick spin means that the equator bulges out, and any small telescope will be good enough to show that the disk is markedly flattened. The upper atmosphere,