Housed in its 17-storey-high dome, the Anglo-Australian Telescope boasts a dished mirror, 3.9 metres in diameter, that collects and focuses the light of faint celestial targets for detailed investigation by arrays of high-tech gadgetry. Its quarry ranges from nearby asteroids in the Solar System to the most distant objects detectable – exploding stars known as supernovae, and delinquent young galaxies known as quasars. And its vantage point on the Universe is a mountain-top called Siding Spring, located in New South Wales’ Warrumbungle Range – a supremely apt Gamilaraay word meaning ‘crooked mountains’. Siding Spring is 450 kilometres north of Canberra, and 350 kilometres north-west of Sydney. Its remoteness from major cities keeps its night skies as pollution-free as they were when the first humans watched the heavens from this place tens of thousands of years ago. Despite that, it’s possible to discern the distant glow of Sydney on the horizon, along with other nearer centres such as Dubbo and Gilgandra.
When it was built in the early 1970s, the Anglo-Australian Telescope was one of the largest on the planet. Now, however, the world’s astronomers have access to over a dozen telescopes with mirrors twice as big. Instruments of this size tend to be multi-national collaborations, but their physical locations concentrate in a handful of places on high mountain-tops not far from the western seaboards of continents. Here, exquisitely stable atmospheric conditions offer freedom from the blurring effects of turbulence, so Hawaii, the Canary Islands, south-western USA, northern Chile and South Africa are where the newest optical astronomy facilities are located. That’s not to say Siding Spring’s work is done: as we will see in chapter 15, clever technology combines with the site’s dark skies to keep the observatory at the cutting edge of modern astronomy.
I was based at Siding Spring for more than two decades, using the Anglo-Australian Telescope and its smaller sibling – a wide-angle instrument known as the United Kingdom Schmidt Telescope. I got to know Siding Spring Mountain in all its guises, from crystal-clear sunsets that foretold perfect night-time conditions to gloomy fog-bound dawns, when the humidity had soared well above the dew-point.
The Earth’s atmosphere plays a critically important role in the science that can be carried out in an observatory, of course. Its characteristics in terms of temperature, pressure, humidity, transparency and freedom from atmospheric turbulence and light pollution are the stock in trade of ground-based astronomers. We are, after all, gathering all our information through this fickle veil of life-giving gas.
Curiously, I discovered during my time at Siding Spring that the atmosphere can be as big an attraction as the sky itself when it comes to the simple pleasures of stargazing. And that is particularly true during that magical period when the dome of the sky is changing from daylight to darkness – or vice versa. Astronomers recognise that this twilight zone corresponds to our passage through the Earth’s ‘terminator’ – a word that astronomers have used for centuries, but which has now been hijacked by the movie industry. In astronomy, it has serenely beautiful overtones; in the movies, it’s a lot less serene.
So what is the terminator in this context? To understand it, you have to imagine yourself looking at a planet (or a satellite of a planet) from a vantage point in space. Moreover, you have to imagine said planet or satellite being illuminated by the Sun, as all Solar System objects are. The rest is easy, because the terminator is simply the line that divides the sunlit portion from the part in darkness.
For worlds with no atmosphere, like the planet Mercury or our own Moon, the terminator is a sharply defined boundary, startlingly abrupt as it delineates the change from darkness to light. But for a planet like Earth, with its blanket of air, the terminator blurs into a fuzzy line, with the illuminated side of the planet gradually merging into the darkness of the night side. That’s because molecules in the atmosphere scatter the light of the Sun beyond the terminator’s geometric boundary.
So let’s now shift the focus back to our vantage point on the Earth’s surface. As the planet spins on its axis, we are carried through the terminator twice in every 24 hours, experiencing either the gradual fall in illumination as the blue (or grey) of the daytime sky metamorphoses into the blackness of night, or, some hours later, the reverse. Just how many hours separate dusk and dawn depends on your latitude and the season of the year.
The period of twilight when the Earth’s rotation carries us through the terminator is something most of us hardly notice. However, if you know how to look, it’s a time when a rich assortment of atmospheric and astronomical phenomena manifest themselves. As you might expect, the sequences of events in the morning and evening twilight zones are perfectly symmetrical – that is, they are identical, but reversed in time. So, the following account applies equally to daybreak as to nightfall, except the order of everything is turned around. Since most of us experience dusk more often than dawn, we’ll stick with the order of events at nightfall. Apart from anything else, it’s a much more romantic time of the day.
IT MIGHT SEEM A BIT BASIC, BUT A GOOD STARTING POINT in understanding twilight phenomena is to ask why the daytime sky is bright. It’s a question great thinkers throughout antiquity pondered, but it was not fully answered until the work of the English scientist, Lord Rayleigh, was published in 1871. Once again, it’s the light-scattering effect of the atmosphere, whose subtleties we’ll encounter in a couple of minutes. But anyone who has seen photos of the Moon’s surface taken by Apollo astronauts in the late 1960s and early 70s will know that while they were taken during the lunar daytime, the sky itself is black. Without an atmosphere, the Moon has no means of scattering light, so the Sun’s rays illuminate nothing until they hit the surface (or anything that might be standing on it – such as the odd astronaut). Actually, that’s not quite true. Under certain circumstances, clouds of fine Moon dust are elevated by electrostatic forces in sufficient quantities to have been noticed by orbiting astronauts just before the Sun appeared over the rim of the Moon. But there’s nowhere near enough of it to make the lunar sky bright.
So, back on Earth, if it weren’t for the clouds that form in the atmosphere, our skies would always be blue. That colour comes from a particular aspect of the way sunlight interacts with air molecules and aerosols (dust particles or very fine droplets). Light is scattered in all directions by its interaction with these particles, but it turns out that its blue component is far more scattered than its red. As sunlight passes through the atmosphere, the blue light is extracted (which is why the Sun itself looks slightly yellow), but turns up again everywhere else in the sky. In fact, the violet light in the Sun’s rainbow spectrum is scattered even more strongly than the blue, but it is also absorbed more strongly by the atmosphere – which is why our skies are a rich blue rather than a psychedelic violet.
When clouds are present, the blue is masked, of course, but the sky is still bright. The clouds themselves have a neutral shade ranging from brilliant white to a foreboding grey. The absence of colour in clouds is no accident: once again, sunlight is being scattered, but this time by water droplets that are much bigger than molecules and aerosols, and they don’t follow Rayleigh’s rule of blue supremacy. The droplets scatter all the colours of the rainbow equally, producing neutral white light (or grey light on dull days).
THE TWILIGHT PHENOMENA I WANT TO INTRODUCE YOU to are best seen when you have a clear horizon in all directions. Get away from buildings, trees, hills, mountains, dust storms, active volcanoes and other distractions. Mid-ocean is absolutely perfect, if you can manage it. But flat areas such as the high Karoo of South Africa, the Steppes of Asia or the deserts of the south-western United States are also ideal for this kind of viewing – not to mention the big sky country of inland Australia. Take a trip out to the Western Plains of New South Wales sometime, and make sure you visit Siding Spring Observatory in the process. But wherever you are, do choose a sunny evening for your twilight experience.
If you can, as the day comes to an end, set aside an hour or so to watch what happens as you cross the Earth’s terminator. Sunset is the first piece of atmospheric enchantment to look for. As the Sun nears the horizon on its way down, you’ll notice a distinct yellowing or even a slight reddening of the sky around it. That’s because its light is travelling through a much greater thickness of atmosphere than when it is high in the sky, intensifying the removal of blue light, and even scattering some of the red light, too.
If there is dust or moisture in the atmosphere, perhaps with a few clouds blocking out the Sun, you can often see shafts of light radiating from its position in the sky. These are known as ‘crepuscular rays’ (evening rays), which, despite appearances, are actually parallel to each other. It is perspective that causes them to fan outwards from the Sun in that spectacular manner so beloved of landscape artists. Occasionally, faint crepuscular rays can be seen after sunset, in a sky that is completely clear. As before, their presence betrays the presence of clouds blocking chunks of the Sun’s light, but these clouds are so far away as to be below the western horizon, and invisible to the viewer.
While the Sun is still low in the sky, turn your back to it, and have a look towards the east. Sometimes, you can see more crepuscular rays, now converging towards a point just below the eastern horizon that is directly opposite the Sun. This point is cleverly called the ‘antisolar point’, and the converging rays are…wait for it…‘antisolar crepuscular rays’. Even more surprising are antisolar crepuscular rays seen after the Sun has set, for their convergence point is now above the eastern horizon and slightly empty-looking – particularly in a cloudless sky. This is a fairly common occurrence at a mountain-top site like Siding Spring.
Just once, I have seen a crepuscular ray arching right across the sky from horizon to horizon, like a gigantic golden rainbow. It was on a humid Sydney summer evening, probably with a high aerosol content in the atmosphere. An amazing sight.
IF YOU HAVE A CLEAR WESTERN HORIZON, AND IT’S FREE from clouds at sunset, it is worth looking for the ‘green flash’. Astronomers’ friends and acquaintances often grumble that this is a figment of said astronomers’ fevered imaginations, but the green flash is a real physical phenomenon. It can even be photographed. It’s caused by sunlight being dispersed into an extremely short vertical rainbow spectrum because the Earth’s atmosphere behaves like a prism. Most of the time, we simply don’t notice it. At sunset, however, we see a diminishing proportion of the Sun’s disc as it crosses the horizon from the first contact of its lower limb to its final disappearance – a process that lasts two to four minutes in North American latitudes. And right at the end, a fine sliver of brightness is left. Just occasionally, when the atmosphere is perfectly stable, this will turn bright green for a second or two before it disappears.
What’s happening at this point is that the red and yellow components of the Sun’s spectrum have now sunk below the horizon, due to the prismatic effect of the atmosphere. That leaves only its green and blue light in the final sliver. Our eyes are more sensitive to green light than blue, so we see an enhancement of green. It lasts only briefly, but it’s quite unmistakeable when it occurs.
One problem with observing the green flash is that your eyes tend to be dazzled because you’re constantly checking to see how near the Sun is to setting. No matter how much you try to avert your gaze, the radiant disc of our star demands your attention. For that reason, the green flash is best seen at dawn, when the first sliver of the Sun’s disc emerges above the distant horizon. Of course, you have to know where to look, but that’s not too difficult to work out from the brightening of the sky. The best green flashes I’ve seen have been at dawn.
GREEN FLASH OR NOT, ONCE THE SUN HAS DISAPPEARED below the horizon, turn again to the east to see one of the most poetic of all sunset phenomena. It’s so commonplace that most of us don’t even notice it, but once you know what you’re actually looking at, you won’t forget it. After sunset on a clear day, all along the eastern horizon you’ll see a blue-grey band topped with a strip of pinkish purple light. As the Sun sinks further below the horizon, the blue-grey band broadens into a shallow arch whose apex is directly opposite the sunset. At the same time the pinkish glow becomes more prominent, separating the grey arch from the blue of the rest of the sky, sometimes with extraordinary brilliance.
What you’re seeing here is the Earth’s shadow cast on its atmosphere, rising majestically in the east as the Sun sets. As soon as the Sun has set, you are actually inside the shadow, and being carried eastwards away from its sharp upper edge by the Earth’s rotation, which, in North American latitudes, ranges from about 800 to 1400 kilometres per hour. For this reason, the shadow soon becomes indistinct, and the grey arch and darkening sky gradually merge into a uniform blue-grey, as the pink glow disappears.
These delightful shadow effects have equally delightful names. The blue-grey arch is known as the ‘twilight wedge’ (because of its three-dimensional shape in the atmosphere), while the pink glow is known more enigmatically as the ‘Belt of Venus’. Apparently it refers to the ‘cestus’ – a girdle or breast-band of the Greek goddess Aphrodite, aka the Roman goddess Venus. Its pinkish-purple colour comes from the fact that the atmosphere scatters the red-rich light of the setting Sun directly back towards the observer, and it mixes with the blue of the still-illuminated sky along the boundary of the Earth’s shadow. It’s a truly beautiful phenomenon that’s commonly visible, but most people miss it.
AS THE SKY DARKENS, THE UNIVERSE STARTS TO REVEAL itself in all its splendour. Of course, the planets, stars and galaxies are there all the time, but they’re hidden under the brilliance of the daytime sky. One or two objects are brighter than the sky itself, so they can be seen in daylight. The Moon is one, of course, and at certain times, the planet Venus is another. Sometimes Venus is visible as a tiny speck of light while the Sun is still in the sky, when the planet is in, or near, a position known as ‘greatest brilliance’, which it occupies for a few days at a time, separated by intervals of a few months. The event’s occurrence is dictated by the elaborate dance of our sister planet relative to Earth, and you can check the details via the internet (just put ‘Venus greatest brilliance’ into your search engine). Take care, though, because when this phenomenon occurs, the planet is relatively close to the Sun in the sky. Better not to risk using binoculars or a telescope for fear of accidentally beaming direct sunlight into your eye. That would be catastrophic.
At sunset, reddened backscattered sunlight mixes with the blue of the sky to form a purple ‘Belt of Venus’ on the upper edge of Earth’s shadow. As the planet rotates, the observer is carried into the shadow, and the Belt of Venus quickly becomes indistinct. The rotating Earth is viewed here from the north.
Author
Back to twilight, though. As the Sun dips further below the horizon, the brighter stars and planets become steadily more visible. You might be intrigued to know that astronomers define three stages of twilight, distinguished by the differing levels of sunlight still being scattered into the atmosphere. ‘Civil twilight’ lasts until the Sun is 6 degrees below the horizon, and while it holds sway, the sky is still quite bright. It’s followed by ‘nautical twilight’, which takes the Sun to 12 degrees below the horizon, and then by ‘astronomical twilight’, which lasts until it’s 18 degrees below. These definitions date from the late 19th century, and while they seem arbitrary, they were chosen so that by the end of astronomical twilight, there would be no scattered sunlight whatsoever in the sky. By then, the sky is ‘officially’ dark.
What happens next depends on the phase of the Moon, and whether you’re skywatching from a place afflicted by light pollution, as most of our cities are. If the Moon is full, it lights up the sky with surprising intensity, allowing you to find your way around easily without artificial light. But if the Moon is a slender crescent, or absent from the evening sky altogether – and especially if you’re well away from city lights – you might see the bright band of the Milky Way crossing the sky. This is our view through the thickness of our Galaxy’s disc, and the Sun is just one of its 400 billion or so stars.
THERE’S ONE MORE PHENOMENON THAT NEEDS A CLEAR, moonless sky, completely free from light pollution, to be visible, but which is unmistakeable once seen. It is a faint pillar of light that projects upwards from the western horizon for half an hour or so after the end of astronomical twilight. The luminous pillar is called the ‘zodiacal light’, and its axis lies along the ecliptic – the path of the Sun and planets through the sky. Look for it on spring evenings after dark, when the ecliptic stands more nearly vertically than at other times of the year in North American latitudes. Don’t confuse it with the Milky Way, however, which is further along the horizon to the north.
It took a long time for scientists to figure out what caused this spectacle. One of my great scientific heroes, the Norwegian physicist Kristian Birkeland, barked completely up the wrong tree during the first decade or so of the 20th century by imagining it was due to an electromagnetic interaction between subatomic particles from the Sun, and the Earth’s atmosphere. Birkeland had correctly identified this interaction as the source of the polar aurorae in the closing years of the 19th century, but in the case of the zodiacal light, his intuition let him down. A sojourn in Egypt to make detailed observations of the light was cut short by the First World War, and Birkeland made plans to return to his native Oslo. To avoid the hostilities (as well as the British, whose scientists had been scornful of his work on the aurora), he elected to travel from Cairo to Oslo via Tokyo. Not the most direct route, and, sadly, it was in Tokyo that he died from an overdose of sleeping medication, on 15 June 1917.
It wasn’t long, however, before scientists worked out that the zodiacal light has a much more prosaic origin than electromagnetic interactions. Like the blue of the sky, it’s a scattering phenomenon, with sunlight being scattered not by the Earth’s atmosphere, but by particles of interplanetary dust in the disc of the Earth’s orbit. These are large enough that they don’t obey Lord Rayleigh’s blue-biased rules of scattering, however, so – just like clouds in the sky – the zodiacal light is colourless.
THERE’S ONE FURTHER ASPECT OF THE ZODIACAL LIGHT that needs to be mentioned. This story goes back to the very early 1970s, when a young astronomy student in the United Kingdom began his PhD research on the topic at Imperial College, London. His task was to observe the rainbow spectrum of the zodiacal light over approximately a year from the high-altitude Observatorio del Teide in Tenerife. He was keen – but his career inconveniently lurched into music, at which he did tolerably well. Thoroughly neglected, his research quickly ground to a halt, until he picked it up again in the early 2000s, and graduated in August 2007 with his long-lost (and undoubtedly well-deserved) PhD. This now-eminent astronomer is better known as the lead guitarist of Queen – one Brian May. And it has to be said that there’s nothing quite like a rock star for bringing street cred to the science of the Universe.
What is surprising, though, is how little interest the academic community had shown in the zodiacal light – so little, in fact, that more than three decades could elapse without Brian’s research becoming totally outdated. But in the event, his timing was perfect. During recent years, the Solar System’s faint dusty disc has received increasing attention from astronomers. It’s now recognised as a fossil of the cloud of gas and dust from which the planets were born. It has much to tell us – far more, perhaps, than could ever have been guessed by a musically minded young astronomer stumbling forth on his research career.