Wonders of the Solar System Text Only. Andrew Cohen

now allows us to track their numbers as they ebb and flow across the face of the Sun.

      Since sunspots are cooler areas than on the rest of the Sun, we might have expected to find that the power of the Sun dimmed when the sunspot activity was at its height. In fact, we’ve found the opposite to be true: the greater the number of sunspots, the more powerful our star becomes. This variation is not just random, as we have studied the Sun in greater and greater detail we have begun to observe patterns emerging; patterns that seem to have direct links to our climate back here on Earth. We’ve discovered that the Sun has seasons.

      THE SUN AND EARTH: SHARING A RHYTHM?

      For decades scientists have sought to understand how these subtle changes in the Sun’s power might be affecting the Earth. It’s a puzzle that led one man to look away from the Sun and focus instead on the rivers around the Iguaçu Falls. Argentinean astrophysicist Pablo Mauas has spent the last decade analysing data that details every aspect of this river system – from water levels to flow rates – from 1904 all the way through the twentieth century. Unlike many of the world’s great rivers, the Parana is so large that it can be navigated by very big ships – and where there are ships there are records. These records enabled Pablo to uncover an extraordinary history and to reveal that, just like sunspots, the river too has a rhythm.

      Pablo and his team found that the stream flow of the river fluctuated dramatically three times during the last century, but the records gave no indication as to what was behind these fluctuations. The amount of water in the river Parana seems to follow a pattern, and Pablo had a hunch that this rhythm might be connected to the rhythms of the Sun. To try to link events on the Sun, 150 million kilometres away, to the flow of the great Parana, Pablo looked first at the most obvious rhythm of our star.

      We’ve known for over 150 years (since the German astronomer Heinrich Schwabe collated the data that stemmed back to Galileo’s earliest observations of sunspots) that the Sun follows a cycle that is repeated approximately every eleven years. This cycle reflects a rhythmic variation in the number of sunspots, which gives us a very clear indication of the amount of radiation given off by the Sun: the greater the number of sunspots, the greater the energy that is reaching the Earth. But when Pablo Mauas looked for a link between the Paranas rhythm and the eleven-year cycle, at first he found nothing. So instead he turned to calculations that described the Sun’s underlying brightness during the last century. We already know that climate change and events such as El Niño can boost the flow of the river, but when Pablo removed both of these effects from the data there appeared to be a strong relationship between the solar data and the stream flow. Superimpose the solar data on the water levels in the river (see below) and you see that when solar activity rises, the volume of water in the river goes up. There is a beautiful correlation between the flow in these rivers and the solar output. Pablo has revealed an amazing link across 150 million kilometres of space that may one day help us to not only better understand the impact of the Sun on our climate, but also to predict the likelihood of floods in the heavily populated waterways of one of South America’s greatest rivers.

      Changes in the Sun seem to move weather systems elsewhere, too. In India, the monsoon appears to follow a similar pattern to the river Parana, boosting precipitation when solar activity is at its greatest, whereas in the Sahara desert the opposite seems to occur: more solar activity means less rain. The exact mechanisms by which our star may affect Earth’s weather remain, for now, a mystery. We know that the energy production rate of the Sun – the power released in the fusion reactions at the core – is very constant, indeed. It doesn’t change, as far as we can tell, and so the changes that we see must be to do with the way in which the energy exits the Sun. And while the amount of radiation that falls onto the surface of the Earth is only at the tenths of a per cent level, it really does reveal the intimacy and delicacy of the connection between the Sun and the Earth.

      HOW TO CATCH A SUNBEAM

      We are tied to our star in the most intimate of ways. All the planets in our solar system are bathed in varying levels of sunlight, but only on one do we know of a phenomenon that does more than just passively receive the warmth of the Sun. Here on Earth, we actually feed on starlight. The Sun is the source of energy for almost all life on Earth; every plant, algae and many species of bacteria rely on the process of photosynthesis to create their own food using the power of the Sun. This in turn creates the foundations for the complex web of life here on Earth; not only does the process of photosynthesis maintain the normal level of oxygen in the atmosphere, but it is also the basis on which almost all life depends for its source of energy.

      We are only just beginning to understand the complex mechanisms by which plants capture sunlight; some of this explanation may take us off into the quantum world, but at its most basic chemical level photosynthesis is a simple process. Inside every leaf are millions of organelles called chloroplasts, and it’s these little units that do something magical when they capture a photon that has taken the eight-minute, 150-million-kilometre (93,000-million-mile) journey from the Sun. The chloroplasts take in carbon dioxide and water, and by capturing the energy from a sunbeam they convert this into oxygen and complex sugars. It’s these complex sugars, or carbohydrates, that are the basis of all the food we eat – whether directly through the consumption of photosynthetic plants or indirectly through the consumption of animals that feed on them. The amount of energy trapped by photosynthesis is immense – around 100 terawatt-years, which is six times larger than the power consumption of human civilisation.

      This intimate link between our planet and the Sun is all around us. Yet although we are surrounded by vast swathes of wonderful green machines that are all feeding on the Sun, the leaves and plants that cover so much of the planet do not just rely on any sunlight. In fact, plants are fussy eaters and have evolved to use just a fraction of the sunlight that makes its way through Earth’s atmosphere.

      On the surface of Earth sunlight may appear white, but when you pass it through a prism, you can see that it is made up of all the colours of the rainbow. Different wavelengths of light have different colours – from the blues with the shortest wavelength to the reds with the longest – but it’s not just their colour that distinguishes them. The prism reveals the recipe of light that is specific to our star; we see the red, green and blue photons that make up the sunlight all around us and each of these photons has very specific characteristics. The red photons don’t carry much energy but there are lots of them, whereas the blue photons, although there are fewer, carry a little more energy. Plants have evolved to gain the maximum energy most efficiently out of the recipe of light our star throws at us, so they don’t just use any photons for photosynthesis but only the ones from the red and blue bit of the spectrum.

      This intricate relationship between the evolution of plants and our star has had a profound effect on one of the defining features of our planet. When a red or blue photon hits a plant it is absorbed and so those wavelengths of light can no longer bounce back into your eye. Whereas when a green photon hits a plant it is not absorbed but reflected, so this wavelength of light bounces off a leaf and into your eye to create a living world that is defined by one colour more than any other: green. So the verdancy of the forests and jungles that cover our planet is all down to how plants have adapted to the quality of our star’s light.

      SOLAR ECLIPSE IN VARANASI

      Nothing prepares you for a total solar eclipse, and nothing prepares you for Varanasi. The old Solar City is never quiet and deserted; it is a little slice of ancient India and feels more hectic and vibrant even than the twenty-first-century version. But on the morning of 22 July 2009, the banks of the holy river were packed with people. There was no room, not a square centimetre of space, amongst a million sandaled feet crammed onto the Ghats. Green, yellow, red and orange saris and bronzed torsos bared to the early morning summer Sun formed a continuous bridge between the stepped shore and the heavily silted Himalayan waters of the Ganges. The ritualistic instinct to wash in the holy river powered a continuous convective flow of bodies down the concrete steps of the Ghats to the water’s edge – a circulating and impenetrable wall of humanity, simultaneously frenzied and calm. As I stood with them I marvelled at the patience of the Indian people – something British film crews dripping with tripods and flight cases will never be able to emulate.

      With immense difficulty, we had found a place