Wonders of the Solar System Text Only. Andrew Cohen

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      THE CLOCKWORK SOLAR SYSTEM

      The story of the Solar System is the story of the emergence of order out of chaos, guided by the simplest law of physics: gravity. The planets and their moons exist in relatively stable orbits because of a delicate interplay between gravity and angular momentum, and this beautiful natural balance is written before our eyes in the spinning patterns and rhythms of the heavens.

      In the small ancient city of Kairouan on the north-eastern plains of Tunisia lies the fourth most holy site in the Islamic world. Founded by Arabs in 670 CE, this city of just 150,000 people is home to the oldest place of Muslim worship in the Western world. The great mosque of Kairouan is both impressive in its beauty and also in its scale. Covering over 9,000 square metres (97,000 square feet), the Mosque resembles a great fortress as well as a place of worship. At its heart is a vast courtyard and near the centre is a beautiful piece of astronomical engineering – an ancient sundial. Humans have used sundials like this one to follow the brightest star in the heavens for over 5,500 years.

      For the last fourteen centuries, the sundial at the centre of this great mosque has measured the relentless passing of the days, marking out the passage of time as the Sun travels across the sky, plotting the call to prayers before dawn, at sunrise, at noon, at sunset and in the evening.

      The sundial is a beautifully simple piece of technology. Originally nothing more complicated than a stick in the ground or the length of a human shadow, sundials have enabled us to measure time by following the movement of the Sun across our sky. For thousands of years this movement appeared to confirm the Earth’s position at the centre of the Universe. From the most simple of observations it seemed to make perfect sense that the Sun orbits the Earth every 23 hours and 56 minutes. Yet the simple, regular rhythm that each one of us witnesses every day is nothing more than an illusion. It is not the Sun that’s moving, what we are observing is the rotation of the Earth as it travels through space.

      It’s wonderful to think that across the planet the rhythms of our lives are governed by our journey through space. From waking up to going to bed, eating strawberries in July or a tangerine in December.

      Travelling at 108,000 kilometres an hour, on a 900-million-kilometre journey around the Sun, our planet completes this epic journey once every 365.25 days. The year in the life of our planet is just one of the endless rhythms by which we live our life – and all of these are governed by the seemingly clockwork motion of our planet. It carries us through cycles of night and day as it turns on its axis, rotating at 1,700 kilometres an hour every twenty-four hours. The length of the day at a particular place on the Earth’s surface is dictated by the precise angle of our planet in relation to the Sun.

      We have seasons here, too, due to the fact that the Earth’s axis is tilted by twenty-three degrees. As we journey around the Sun this angle creates the changing dynamic that defines the cycles of many of the creatures that live both on the land and in the oceans. In the Northern Hemisphere the summer months coincide with the North Pole leaning towards the Sun; when, at this time of year, the angle favours the northern half of our planet with extra energy from our star. By winter the dynamic has changed; the North Pole is pointing away from the Sun and the Southern Hemisphere is bathed in additional sunlight.

      It’s wonderful to think that across the planet the rhythms of our lives are governed by our journey through space. From waking up to going to bed, wearing a jumper one month and a T-shirt the next, eating strawberries in July or a tangerine in December, each of these everyday events is intimately connected to a journey through space that catapults us at 108,000 kilometres (67,000 miles) per hour around a star, but leaves most of us completely unaware of this rollercoaster ride through the cosmos.

      It’s not only Earth that is subject to these rhythms – the whole Solar System is full of these cycles, with each planet orbiting the Sun at its own distinct tempo. Mercury is the fastest; closest to the Sun, it reaches speeds of 200,000 kilometres (124,000 miles) per hour, completing its orbit in just eighty-eight days. Venus rotates so slowly that it takes longer to spin on its axis (225 days) than it does to go around the Sun, so that on Venus (and also on Mercury) a day is longer than a year. Further out, the planets orbit more and more slowly. Mars completes one orbit of the Sun every 687 days, just a couple of months short of two Earth years. Jupiter, the largest planet, takes twelve Earth years to complete each orbit, Saturn almost thirty years, Uranus eighty-four years and at the very furthest reaches of the Solar System, four and a half billion kilometres from the Sun, Neptune travels so slowly that by 2009 it hadn’t completed a single orbit since it was discovered in 1846.