Wonders of the Solar System Text Only. Andrew Cohen

total eclipse of the Sun since June 1991 was about to be visible to a lucky few. For three and a half minutes the Moon would cover the face of the Sun and plunge this ancient city into darkness.

      SOLAR ECLIPSES

      A total solar eclipse is possibly the most visual and visceral example of the structure and rhythm of our solar system. It is a very human experience, and one that lays bare the mechanics of this system.

      At the centre is the Sun, reigning over an empire of worlds that move like clockwork. Everything within its realm obeys the laws of celestial mechanics discovered by Sir Isaac Newton in the late seventeenth century. These laws allow us to predict exactly where every world will be for centuries to come. And wherever you happen to be, if there’s a moon between you and the Sun, there will be a solar eclipse at some point in time.

      Eclipses occur all over the Solar System; Jupiter, Saturn, Uranus and Neptune all have moons and so eclipses around these planets are a frequent occurrence. On Saturn, the moon Titan passes between the Sun and the ringed planet every fifteen years, while on the planetoid Pluto, eclipses with its large moon, Charon, occur in bursts every 12o years. But the king of eclipses is the gas giant Jupiter; with four large moons orbiting the planet, it’s common to see the shadow of moons such as Io, Ganymede and Europa moving across the Jovian cloud tops. Occasionally the eclipses can be even more spectacular. In spring 2004, the Hubble telescope took a rare picture (opposite top) in which you can see the shadows of three moons on Jupiter’s surface; three eclipses occurring simultaneously. Although this kind of event happens only once every few decades, the timing of Jupiter’s eclipses is as predictable as every other celestial event. For hundreds of years we’ve been able to look up at the night sky and know exactly what will happen when. Historically, this precise understanding of the motion of the Solar System provided the foundation upon which a much deeper understanding of the structure and workings of our universe rests. A wonderful example is the extraordinary calculation performed by the little-known Dutch astronomer Ole Romer in the 1670s. Romer was one of many astronomers who attempted to solve a puzzle that seemed to make no sense.

      The eclipses of the Galilean moons, Io, Europa, Ganymede and Callisto, by Jupiter were accurately predicted once their orbits had been plotted and understood. But it was soon observed that the moons vanished and reappeared behind Jupiter’s disc about twenty minutes later than expected when Jupiter was on the far side of the Sun (the accurate modern figure is seventeen minutes). When the predictions of a scientific theory disagree with evidence the theory must be modified or even rejected, unless an explanation can be found. Newton’s beautiful clockwork Solar System was on trail.

      Romer was the first to realise that this delay was not a glitch in the clockwork Solar System. Instead, it was caused because light takes time to travel from Jupiter to Earth. The eclipses of the Galilean moons happen just as Newton predicts, but we don’t see the eclipses on earth until slightly later than predicted when Jupiter moves further away from the Earth, simply because it takes more time for light to travel a greater distance.

      From this beautifully simple observation of the eclipses of Jupiter, fellow Dutch astronomer, Christian Huygens, was able to make the first calculation of the speed of light. The speed of light, we now know, is a fundamental property of our universe. It is one of the universal numbers that is unchanging and fixed throughout the cosmos. Ultimately, an understanding of its true significance had to wait until Einstein’s theory of space and time, Special Relativity, in 1905, but the long and winding road of discovery can be traced back to Romer and his eclipses.

      Closer to home, eclipses become even more familiar. In 2004, the Mars Exploration Rover Opportunity looked up from the surface of Mars and took possibly the most beautiful picture of any extraterrestrial eclipse (opposite bottom). In this remarkable image you can see Mars’ moon Phobos as it makes its way across the Sun – an image of a partial solar eclipse from the surface of another world.

      Eclipses on Mars are not only possible but commonplace, with hundreds occurring each year, but one event we will never see on Mars is a total solar eclipse. Here on Earth, though, humans have the best seat in the Solar System from which to enjoy the spectacle of a total eclipse of the Sun – all thanks to a wonderful quirk of fate.

      For a perfect total solar eclipse to occur, a moon must appear to be exactly the same size in the sky as the Sun. On every other planet in the Solar System the moons are the wrong size and the wrong distance from the Sun to create the perfect perspective of a total solar eclipse. However, here on Earth the heavens have arranged themselves in perfect order. The Sun is 400 times the diameter of the Moon and, by sheer coincidence, it’s also 400 times further away from the Earth. So when our moon passes in front of the Sun it can completely obscure it.

      With over 150 moons in the Solar System you might expect to find other total solar eclipses, but none produce such perfect eclipses as the Earth’s moon. It won’t last forever, though; The clockwork of the Solar System is such that the raising of the tides on Earth caused by the Moon has consequences. As the Earth spins beneath tidal bulges raised by the Moon, its rate of rotation is gradually, almost imperceptibly, reduced by friction, and this has the effect of causing the Moon to gradually drift further and further away from Earth. This complex dance, in precise accord with Newton’s laws, is also responsible for the fact that we only see one side of the Moon from the Earth – a phenomenon called spin-orbit locking.

      The drift is tiny, only around 4 centimetres (1.6 inches) per year, but over the vast expanses of geological time it all adds up. Around 65 million years ago the Moon was much closer to Earth and the dinosaurs would not have been able to see the perfect eclipses we see today. The Moon would have been closer to Earth and would therefore have completely blotted out the Sun with room to spare. In the future, as the Moon moves away from the Earth, the unique alignment will slowly begin to degrade; while drifting away from our planet the Moon will become smaller in the sky and eventually too small to cover the Sun. This accidental arrangement of the Solar System means that we are now living in exactly the right place and at exactly the right time to enjoy the most precious of astronomical events.

      IN THE REALM OF THE SUN

      Our closest star is the strangest, most alien place in the Solar System. It’s a place we can never hope to visit, but through space exploration and a few chance discoveries our generation is getting to know the Sun in exquisite new detail. For us it’s everything, and yet it’s just one ordinary star among 200 billion starry wonders that make up our galaxy. To explore the realm of our sun requires a journey of over thirteen billion kilometres; a journey that takes us from temperatures reaching fifteen million degrees Celsius, in the heart of our star, to the frozen edge of the Solar System where the Sun’s warmth has long disappeared.

      On 14 November 2003, three American scientists discovered a dwarf planet at the remotest frontier of the Solar System. Sedna is a planetoid three times more distant from the Sun than Neptune. Around 1,600 kilometres (1,000 miles) in diameter, Sedna is barely touched by the Sun’s warmth; its surface temperature never rises above minus -240 degrees Celsius. For most of its orbit Sedna is further from our star than any other known dwarf planet. On its slow journey around the Sun, one complete orbit – Sedna’s year – takes 12,000 Earth years. From its frozen surface at least thirteen billion kilometres from Earth, a view of the Sun rising on Sedna would give a very different perspective on our solar system and a clear depiction of how far the Sun’s realm stretches. Sunrise on Sedna is no more than the rising of a star in the night sky: from this frozen place, our blazing sun is just another star.

      To travel from the outer reaches of Sedna’s orbit to one of the first true planets of the Solar System we would need to cover over ten billion kilometres. Uranus was the first planet to be discovered with the use of a telescope, in 1781, by Sir William Herschel, and like all the giant planets (except Neptune) it is visible with the naked eye. Even so, sunrise on Uranus is barely perceptible; the Sun hangs in the sky 300 times smaller than it appears on Earth. Only when we have travelled the two and a half billion kilometres past Jupiter and Saturn do we arrive at the first world with a more familiar view of the Sun. Over 200 million kilometres out, sunset on Mars is a strangely familiar sight. On 19 May 2005, the Mars Exploration Rover Spirit captured this eerie view as the Sun sank below the rim of the Gusev crater. The panoramic mosaic image was taken