A rare sight; in this picture Earth’s crescent moon is visible above Venus (bottom) and Jupiter (right) in the night sky. As light takes longer to reach Earth from other planets and moons, depending on how far away they are, we see further into their respective pasts.
© JASON REED/Reuters/CORBIS
Look up at the Moon and you are looking at our closest neighbour a second in the past, because it is on average around 380,000 kilometres (236,120 miles) away; perceptible certainly, but not important. However, take a look at the Sun and you really are beginning to bathe in the past.
The Sun is 150 million kilometres away (93 million miles) – this is very close by cosmic standards, but at these distances the speed of light starts to feel rather pedestrian. We are seeing the Sun as it was eight minutes in the past. This has the strange consequence that if we were to magically remove the Sun, we would still feel its heat on our faces and still see its image shining brightly in the sky for eight minutes. And because the speed of light is actually the maximum speed at which any influence in the Universe can travel, this delay applies to gravity as well. So if the Sun magically disappeared, we would not only continue to see it for eight minutes, we would continue to orbit around it too. We are genuinely looking back in time every time we look at the Sun.
However, this is just the beginning of our time travelling. As we look up at the planets and moons in our solar system, we move further and further into the past. The light from Mars takes between four and twenty minutes to reach Earth, depending on the relative positions of Earth and Mars in their orbits around the Sun. This has a significant impact on the way we design and operate vehicles intended for driving on the surface of Mars. When Mars is at its furthest point from Earth it would take at least forty minutes to be told that a Mars Rover was driving over a cliff and then be able to tell it to stop, so Mars Rovers need to be able to make up their own minds in such situations or must do things very slowly. Jupiter, at its closest point to Earth, is around thirty-two minutes away, and by the time we journey to the outer reaches of our solar system, the light from the most distant planet, Neptune, takes around four hours to make the journey. At the very edge of the Solar System, the round-trip travel time for radio signals sent and received by Voyager 1 on its journey into interstellar space is currently thirty-one hours, fifty-two minutes and twenty-two seconds, as of September 2010.
But look beyond our solar system and the time it takes for light to travel from our nearest neighbouring stars is no longer measured in hours or days, but years. We see Alpha Centauri, the nearest star visible with the naked eye, as it was four years in the past, and as the cosmic distances mount, so the journey into the past becomes ever deeper
TO THE DAWN OF TIME
When filming a series like Wonders of the Universe, the locations are chosen to be visually spectacular, but they must also have a narrative that enhances the explanation of the scientific ideas we want to convey. Occasionally, the locations deliver more. There is a resonance, a symbiosis between science and place that serves to amplify the facts and generates something deeper and more profound on screen. For me, the Great Rift Valley was such a place.
We arrived in Tanzania on 10 May 2010 for the first day of filming. After a brief overnight stay close to the airport at Kilimanjaro, we were driven out into the Serengeti in vintage dark green Toyota Land Cruisers, complete with exaggerated front cattle bars and shovels tied to the rear doors. The landscape is unmistakably African; the warm, damp light still wet from the rains illumines plains seemingly too vast to fit on our planet. The horizon, darkened by scattered thunderclouds stark against the early summer skies, is simply more distant than it should be. The rains have brought with them journeys, and as you drive you experience first-hand the thousand-mile migration of the Serengeti wildebeest. The relentless advance of these herds creates ruts in the drying savannah along the precise and ancient roads that always seem to run at right angles to your direction of travel, shaking the Land Cruisers to the edge of their design tolerance. Zebras, giraffe and Grant’s gazelles graze, unconcerned, as our intrepid film crew rattles by.
The Great Rift Valley is not just an extraordinary geological feature…there is more to this place because the echoes of the history of humanity ring louder across these plains than anywhere else on the planet.
The Great Rift Valley, Tanzania, is one of the most spectacular geological locations on Earth. The summer skies were darkened by rainclouds, but these soon departed to reveal dusty, unmistakably African landscapes and breathtaking vistas.
The Great Rift Valley, Tanzania, is one of the most spectacular geological locations on Earth. The summer skies were darkened by rainclouds, but these soon departed to reveal dusty, unmistakably African landscapes and breathtaking vistas.
Our camp is idyllic by the strictest definition of the word. Khaki tents nestle beneath acacia trees in the shadow of a giant copper-striped rock populated by a tribe of itinerant baboons intent on stealing our tape stock. Fortunately, we are guarded by the Masai, who, all cliché aside, are as tough as hell and scare not only the baboons but also the Serengeti lions and the BBC in equal amount.
So much for the visuals; the reason for the resonance of this place lies in the deep past of this dramatic landscape of life. The Great Rift Valley is not just an extraordinary geological feature that stretches 6,000 kilometres (3,700 miles) from Syria to Mozambique; there is more to this place because the echoes of the history of humanity ring louder across these plains than anywhere else on the planet. To walk this earth is to walk in the footsteps of the true ancients. Ancestors like Lucy, one of the most important fossils ever discovered, a skeleton uncovered in the Ethiopian section of the valley in 1974 by Donald Johanson. Lucy is 3.2 million years old; the remains of an Australopithecus, an extinct hominid species many anthropologists believe links directly to our own heritage. Further down the rift, in Tanzania, more closely related human ancestors have been discovered. In the early 1960s, Mary and Louis Leakey unearthed the remains of the earliest known species of our genus, Homo. Homo habilis is thought to have been a direct descendant of Australopithecus, and may be the first of our ancestors to have made tools. It’s all in the mind, I suppose, but sitting around a fire on a cool evening in the Serengeti I felt as if I had returned to the place where I had been born after many years away. There is something about geographic origins that resonates, over a lifetime or a hundred thousand lifetimes
FINDING ANDROMEDA
The connection between the history of the Serengeti and the science of light is a dimly glowing jewel in the velvet Tanzanian sky. With no cities to pollute the darkness, the plains of the African night are bathed in the light of a billion suns. The glowing arc of the Milky Way Galaxy dominates the sky, a silver mist of stars so numerous, they are impossible to count. Every single point of light and every patch of magnificent mist visible to the unaided human eye have as their origin a star in our own galaxy, or the misty clouds known as the Magellanic clouds – two small dwarf galaxies in orbit around the Milky Way. All except for one…
To find it, you first need to recognise the distinctive ‘W’ shape of the constellation of Cassiopeia. It sits on the opposite side of Polaris, the North Star, to the constellation Ursa Major, otherwise known as The Great Bear or The Plough. Cassiopeia, being so close to Polaris,
