distance. Put very simply, the further away an object is, the dimmer it appears! Leavitt and Pickering published a more detailed study in 1912, in which they proposed a simple mathematical relationship between the period and intrinsic brightness of 25 variable stars. This relationship is known as the period-luminosity relation. All that was required to calibrate the relation was a parallax measurement of the distance to a single variable of the type observed by Leavitt. If this could be achieved, then the distance to the Small Magellanic Cloud could be obtained. In 1913, the Danish astronomer Ejnar Hertzsprung, in a spectacularly accurate piece of astronomical observing, managed to measure the distance by parallax to the well-known variable star Delta Cephei. Delta Cephei has a period of 5.366341 days, and lies 890 light years from Earth, according to modern measurements by the Hubble Space Telescope. Because of its historic place as the first of Leavitt’s variable stars to have its distance measured, these stars are now known as Cepheid variables. Inexplicably, even though Hertzsprung managed to get the parallax measurement and the distance to Delta Cephei correct, his published paper quotes the distance to the Small Magellanic Cloud as 3000 light years, which is badly wrong; the modern-day measurement is 170,000 light years. There is speculation that Hertzsprung made a simple typographical error in the paper, and for some reason couldn’t be bothered to correct it. In any case, the technique had been established, and two years later Harlow Shapley published the first of a series of papers that refined the method and led him to the first measurements of the size and shape of the Milky Way. He concluded that the galaxy is a disc of stars, around 300,000 light years in extent, with the Sun positioned around 50,000 light years from the centre. This is roughly correct – the Milky Way is around 100,000 light years across and the Sun is about 25,000 light years from the centre. This was an important moment in the history of astronomy, because it was the first measurement that relegated the solar system from being the centre of everything. It’s true that few if any astronomers would have claimed otherwise by the turn of the twentieth century, but science is a subject that relies on measurement rather than opinion. The journey into insignificance had begun.
With the size and shape of the galaxy measured, the question of our place in creation now shifted from the position of the Sun within the galaxy to the nature of the universe itself. If the progress from Copernicus through Newton to Leavitt and Shapley appears relatively fast, certainly when viewed in the context of the glacial progress throughout the 2000-year dominance of Aristotelian thinking, then the decade that followed Shapley’s determination of the size of the Milky Way might be described as an intellectual avalanche. The revolution was fuelled from two sides. A new generation of telescopes and the increasingly sophisticated observational techniques developed by astronomers like Leavitt, Hertzsprung and Shapley provided the data, and in parallel theoretical physics experienced a revolution. Claims of revolutions or paradigm shifts have to be made with great care in science – indeed the terminology is quite unfashionable in certain academic circles. But from a physicist’s perspective there is no doubt that physics experienced a revolution in 1915, because in November of that year Albert Einstein presented a new theory of gravity to the Prussian Academy of Science.
The theory is known as General Relativity, and it replaces Newton’s law of universal gravitation. Many physicists regard General Relativity as the most beautiful piece of physics yet devised by the human mind, and we will explore why this is so a little later. For now, let us note that the Big Bang, the expanding universe, black holes, gravitational waves and the whole evocative landscape of twenty-first-century cosmological language began, absolutely, with the publication of General Relativity. The parallels with the Newtonian revolution are clear. Without Newton’s laws, there is no deep understanding of the solar system and the motions of the planets. Without General Relativity, there is no deep understanding of the large-scale structure and behaviour of the universe. But we are getting ahead of ourselves. As the second decade of the twentieth century dawned, the size and shape of the Milky Way galaxy was established, albeit with rather large errors, but the true extent of the universe beyond our galaxy was still hotly debated. Could we, at least, cling to a sort of token pre-Copernican fig leaf and place our galaxy at the centre of the universe? The desire to be special runs deep. The last intellectual rearguard action against our demotion can, rather theatrically, be said to have played out on a single evening on 26 April 1920 in the Baird auditorium at the Smithsonian Museum of Natural History, Washington DC. This is, of course, an oversimplification, but allow me a minute to enjoy the sound of the outraged shaking jowls of a thousand historians of science before I qualify and partially justify this hyperbolic claim.
The history of science is littered with crunching moments of conflict, debates and disagreements that divided opinion in the most passionate of battles. The wonderful thing about science, however, is that the debates can be settled when facts become available. Science and ‘conservative common sense’ famously clashed in 1860 when Thomas Huxley and Samuel Wilberforce fulminated over the new theory of evolution published by Darwin seven months earlier. I imagine Wilberforce’s indignant reddening cheeks shaking with righteous outrage as he denied the repugnant possibility that his grandfather was a monkey. None of his relatives was a chimpanzee, by the way; we simply share a common ancestor with them around 6 or 7 million years ago. But the ‘unctuous, oleaginous and saponaceous’ bishop, as Disraeli once called him, was having none of it. This might be a little unfair to the great Victorian orator and bishop of the Church of England, but in the case of evolution he was firmly on the wrong side of reality. Few great leaps in knowledge occur without dividing opinion, and this is entirely appropriate. Extraordinary claims require extraordinary evidence, and the great scientific discoveries we are celebrating here are utterly extraordinary. The trick as an educated citizen of the twenty-first century is to realise that nature is far stranger and more wonderful than human imagination, and the only appropriate response to new discoveries is to enjoy one’s inevitable discomfort, take delight in being shown to be wrong and learn something as a result.
The world of astronomy had its moment of intellectual sumo in what has become known as the Great Debate. The year was 1920, and two eminent astronomers found themselves stuck on a train together travelling the 4000 kilometres from California to Washington to discuss the greatest cosmological question of the day. The younger of the two men, Harlow Shapley, we have already met. He had just published his data suggesting that the Milky Way galaxy was much larger than previously suspected. This, however, was where he believed the universe stopped; Shapley was convinced our galaxy was the beginning and end of the cosmos. His travelling companion thought otherwise. Heber Curtis had been studying a misty patch of light known as the Andromeda Nebula. He was convinced that this was not part of our galaxy, but a separate island universe of billions of other stars.
It is not known what they discussed on the train, but the debate itself took place at the Smithsonian Museum of Natural History throughout the day and night of 26 April. At stake was the scale of the universe itself, and both men knew that the question would ultimately be settled by evidence rather than debating skills. The human race had already been shunted from the centre of the universe by Copernicus, and now faced the possibility that the Milky Way galaxy itself was part of a multitude, stretching across millions of light years of space. The question wasn’t settled that evening, but the experienced Curtis, perceived as the underdog because of the magnitude of what he was suggesting, landed significant blows. Curtis observed that the Andromeda Nebula contains a number of novae – exploding stars that shine temporarily, but brightly, in the night sky – but he also noted that the novae in Andromeda appeared on average to be ten times fainter than any others. Curtis asserted that Andromeda’s novae appear dimmer simply because they are perhaps half a million light years further away than those in the Milky Way. Andromeda is therefore another galaxy, claimed Curtis, which strongly implied that the other so-called nebulae were other galaxies too. This was the very definition of an extraordinary claim, and the extraordinary evidence came only four years later.
In 1923 a photo of Andromeda, taken by a 33-year-old astronomer called Edwin Hubble, further fuelled the