This article was published by UNESCO on the occasion of the International Year of Astronomy 2009
Millions around the world held their breath on 20 July 1969, as the United States’ Eagle lunar module touched down in the Moon’s Sea of Tranquillity – without making a splash! Despite its name, the Sea of Tranquillity does not contain a single drop of water. Instead, it is covered with volcanic rocks which flowed some 3 to 4 billion years ago, as lava. The quirky name, however, is reminiscent of a once popular theory explaining the Moon’s appearance from Earth. As Italian painter and scientist Leonardo da Vinci wrote in his Codex Leicester, “The luminous part of the Moon is water, which has been stirred up by the winds.”
You might be forgiven for thinking that a scientific explanation for the Moon’s appearance was not practically possible – that is until Italian astronomer Galileo Galilei turned his telescope towards the night sky 400 years ago. Now enter Alhazen, or Ibn Haitham (965-1040), an Iraqi scientist working at the turn of the first millennium near Al-Azhar mosque in Cairo, and dubbed the founding father of modern optics. Among a number of major contributions, he answered one of the most intriguing questions faced by science: what gives rise to the dark figure in the Moon’s face.
Mystery solved
This phenomenon had been the subject of much speculation since antiquity, resulting in numerous extravagant theories. Half a dozen of these, at least, were explored by Alhazen in his work The Trace on the Moon’s Face. Here, he showed that none of the theories examined made predictions that agreed with observational evidence. He put each candidate theory to the test using some crucial observations, most notably the fact that the figure in question always appears constant in terms of positioning, size, shape, and darkness. For instance, he ruled out the theory that the dark figure was an image of Earth’s oceans and mountains reflected on the Moon’s mirror-like surface. Based on the law of reflection, he showed that the Moon’s changing angle with respect to an observer on Earth meant that any such image had to change with time – which is obviously not the case. A couple of other theories were written off based on similar arguments. Firstly, that the dark figure is a shadow caused by dramatic lunar features such as mountains and craters. This time, Alhazen argued that the Moon’s changing orientation with respect to the Sun meant that any shadowy patterns had to also change with time – flying in the face of empirical evidence.
And secondly, that the dark figure is caused by a vaporised substance ever present between the Moon and Earth. He argued that if this were true, observing the Moon from different Earthly locations would show the vaporised substance against different parts of the Moon, if not outside the Moon altogether! Solar eclipses – something of a laboratory for physicists and astronomers – enabled Alhazen to rule out yet another exotic theory: the dark figure is a transparent region in the lunar body. If this were true, asked Alhazen, why doesn’t sunlight shine through it during an eclipse of the Sun? Moonlight, Alhazen concluded, can only be adequately explained using the phenomenon of diffuse reflection – that is, reflection from a rough surface. Further, the Moon does not reflect light in any other way. What gives rise to the dark figure, he explained, is the fact that its material, because of its different optical properties simply reflects less light!
Date with the Moon
Although, astonishingly, Alhazen made his lunar discoveries based on observations with the naked eye, the study of the visual and magnifying properties of lenses was in fact launched with his Book of Optics. This new understanding of the lens, based on geometry and experiment, underpinned the craft of the Dutch spectacle-makers who, by holding one lens in front of another, invented the telescope, enabling Galileo to revolutionise astronomy. In December 1609, hunting for the unexpected, Galileo used a 20-fold magnification telescope to observe the Moon. He could make out mountains, craters, and what he wrongly thought were seas (i.e. water). We now know, from lunar samples collected by Apollo missions (1966-1972), along with a certain amount collected by Russia’s robotic Luna missions (1958-1976) that lunar maria are covered with dark rocks (basalts) – confirming Alhazen’s basic conclusions about the makeup of our closest neighbour.
The two men are immortalized on the Moon: there is a lunar crater, Galilei, celebrating Galileo’s discoveries. Another crater, Alhazen, celebrates Ibn Haitham’s. Ambitious projects such as the IYA2009 could inspire the “next big thing” in astronomy. Soon the entire night sky will be continuously scanned by space as well as Earth-based telescopes, across many wavebands, churning out terabytes of data. The next 40 years, let alone the next 400, could well see another revolution in astronomy. Will this coincide with the realisation of the first large-scale quantum computers capable of accomplishing, so to speak, astronomical feats? Only time will tell.
The Miracle of Light, A World of Science, Vol. 3, No. 4, October-December 2005.
Hatim Salih is a Sudanese researcher in quantum computing and a member of the York Astronomical Society (United Kingdom).