At the end of the first century AD, Plutarch wrote a short work about the Moon, entitled On the Face in the Moon’s Orb where he suggested that the markings on the face of the Moon were caused by deep recesses, too deep to reflect sunlight. He proposed that the Moon had mountains and river valleys and even speculated that people might live there.
Although a Hindu astronomer, Aryabbata, repeated and confirmed the experiment conducted by Hipparchus as late as 500 AD, Christian authorities of the time maintained a biblical approach to the Moon and only information about our near neighbour that didn’t contradict the scriptures was countenanced. With the arrival of Christianity the world entered a dark age where scripture rather than science was the only permitted guide to human existence.
The grip of the Church slipped somewhat during the fifteenth and sixteenth centuries and the Renaissance (literally meaning ‘rebirth’) emerged bringing radical and comprehensive changes to European culture. The Renaissance brought about the demise of the Middle Ages and for the first time the values of the modern world made an appearance. The consciousness of cultural rebirth was itself a characteristic of the Renaissance. Italian scholars and critics of this period proclaimed that their age had progressed beyond the barbarism of the past and had found its inspiration, and its closest parallel, in the civilizations of ancient Greece and Rome. By the end of the sixteenth century, a genius from the town of Pisa called Galileo Galilei became one of the most important scientists of the Renaissance carrying out experiments into pendulums, falling weights, the behaviour of light and many other subjects that captured his imagination. Above all, for most of his adult life Galileo was an avid astronomer.
In May 1609, Galileo received a letter from Paolo Sarpi telling him about an ingenious spyglass that a Dutchman had shown in Venice. Galileo wrote in April 1610:
‘About ten months ago a report reached my ears that a certain Fleming had constructed a spyglass by means of which visible objects, though very distant from the eye of the observer, were distinctly seen as if nearby. Of this truly remarkable effect several experiences were related, to which some persons believed while others denied them. A few days later the report was confirmed by a letter I received from a Frenchman in Paris, Jacques Badovere, which caused me to apply myself wholeheartedly to investigate means by which I might arrive at the invention of a similar instrument. This I did soon afterwards, my basis being the doctrine of refraction.’
From these reports, and by applying his skills as a mathematician and a craftsman, Galileo began to make a series of telescopes with an optical performance much better than that of the Dutch instrument. His first telescope was made from available lenses and gave a magnification of about four times, but to improve on this Galileo taught himself to grind and polish his own lenses and by August 1609 he had an instrument with a magnification of around eight or nine. He quickly realized the commercial and military value of his super-telescope that he called a perspicillum, particularly for seafaring purposes. As the winter of 1609 brought colder, clearer nights Galileo turned his telescope towards the night sky and began to make a series of truly remarkable discoveries.
The astronomical discoveries he made with his telescopes were described in a short book called The Starry Messenger published in Venice in May of the following year – and they caused a sensation! Amongst many other findings Galileo claimed to have proved that the Milky Way was made up of tiny stars, to have seen four small moons orbiting Jupiter and to have seen mountains on the Moon.
As with many of his scientific investigations Galileo could easily have fallen foul of the Church authorities if his drawings of the Moon had been made public. According to Christian tradition both the Sun and the Moon were perfect, unblemished spheres. They simply had to be so because God had created them – and none of the Almighty’s creations could be flawed. Eventually Galileo was put under perpetual house arrest by the Papacy for his blasphemous claim that the Sun was at the centre of the solar system. It is therefore quite possible that he knew more about the Moon than he was willing to admit in public.
In order to explain the markings on the Moon without treading on the toes of the Church, a number of ideas were put forward in Christian countries. Perhaps the most popular of these, at least for a while, was the suggestion that the Moon was a perfect mirror. If this was the case there were no markings on the Moon but rather reflections of surface features on the Earth. It didn’t seem to occur to anyone that as the Moon orbited the Earth the markings should change, since the land beneath it would not remain constant.
Another suggestion, and one that was accepted in some circles, was that there were mysterious vapours between the Earth and the Moon. The images, it was suggested, were present in sunlight and were merely being reflected from ‘the vapours’. But the most popular theory, probably because it didn’t impinge on Christian doctrine, was that there were variations in the density of the Moon and that these created the optical illusions we see as markings on the Moon’s surface. This unlikely explanation was safe, though it probably did little to convince early scientists, and certainly would not have impressed Galileo.
After Galileo’s time, telescopes improved markedly and it was obvious to anyone who studied the Moon that it was a sphere with a rocky and undulating surface. As the Church gradually lost its power to direct scientific thought, many of the earlier ideas regarding the Moon became unthinkable. But no one had any idea how the Moon had come into being and why it occupied the orbit it did around the Earth.
It didn’t take long for the subject of the Moon to become very important to astronomers. Empires such as those created by Britain, France and Spain, were expanding. This necessitated long sea voyages and led to that most urgent of searches – a way to plot ‘longitude’ whilst at sea. It is quite easy to establish one’s position on the planet in a north–south line (latitude) but it was impossible to know where you were in terms of east–west (longitude). In the northern hemisphere, for example, latitude can be quickly gauged by measuring the angular distance between the horizon and the Pole Star. This angle also defines one’s position north of the equator.
The longitude problem was eventually solved by having an extremely accurate clock on board a ship that was set to the time at one’s point of departure. It wasn’t difficult to work out the difference between local time, say at midday, and the time at the home port. It was then simply a matter of adding or subtracting to discover one’s true position on the Earth’s surface. This was fine but it took many decades before a suitably accurate clock could be created. In the meantime, astronomers sought for other methods to determine longitude, not least of all because there was a fabulous prize on offer for anyone who could crack the problem. And the place where many of them turned to establish longitude was the Moon.
Astronomers proposed that if really accurate tables were kept of the Moon’s position relative to the background stars it would be possible to assess the true time of day in one’s home port. The reason this could work was that the Moon, being very close to the Earth and orbiting quickly, moved across the heavens by around thirteen degrees of arc per day. Using the Moon it was a fairly simple matter to establish ‘local time’ and then to do the necessary computations to discover one’s position.
The lists of tables necessary to accomplish the task were not so simple, however, and as soon as good chronometers were available the Moon was abandoned as a means for longitude assessment. However, the desire to solve this problem, and the potential profitability of doing so, meant that the Moon was receiving a great deal of attention during the seventeenth century and very accurate maps of its surface began to appear.
It wasn’t until the nineteenth century, however, that probably the first reasoned explanation as to the Moon’s origin was put forward. George Darwin, the son of Charles Darwin, the controversial Englishman who first proposed the theory of natural selection, was a known and respected astronomer who studied the Moon extensively and came up with what became known as the ‘fis
sion theory’ in 1878. George Darwin may have been the first astronomer to ascertain that the Moon was moving away from the Earth. Working backwards from his knowledge of the rate the Moon was receding from the Earth, Darwin proposed a time that the Earth and the Moon could have been part of the same common mass. He suggested that this molten, viscous sphere had been rotating extremely rapidly in about five and a half hours.
Darwin further speculated that the tidal action of the Sun had caused what he termed as ‘fission’ – a Moon-sized dollop of the molten Earth spinning away from the main mass and eventually taking up station in orbit. At the time this seemed very reasonable and was the favoured theory by the beginning of the twentieth century. In fact the fission theory did not come under serious attack until the 1920s when a British astronomer called Harold Jeffries was able to show that the viscosity of the Earth in its semi-molten state would have dampened the motions required to generate the right sort of vibration necessary to fulfil Darwin’s fission.
A second theory that once convinced a number of experts was the ‘coaccretion theory’. This postulates that the Earth, having already been formed, accumulated a disc of solid particles – a little like the rings of Saturn. It was suggested that, in the case of the Earth, this disc of particles ultimately came together to form the Moon. There are several reasons why this theory can’t be the answer. Not least is the problem of the angular momentum of the Earth–Moon system that could never have been as it is, if the Moon had formed in this way. There are also difficulties regarding the melting of the magma ocean of the infant Moon.
The third theory regarding the origin of the Moon that was in circulation around the time that the first lunar probes were launched was the ‘intact capture theory’. At one time seeming to be the most attractive possibility, the intact capture theory suggested that the Moon originated far from the Earth and that the Moon became a ‘rogue’ body that was simply captured by the gravitational pull of the Earth and that it took up orbit around the Earth.
There are many reasons why the intact capture theory is now disregarded. Oxygen isotopes of the rocks on the Moon and on the Earth prove conclusively that they originated at the same distance from the Sun, which could not be the case if the Moon had been formed elsewhere. There are also insurmountable problems in trying to build a model that would allow a body as big as the Moon to take up orbit around the Earth. Such a huge object could not simply drift neatly into an Earth orbit at low speed like carefully docking a super-tanker – it would almost certainly smash into the Earth at a massive speed or possibly skim off and hurtle onward.
By the middle of the 1970s all previous theories about the way the Moon had been formed were running into trouble for one reason or another and this created a virtually unthinkable situation in which acclaimed experts might have to stand up in public and admit that they simply didn’t know how or why the Moon was there. As acclaimed science writer William K Hartmann, senior scientist at the Planetary Science Institute, Tucson, Arizona said in 1986 in his book Origin of the Moon:
‘Neither the Apollo astronauts, the Luna vehicles, nor all the king’s horses and all the king’s men could assemble enough data to explain the circumstances of the moon’s birth.’ 9
Out of this miasma came a new theory and, in fact, the only one that is presently widely accepted despite some fundamental problems. It is known as the ‘Big Whack theory’.
The idea came out of theories that originated in the Soviet Union in the 1960s – specifically the work of Russian scientist V S Savronov, who had been working on the possibility of planetary origins from literally millions of different-sized asteroids known as planetesimals.
As a divergence from the Soviet ideas, Hartmann, together with a colleague, D R Davis, suggested that the Moon had come into being as a result of the collision of two planetary bodies, one being the Earth and the other a rogue planet at least as large as the planet Mars. Hartmann and Davis hypothesized that the two planets had collided in a very specific way that allowed jets of matter to be ejected from the mantles of both bodies. This matter was thrown into orbit, where it eventually came together to form the Moon.10
The suggestion seems to have many merits. First and foremost it appears to address the greatest puzzle that the recovery of Moon rock had thrown up: How was it that the composition of the Moon was so similar to that of the Earth, but only in part?
A close analysis of Moon rock has shown that it is very similar to the rock that forms the mantle of the Earth, yet the Moon is nowhere near as massive as the Earth in proportional terms. (The Earth is only 3.66 times as big as the Moon but has eighty-one times the Moon’s mass.) It was obvious that the Moon could not contain many of the heavy elements that are found inside the Earth and the Big Whack theory purported to explain why this was the case. The Earth and the rogue visitor had come together in a very specific way. Although they would eventually form one planet it was reasoned that they must have impacted, drawn apart and then come together again. Computer modelling showed that under these very special circumstances it would have been possible for the material thrown off to have been mantle material, from close to the surface of the two bodies.
Although the theory eventually gained ground, at first it seemed so improbable that it was generally rejected. But with the passing of time, further work showed that such an unlikely scenario could conceivably have taken place. In 1983 an international conference was held at Kona, Hawaii, to try and solve the problems regarding the origins of the Moon. It was at this meeting that the Big Whack theory, also known as the Giant Impact Hypothesis of the Collision Ejection theory, began to gain ground. Hartmann’s own suggestions, together with those of other scientists at the conference, formed the nucleus of the 1986 book, Origin of the Moon, which was edited by Hartmann himself.
In the intervening period several experts have created computer models that purport to add weight to the Big Whack theory and the most convincing of these are those of Dr Robin Canup, who is now Assistant Director of the Department of Space Studies in Colorado, USA. Canup wrote her PhD dissertation on the Moon’s origin and specifically the Big Whack theory. Her early work led to the conclusion that the suggested impact would have actually led to a swarm of moonlets, rather than the Moon, but by 1997 further computer modelling resulted in a model of the impact that would lead to the Moon’s presence.
Despite the fact that the Big Whack theory is now generally accepted by most authorities, it has many problems. Not least of all is that recognized by Robin Canup herself as she admits that there is one key aspect of the theory that doesn’t make sense. This stems from the fact that other researchers have pointed out that such a massive impact as that proposed could not have failed to speed up the rotation of the Earth to a level far beyond today’s situation. Canup agrees and the only way that she could deal with this anomaly is to propose a second major impact – which was designated ‘Big Whack II’. This suggests that the second planetary collision happened perhaps only a few thousand years after the first one but, quite incredibly, this incoming object came from the opposite direction and so cancelled out the huge spin imparted to the Earth by the first cataclysmic event. This balanced double act sounds unlikely in the extreme. Two cosmic collisions that just happen to precisely return the planet to its natural rhythm? To us, this explanation smacks of desperation!
Canup herself is not happy with Big Whack II and is hopeful of modifying the original theory so that it can account for the present rate of spin of the Earth.
There is another big problem to overcome if the Big Whack theory is to be taken seriously. When rocks were brought back from the Moon, both by American astronauts and Soviet unmanned Moon missions, they were subjected to every conceivable test. The observed fact that put paid to the captured asteroid theory of the Moon is also a gigantic stumbling block to the Big Whack theory. It has been observed that the oxygen isotope signatures of Moon rocks are identical with those of rocks from the Earth – and that fact has some serious implicat
ions: Moon rocks and Earth rocks can only have the same oxygen isotope signature if they originated at the same distance from the Sun. This would mean that the Mars-sized body that hit the Earth must have occupied a similar orbit to that of the Earth and yet had already managed to survive for many millions of years before it hit the Earth.
That does not sound reasonable.
This situation is extremely unlikely and it throws up other difficulties. The present obliquity of the Earth (its twenty-three degree tilt against the plane of its orbit around the Sun) is usually deemed to be the result of the giant impact, but any body of the size of Mars that was in an orbit similar to that of the Earth could not have had sufficient momentum to knock the Earth’s angle of rotation back so severely. Either the rogue planet was Mars-sized, and came from way out in the solar system and was therefore travelling extremely fast, or else it had to be at least three times the size of Mars, which doesn’t tie in with the computer models as they stand.
Some of the other problems were cited by Jack J Lissauer, a well-respected scientist from NASA’s Ames Research Center in an article he wrote for Nature in1997.11 Lissauer is said to have joked to his students about a remark made by another scientist, Irwin Shapiro from the Harvard-Smithsonian Center for Astrophysics: ‘The best explanation for the Moon is observational error – the Moon does not exist!’
Lissauer’s article pointed out some of the problems with the Big Whack theory. He made it clear that in his opinion the latest research demonstrated that much of the material blown out by the impact (the ejecta) would have fallen back to the Earth. He says:
‘The implication here is that lunar growth in an impact-produced disk is not very efficient. So, to form our Moon, more material must be placed in orbit at a greater distance from Earth than was previously believed.’
Who Built the Moon? Page 5
