by Marcus Chown
The rock and ocean are stretched at high tide and squeezed at low tide. Consequently, at high tide, water is sucked out of a well, lowering its water level, and at low tide water is squirted back into a well, raising its water level. This is precisely the phenomenon observed by Poseidonios. It took 2,000 years but Pekeris finally explained it.
Tides in the rock: the LHC
There is a more contemporary and more high-tech example of the effect of the tides on the solid planet. At CERN, the European laboratory for particle physics, near Geneva, subatomic particles are whirled at dizzying speed around a subterranean racetrack 26.7 kilometres in circumference. While cows graze peacefully in fields spanning the border between France and Switzerland, 100 metres or so below them the microscopic building blocks of matter are slammed together in collisions of unimaginable violence. The energy of motion of the incoming particles is converted into the mass-energy of new particles, which are conjured out of the vacuum like rabbits from a hat.13 As the subatomic shrapnel speeds outwards from the collision point, it is detected by cathedral-sized detectors. It was in such collision debris, for instance, that the Higgs particle (the ‘quantum’ of the Higgs field, responsible for endowing all other subatomic particles with their masses), was discovered in July 2012.
The Higgs was found with the Large Hadron Collider, which whirls beams of protons both ways around the underground ring at 99.9999991 per cent of the speed of light before slamming them into each other.14 But the LHC occupies the circular tunnel previously used by another particle accelerator: the Large Electron-Positron Collider, which instead smacked together electrons and their antiparticles, positrons. And it was while using the LEP in 1992 that physicists noticed something peculiar about the energy of the particle beams.15
More than 3,000 electromagnets distributed around the circular LEP tunnel constrained the electrons and positrons, continually bending their trajectories away from the straight-line trajectory their inertia wanted them to take. But the LEP physicists noticed that, twice every 25 hours, the beams drifted slightly from their path and back again. In order to keep the beams from wandering outside the giant ring, the physicists had to continually compensate for the drift by slowly increasing the energy of the particles, then reducing it again. The necessary change in energy of the beams was tiny – about a hundredth of a per cent.
What could possibly be causing the particle beams to drift periodically from their circular path? After the physicists had puzzled for a while, they finally realised. The tides rise and fall twice every 25 hours. Incredible as it seemed, the effect observed at the LEP was connected to the tides.
Twice every 25 hours the rock into which the LEP ring was bored bulged upwards. This stretching of the rock caused the LEP to shrink. And twice every 25 hours the crust dropped back down, compressing the rock and expanding the LEP. The crust moved up and down by only 25 centimetres, roughly the height of the book you are reading, and this changed the circumference of the LEP by at most 1 millimetre.16 Nevertheless, it was enough to require the energy of the circulating particles to be periodically adjusted by about 0.01 per cent lest they wander from the ring.17
The effect was of course largest when the Moon was full or when there was a new Moon – times when the Sun and the Moon are aligned and reinforce each other’s effect on the Earth. It is hard to imagine a more high-tech manifestation of the effect of the tides on the solid Earth.18
Moonquakes
But the rocks on the Earth are not alone in experiencing tidal stretching and squeezing. So too do the rocks on the Moon. In fact, the tides the Earth pulls on the Moon are much bigger than the tides the Moon pulls on the Earth because the Earth is about 81 times more massive than the Moon. It might be expected that the tides on the Moon are also 81 times bigger than those on the Earth. But, remember, tides are not caused merely by gravity but by differences in gravity. And the Moon is only about a quarter the diameter of the Earth, which means it has only a quarter of the length span over which such a difference in gravity can manifest itself. So the tidal stretching of the Moon by the Earth is not 81 times bigger than the tidal stretching of the Earth by the Moon but only a quarter of that figure, or about 20 times.19 Nevertheless, it is enough to stretch the Moon by about 10 metres.
We tend to think of the Moon as stone-cold dead, its grey, crater-strewn desolation untouched by the hand of change. But this tidal stretching and squeezing means that the Moon is not quite the inert world of popular imagination. In fact, since well before the invention of the telescope, people have reported seeing strange lights on the Moon at a rate of once every few months. One of the earliest sightings, for instance, was made on 18 June 1178 by five monks at Canterbury Cathedral who reported an explosion on the Moon. The mysterious lights, known as Transient Lunar Phenomena, are one of the greatest mysteries of the Moon.
TLPs that have been observed in the age of the telescope share a number of common features. They are localised, slightly bigger than the resolution limit of the human eye, implying they cover an area of at least 1 square kilometre. They last from a minute to a few hours. They involve a brightening, dimming or even blurring of the lunar surface. And before they disappear they sometimes change colour to a ruby red.
For a long time many astronomers believed that TLPs were ‘in the eye of the beholder’ and not a phenomenon intrinsic to the Moon. But, in 2002, Arlin Crotts of Columbia University in New York sifted through the records of 1,500 historical sightings. He discovered that most reliable TLPs occur at just six locations on the Moon – half at the 45-kilometre-diameter Aristarchus crater and a quarter at the 100-kilometre Plato crater.20
The six locations are all places where the lunar crust has been violently fractured, either by relatively recent asteroid or comet impacts – within the last few hundred millions of years – or by the flurry of mega-impacts which occurred 3.8 billion years ago and caused lava to well out of the Moon’s interior and form the lunar ‘seas’, or Maria.21
Seismometers left on the Moon by all but one of the Apollo missions have recorded several hundred ‘moonquakes’, which, not surprisingly, are more common when the tidal effect of the Earth is greatest. Most have been located along the boundaries of the mare basins where the rock is most fractured. Not only that but Apollo 15, Apollo 16 and the Lunar Prospector probe, which orbited the Moon in 1998, all detected occasional bursts of radioactive radon-222 gas on the surface, and these were associated exclusively with the six TLP sites.
Radon-222 is a decay product of uranium, which is distributed throughout the rocks of the Moon’s interior. Crotts consequently speculates that TLPs occur when moonquakes cause gas from deep in the lunar interior to vent through fissures and cracks. The gas often builds up pressure before bursting its plug of lunar soil, or ‘regolith’, and exploding into space.
Crotts thinks a mere half a tonne of gas escaping into the vacuum would be enough to puncture the regolith, creating a cloud a few kilometres across that persists for between 5 and 10 minutes. The gas cloud either plunges the surface below into shadow or shines brightly because the dust grains it contains reflect more light when scattered through the vacuum than when clumped together on the surface. It is also possible that friction between the grains separates out negative and positive electrical charge, eventually triggering a ‘breakdown discharge’ like lightning which energises the atoms of the gas, causing then to emit characteristic red light.
According to Crotts’ calculations, the periodic tidal stretching and squeezing of the Moon by the Earth’s gravity grinds up about 100,000 tonnes of rock a year — a mass equivalent to one aircraft carrier. And from this is released about 100 tonnes of gas.
None of this speculation is academic because there are plans for humans to go back to the Moon. Apollo 18, which was cancelled before its launch, was actually scheduled to land at one of the principal TLP sites. If a TLP happened at a landing site it would be very dangerous for any astronauts. Picture the scene:
20 July 2025, Arist
archus crater, lunar nearside: exactly 56 years after Apollo 11, NASA’s Altair 2 landing craft touched down only hours ago and astronauts are now walking on the Moon again for the first time in more than half a century. Suddenly, a large area of the crater floor begins to convulse and a titanic explosion of gas sends dust fountaining up into the vacuum. Knocked off their feet by the blast, the astronauts look back towards their landing craft. But it is no longer there. It has disappeared in a roiling cloud of silver dust.
If Crotts is right, the Moon is a more dangerous place for humans than anyone suspected. And it is entirely a consequence of Newton’s theory of tides.
Since moonquakes can be triggered by tides pulled by the Earth in the Moon’s rock, it is natural to wonder whether terrestrial earthquakes are triggered by tides pulled by the Moon in the Earth’s rock. It seems they are not – at least not the big earthquakes. But, interestingly, the aftershocks of the devastating earthquake which struck Christchurch in New Zealand on 22 February 2011 were found to be correlated with the location of the Moon in the sky.22 A possible reason for this may be that the big quake left the rock in an unstable state, ready to move again if nudged by even the slightest force.
Tidal spin-down of the Moon
The tides on the Earth and Moon do more than simply distort the shape of each body, causing the rise and fall of the seas on Earth and moonquakes on the Moon. They have profound consequences for the system of the Earth and Moon as a whole. Once upon a time, for instance, the Moon spun faster than it does today. Its rotation was slowed by the tidal interaction with the Earth.
When the Moon was spinning faster, the bulge pulled in the Moon by the Earth was dragged around with the rotation of the Moon so that the bulge no longer quite faced the Earth. The Earth’s gravity pulled back on this receding bulge and the effect of this was to brake the rotation of the Moon. Eventually, a point was reached when the Moon was spinning so slowly that it was turning only once on its axis during each orbit of the Earth.
This is the case today. One face of the Moon – the lunar nearside – perpetually points towards the Earth while the lunar far side perpetually faces away from the Earth. In fact, the far side of the Moon was seen for the first time only on 7 October 1959 when the Soviet ‘Luna 3’ space probe flew over it.23
Because of the Moon’s ‘synchronous’ rotation, the tidal bulge caused by the pull of the Earth points perpetually towards the Earth. Since the bulge is no longer being dragged around by the Moon’s rotation, the Earth’s gravity, which formerly pulled back on the receding bulge, braking the Moon’s spin, no longer has any effect on the Moon’s rotation. In fact, the Moon’s rotation has been ‘locked’ in this state ever since the moment the Moon’s rotation period first matched its orbital period.
Tidal spin-down of the Earth
But the Moon is not alone in having its rotation slowed by a tidal interaction. The rotation of the Earth is also slowed. The effect is less dramatic than in the case of the Moon because the Earth, being a far heavier flywheel, is more resistant to having its motion changed. Consider the bulge created in the ocean on the side of the Earth facing the Moon. Because the Earth is spinning quickly, the bulge tends to get ahead of the line joining the Earth to the Moon.24 The Moon’s gravity pulls back on this receding bulge, braking the Earth’s rotation.
The unavoidable conclusion is that the Earth must have spun faster in the past. And evidence supports this. It comes from corals. Such marine organisms, most commonly found in tropical seas, secrete calcium carbonate to form a hard skeleton. The daily and seasonal growth of the skeletons creates regular bands in much the same way that the yearly growth of trees creates tree rings. By counting the bands, it is possible to determine how many days there are in a year. The evidence from fossil corals which lived about 350 million years ago is that at the time there were about 385 days in a year. Since the year – the time taken for the Earth to circle the Sun – is unlikely to have been different, it must mean that the day 350 million years ago was less than 23 hours in length.25
A reduction of just over an hour in the day in 350 million years indicates a relatively modest slowing down of the Earth’s spin. But the slowdown is remorseless and ongoing. We know, for instance, that the day today — if that makes sense – is longer than the day of a century ago by about 1.7 milliseconds. In fact, we can be sure that for the past 2,500 years the length of the day has been increasing at 1.7 milliseconds per century. The evidence, remarkably, comes from Babylonian clay tablets.26
Babylonian astrologers used such tablets to record total eclipses of the Sun, when the disc of the Moon slides across the solar disc, plunging the world into darkness in the middle of the day. Most of the tablets were unearthed in the nineteenth century by peasants looking for bricks and sold to antique collectors in Baghdad, 85 kilometres to the north of the ancient city of Babylon. From there they found their way to the British Museum in London, which boasts an almost complete collection. Many of the clay tablets record the precise times of the total eclipses.
But the timings pose a puzzle.
In 136 BC, for instance, an astrologer recorded that, at 8.45 a.m. on the morning of 15 April, Babylon was plunged into darkness when the Moon passed in front of the Sun. There is no reason to doubt the astrologer’s account. But, if present-day astronomers use a computer to wind back the movements of the Earth, Moon and Sun, like a movie in reverse, they find something puzzling. The total eclipse of 15 April 136 BC should not have been visible from Babylon. The Earth, Sun and Moon were simply not lined up in such a way to create a total eclipse. In fact, the ‘zone of totality’ should have passed over the island of Mallorca, 48.8 degrees west of Babylon.
A difference of 48.8 degrees amounts to one-eighth of a complete rotation of the Earth, or 3.25 hours. It seems that, during the total eclipse of 15 April 136 BC, the Earth was one-eighth of a turn to the east of where it should have been. There is only one way to explain this. Over the past millennia, the Earth’s spin must have slowed down. Since 136 BC there have been about a million days, so, even if the day was only a fraction of a second longer back then, all those fractions of a second would have added up to explain the 3.25-hours discrepancy in the timing of the 136 BC total eclipse. In fact, the only way to make sense of the Babylonian eclipse records is if the day in 500 BC was about 1/20 of a second shorter than it is today, and that ever since it has been lengthening by 1.7 milliseconds per century.
It is wonderful that marks scratched onto a clay tablet by an ancient civilisation can yield such super-precise astronomical information. The phenomenal accuracy of the techniques is down to the astronomical coincidence that the Moon and Sun appear the same size in the sky. This leads to an eclipse ‘track’ which is at most 250 kilometres wide, making total eclipses at any given spot on Earth very rare indeed. So if someone in antiquity recorded an eclipse at a particular location, it is not necessary for astronomers today to know a precise date in order to identify it. Knowing the year to within 20 years either way is usually good enough.
There is a twist to this story. Subtle changes in the shapes of the orbits of artificial satellites caused by the Earth’s tidal bulge imply that tidal braking of the Earth should in fact be lengthening the day by 2.3 milliseconds per century not 1.7 milliseconds. Something else must be affecting the Earth’s spin. The something else turns out to be connected with the last ice age, which finished about 13,000 years ago.
During the ice age, the tremendous weight of the ice sheets bore down on the Earth, flattening the planet slightly at the poles and making it fatter. At the end of the ice age, when the ice began to melt, the land began slowly to rise. This process of ‘postglacial rebound’ is still going on today. Its effect is to make the Earth more circular and less fat. Consequently, like an ice skater pulling in their arms, the planet spins ever faster. The effect causes a shortening of the day by between 0.5 and 0.6 milliseconds per century, and explains why the day is currently lengthening not by 2.3 milliseconds but by only 1.7 mil
liseconds per century.
In the long term, the braking effect of the Moon on the Earth’s rotation might be expected to slow it to the point at which one face of the Earth perpetually faces the Moon just as today one face of the Moon perpetually faces the Earth. If this were to happen, the Moon would be invisible from one half of the planet just as the Earth is today invisible from one half of the Moon. Calculations show that such a ‘locking’ of the Earth’s rotation will occur when the Earth’s spin has slowed so much that it turns on its axis once every 47 present-day days.
It will take more than 10 billion years for the Earth’s rotation to slow down this much. By that time, the Sun will have run out of hydrogen fuel in its core, swelled into a red giant and either incinerated or swallowed both the Earth and Moon. The truth is that the Earth’s rotation will never become locked liked that of the Moon. There is simply not enough time available. Nevertheless, there are systems out in space where this has indeed happened. Stars whirling around each other in close ‘binaries’ are expected to have their rotations tidally locked, with each star perpetually showing its partner the same face. And, closer to home, Pluto and its moon, Charon, are both tidally locked.
The fleeing Moon
The tidal effect of the Moon on the Earth slows the rotation of the Earth, reducing its ‘angular momentum’. There is a fundamental edict of physics known as the ‘conservation of angular momentum’, which says that the angular momentum of an isolated, or ‘closed’, system can never change. So, if the angular momentum of the Earth goes down, the angular momentum of something else must go up to exactly compensate for it. That something else can only be the Moon.
The Moon’s gravity creates two bulges in the ocean – on opposite sides of the Earth – but the one closest to the Moon is the one with the strongest and most significant gravitational pull on the Moon. As mentioned before, this tidal bulge tends to race ahead of the Moon’s orbit because the Earth revolves on its axis in less time than it takes the Moon to go around the Earth. So its gravity tends to drag the Moon along in its orbit, speeding it up.
