Who Built the Moon?

by Knight, Christopher

  What most of us don’t stop to think about is why seasons happen at all. It is a common misunderstanding to imagine it has something to do with how close the Earth is to the Sun. It is not – it is due to the angle of the planet in relation to the Sun, which is about 22.5 degrees from what might be described as a vertical position. The diagram below shows how the Earth would look if it was standing upright as it goes around the Sun, which would mean that the equator of the Earth would always point straight at the equator of the Sun.

  Figure 5

  If our planet really did stand in this position, the bulge of the Sun’s equator and that of the Earth would be closer together than the Sun’s poles and the Earth’s poles. The result of this would be a super-hot equatorial temperature on the Earth, whilst the polar regions of the Earth would be much colder than they presently are. Strangely enough it’s not so much a case of the difference in distance between the Earth and the Sun that matters; it is more to do with the thickness of the atmosphere above any given part of the Earth in relationship to the direction of the Sun. In the imaginary situation above, sunlight has to travel through far more atmosphere to get to the poles of the Earth than it does to reach the equator, thus greatly reducing the heat.

  Figure 6

  Another important factor that reduces the heat at the poles is diminished power density, where the Sun’s energy is dissipated across a greater area as the Earth curves away from an upright position. For example, a circle of sunlight with a one-kilometre diameter will hit the Earth’s surface as a near perfect circle at the equator, but in extreme northern or southern latitudes it will be distorted into a long oval due to the curvature of the planet. This means that the heat of the sunlight at the poles will be spread over several times the area and therefore be several times weaker.

  The planet Mercury is an excellent example of a world that is standing virtually upright, in relation to its orbit around the Sun. Apart from the fact that little Mercury is so close to the Sun, its angle of inclination, or ‘obliquity’ as it is more properly called, would make it a very uncomfortable place for humans. If it were possible to stand on Mercury during one of its very short eighty-eight-day years, the Sun would rise due east every day (which is equal to fifty-eight Earth days) at the equator and set due west. Mercury has equatorial temperatures that would keep lead boiling, yet probes sent from Earth have shown that the polar regions of Mercury are constantly covered in ice.

  So, if the Earth were in this upright mode, life would be almost impossible across much of the planet, with extremes of temperature providing only a narrow band suitable for mammals such as humans to survive. Even then, the sea and air currents would move wildly between the hot and cold zones causing catastrophic weather conditions with regions of permanent rainfall and others with none at all. Hurricanes and tornadoes would ravage many areas and overall it seems extremely unlikely that higher life forms would ever develop on such a planet.

  Now consider another imaginary scenario in which the Earth is tilted on its axis a full 90 degrees relative to its orbit around the Sun so that one pole faces the Sun at all times.

  Figure 7

  One of the poles, say the South Pole, would be permanently in daylight – stuck for ever in a position equivalent to noon on midsummer’s day in central Africa. The Sun would blaze down from directly overhead every minute of every day! The North Pole on the other hand, would be in a state of constant midnight. Indeed, all of the northern hemisphere would be in constant night and the southern in constant day.

  The dark side of the planet would never warm up and it would be frozen solid with temperatures far below anything we actually experience. The region that is currently between our equator and the Tropic of Capricorn would see the Sun circling right around, low on the horizon once each day. Because of the angle of the sunlight through the atmosphere, there would be very little warmth getting through and the entire region would be covered in glaciers and swept with snowstorms driving down from the dark northern hemisphere.

  Antarctica would be utterly uninhabitable, being far hotter than anywhere on our planet as we know it today. Only the southern tip of South America, Tasmania, New Zealand and maybe the southern section of Australia would have temperatures that were within a tolerable range. But it is hard to imagine what kinds of terrible weather anyone living there would have to endure, with freezing ocean currents moving from the north and very hot ones arriving from the south. A state of permanent fog seems certain; which would in turn block out the Sun.

  If the Earth orbited the Sun in either of the two modes we have just described, there would be no seasons at all – and almost certainly no higher life forms.

  Thankfully we do have seasons, courtesy of the fact that the Earth is actually at an angle of around 22.5 degrees relative to the equator of the Sun. And that angle is maintained by the Moon, which acts as a gigantic planetary stabilizer.

  Figure 8

  Because of this tilt, the northern hemisphere experiences summer when the Earth is on that part of its orbit that angles it more towards, the Sun. Therefore the Sun rises higher in the sky and is above the horizon longer, and the rays of the Sun strike the ground more directly. Conversely, when the northern hemisphere is oriented away from the Sun, the Sun only rises low in the sky, is above the horizon for a shorter period, and the rays of the Sun strike the ground more obliquely.

  Figure 9

  Whilst it is true that the extreme polar regions of the Earth are frozen throughout the year, the tilt angle of 22.5 degrees ensures that most parts of the Earth’s surface get a fair share of warmth throughout each year. This in turn means that by far the vast majority of water on the surface of the planet remains in a liquid state. All of life is utterly dependent on water and cannot exist without it. The band of temperatures at which water is liquid is really very narrow. The oceans of the Earth would freeze at around 1.91°C, with boiling point occurring at 100°C.

  The Earth is therefore extremely well balanced. The coldest temperature ever recorded was -89.2°C (-128.6°F) at the Vostok Station in Antarctica and the highest was 58°C (136°F) at El Azizia in Libya. That is a range of absolute extremes of less than 148°C, which is very little indeed in terms of the entire spectrum. The coldest anything can get is known as ‘absolute zero’ when all molecular motion stops. This occurs at a rather chilly -273.15°C (-459.67°F).

  On the other hand there is no known upper limit for temperature but the hottest temperature in our solar system is the Sun’s core, which comes in at an impressive 15,000,000°C (27,000,000°F).

  The normal temperature range on Earth is such that there are very few parts of the globe that cannot support human life. We have a normal range of body temperature between 36.1 to 37.8°C (97 to 100°F) and yet the Inuit people live happily within the Arctic Circle and the Bedouin travel the deserts of North Africa.

  The world’s average temperature fluctuates slightly around the 14.5°C (58°F) mark, which is comfortable for physical work. Of course, some people will say that the world ‘is’ that temperature and that we would not have evolved as we have if it were any different – but this is flawed logic. We could just as well have evolved in a world where only small sections of the planet were available to us to inhabit. No other known planet has such a narrow temperature band – and a range of temperature that permits water to be liquid most of the time.

  In fact water is a very curious substance altogether. On Earth we can see it at the same time in its three states – as solid ice, as liquid water and as a gas in clouds. Each water molecule is composed of just two atoms of hydrogen and one of oxygen and yet it acts as a universal solvent with a high surface tension.

  Perhaps most surprising of all is how its density changes. Water has its maximum density at 4°C which means that it not only gets lighter as it warms from that point – it also gets lighter as it cools. As everyone knows, warm water rises as convection currents but it is also true that ice floats. Other planets in our solar system may have ice
or steam but only the Earth is awash with life-giving liquid water.

  Liquid water has been absolutely crucial in creating the world we know today and, as far as is known, life cannot exist without it. As surely as plate tectonics and the Earth’s hot core constantly create new mountain ranges, via volcanoes and the pushing up of mountains as land masses meet, so water is mainly responsible for flattening them again. Constant weathering crumbles away the rocks as mountains age and water, in the form of rain, ice and snow, is primarily responsible. Liquid water, as streams and rivers, also disperses the weathered rock, carrying it down to the plains where it is distributed across flatter land, bringing much needed nutrients to nourish life. Even more nutrients are carried by the rivers to the oceans where they offer the necessary food for aquatic plants that stand at the bottom of the oceanic food chain.

  Of course, none of this would be possible if the vast majority of water on the Earth was not in a liquid state. Only two per cent of Earth’s water is locked up in glaciers and the icecaps, with ninety-seven per cent being the water of our seas and oceans and just one per cent available for human consumption as fresh water. With only a small change in the overall temperature of the Earth, or an alteration in the seasonal patterns, the nature of the water on our planet would change. As we have seen, a more pronounced planetary tilt could well lead to a freezing of the oceans. This would result in an overall loss of temperature at the surface of the planet, with even greater freezing.

  On the other hand, if the Earth were not tilted at all, the equatorial regions would become unbearably hot and weather patterns across the planet would be radically changed. In addition, the biodiversity, that scientists are now certain has been so important to our evolution, might never have developed in a world with more polarized areas of temperature.

  It has therefore been vital for our existence that the tilt of the Earth has been maintained at around 22.5 degrees for an extremely long period of time, and yet, bearing in mind the composition of the planet this is a very unlikely state of affairs. Venus is the nearest planet to Earth and the most similar to our own, but it has toppled over in the past and other planets in the solar system show signs of having varied markedly in their tilt angle across time. The Earth is very active internally and highly unstable, yet, despite a few periodic wobbles, it keeps the same angle relative to the Sun.

  Astronomer Jacques Laskar, a Director of Research at the National Scientific Research Centre (CNRS) and head of a team at the Observatory of Paris is in no doubt that the Earth would indeed topple over, if it were not for the presence of the Moon!18

  With computer modelling, Laskar showed in 1993 that all the other Earth-like planets (Mercury, Venus and Mars) have highly unstable obliquity, which, in the case of Mars for example, varies wildly across time between 0 degrees and 60 degrees. The same computer modelling indicates that in the case of the Earth the obliquity would vary even more, between 0 degrees and 85 degrees – but for the stabilizing influence of our incredibly large Moon.

  Nobody knows for certain how long it would take for the Earth’s obliquity to change significantly if the Moon was not exerting such a massive influence. There is a constant transfer of energy taking place between the two bodies, which in addition to stabilizing Earth’s obliquity has also significantly slowed our planet’s rate of spin. This constant obliquity has made the Earth a perfect crucible for advanced life by providing many millions of years of stability for life to develop from its simplest form to the complex patterns it adopts today.

  Although the Earth is significantly more massive than the Moon, the Moon is still a very large body. Tides in Earth’s oceans, seas and lakes are caused by the gravitational interaction of the Earth, the Moon and the Sun. Tides have an effect on dry land as well as oceans but this effect can only be detected by careful measurement. Solar tides (the point of greatest gravitational pull by the Sun) are twelve hours apart but since the Moon is also moving, lunar tides are slightly more irregular, occurring every 12.42 hours on average.

  The height of tides in any particular part of the ocean is dependent on a number of factors such as the shape of any nearby landmasses and the depth of the seabed. In some areas of the world, tides hardly seem to lift the level of water at all – this is just as well for some low-lying places such as the islands of the Maldives in the Indian Ocean because their average height above sea level is less than one metre. In other places, like the British coast, tides can have a huge range between high and low water.

  Figure 10

  The Moon has sufficient gravity to pull a bulge of water from the oceans of the Earth closest to its position towards it. It also distorts the Earth, creating a corresponding bulge in the oceans on the opposite side of the Earth. Because of the Earth’s rotation the bulge on the Moon side runs slightly ahead of the Moon.

  Tides would not cease if the Moon were not present because they are also created by the Sun. However, they would be very much lower than they are now because although the Sun is massive and the Moon much smaller, the Moon is extremely close and the Sun much more distant. It is the interaction of solar and lunar tides that makes it rather complicated to predict when tides will occur and how high or low they are likely to be.

  The highest of the lunar tides occur when the Moon is either in its full or new mode, because at such times it is in line with the Sun and its gravitational forces are added to those of the Sun. Much lower tides are on the first and last quarters of the Moon when the gravity of the Moon and the Sun are working against each other.

  Life in the tidal margins of the oceans and seas has evolved to take advantage of tides, either in a daily or a monthly sense. Some species of crabs for example, lay their eggs in the sand at the high-water mark at the time of the full or new Moon so that they will be safe from marine predators during incubation. There are also many creatures that leave the ocean on the high tide at night to scavenge in the inter-tidal margins, before seeking safety with the next high tide.

  Many shellfish are absolutely dependent on the ebb and flow of the tides for the purpose of feeding and it was shown in the 1960s that oysters are sensitive enough to be aware of the Moon’s position, either overhead or at the opposite side of the planet. Oysters, which obviously have no eyes, were taken from the ocean and placed in tanks in the Rocky Mountains where they began to open and close, as they would have done in the ocean, had it extended so far inland. Because other stimulus such as current or wave motion were absent, it suggests that they are able to feel minute increases and decreases in the gravitational pull of the Moon and the Sun.

  If molluscs, our very distant evolutionary cousins, can somehow sense such astronomical movements – then there would seem to be no reason why humans would not be able to do the same. This just might point the way forward in investigating a possible causation for variations in human behaviour according to the phase of the Moon.

  It probably is not too surprising that some creatures have learned to exploit tides, which are tiny these days in comparison with the remote past when the Moon was much closer to the Earth. The tremendous forces created by a very close Moon would have generated much heat and might even have caused parts of the Earth’s surface to melt. However, this phase did not last all that long because the very transfer of energy that promotes tides is also causing the Moon to drift further and further away from the Earth. This happens because the Earth rotates around its own axis more quickly than the Moon revolves around the Earth. The rapid rotation means that the tidal bulge of the Earth forward of the Moon, (see figure 11) is always ahead of the Moon’s position. The tidal bulge exerts a pull on the Moon and this increases the Moon’s overall energy. Meanwhile, friction between the Earth’s surface and its own oceans is actually slowing the rate of Earth rotation. It is not much, but it does amount to around 0.002 seconds in a century.

  The end result of this dance will be that the Moon will continue to move away from the Earth until a situation of equilibrium is achieved, which is expected to happen
in about fifteen billion years. The Moon will then be 1.6 times further out from the Earth than it is now and the Earth will have a solar day that is equal to the orbit of the Moon, which by then will be fifty-five days. However, we do not have to lose too much sleep about this eventuality because the Sun will have become a red giant about a billion years before that, at which time the Earth will have ceased to exist in any case.

  Figure 11

  As the Earth revolves, it takes the tidal bulges with it, but because of the gravity of the Moon, the water in the bulges is trying to travel in the opposite direction. As a result, waves ground on the bottom of the oceans and on seashores, causing friction. The friction slows the Earth and the energy is passed to the Moon, which responds by speeding up. As it does so, the laws of physics dictate that its orbit must widen.

  Over huge periods of time the relationship between the Earth and the Moon changes, so we find ourselves living in what amounts to a ‘tiny snapshot’ of the overall situation. At present the Moon takes 27.322 days to go around the Earth and because the Earth is also going around the Sun, full and new Moons are ruled by a slightly longer cycle that takes 29.53 days. Both these figures have been significantly different in the past and will be different again in the future but the changes are very slow and, according to NASA, the Moon is becoming more distant from the Earth by around 3.8cm per year.