While the Sun gives out the heat and light that makes life on Earth possible, it also contains almost all the mass in the solar system, which means that it has almost all of the gravitational pull. This keeps the Earth in its orbit, but that is not the whole story. Set loose on the rotating Earth, the Suns gravitation has more subtle effects. More to the point, so does the gravitation of the Moon, which is more powerful because although the Moon is far smaller, the Sun is four hundred times as far away.
Acting on the Earth, the gravitation of these two bodies produces effects known as tides. You probably think of tides – rightly – as the thing that makes the sea go up and down the beach when you are on holiday. Only if you sail – anything from a dinghy to an aircraft carrier – do you need to pay close attention. However, the tides in the oceans are only part of the story. Lunar and solar gravitation also raise detectable tides in the atmosphere, and in the solid Earth itself.
Air tides seem to have been detected for the first time in 1918. There have been suggestions that they are related to extreme weather. But these claims are disputed, since the effects of air tides are very small, far smaller than the day-to-day variation in the Earths atmospheric pressure that you hear about on the weather forecast. Another suggestion is that air tides in the high upper atmosphere may be significant in forming weather patterns.
More solidly established are Earth tides. The Moon raises a tide about 1m high at the Earths Equator while the Sun produces one about half as big. This may not seem much in a planet which is 12,700km across. But this movement has been associated with the timing of volcanic outbreaks and earthquakes. US and Japanese scientists have studied the timing of both and found that when the solar and lunar earth tides coincide to form one major tide, a significant earthquake or volcano may be more likely.
But it is of course the tides in the Earths oceans that are the most noticeable, and matter most, to humans and the rest of life on Earth. Lets take a closer look.
Although no other body in the universe is close enough to raise a detectable tide on the Earth, even the tides produced by the Sun and Moon are complex. For one thing, you might expect the basic shape of a tide to be a bulge towards the Sun or Moon caused by their gravitational pull. In fact, the interaction between the Moon and Sun and the rotating Earth in effect stretches the Earth, plus the water and air around it, along the lines joining the Earths centre to the Moon and Sun. This means that the Moon and Sun each set up two bulges in the oceans, the atmosphere and the Earth itself, one pointing directly towards them and another directly away from them. That is why you get two tides a day, not just one. The Sun tides are 12 hours apart, but the Moons are about 12 hours and 25 minutes apart, the difference being caused by the Moons movement around the Earth.
The height of the ocean tide on any particular day is determined mainly by the interaction between the lunar and solar tides – whether they are working together or in opposition.
The easiest way to think about it is to imagine that the two pairs of bulges raised in the Earths oceans by the Moon and the Sun can either be aligned, or at right angles (at quadrature, in astro-speak). When they are aligned, the result is a very high tide followed by a very low one, as the low-water parts of the ellipse match just like the high-water ones. This is a spring tide.
It has nothing to do with the season associated with rabbits. Instead it gets its name from the way the water springs back and forth a long way when it occurs.
If the two bulges are at right angles, there are still tides because the solar tide is much weaker than the lunar one and is too small to counteract it entirely. But they are much lower, and are called neap tides.
As usual, there are further complications. As we saw above, the Earths orbit around the Sun is not exactly circular and we are closest to the Sun in early January. So the solar tides at that time are higher than six months later, and can therefore go further towards cancelling out or reinforcing the lunar tides.
The Moons orbit round the Earth is even more eccentric, 5.5 percent away from circularity compared to 1.7 percent for the Earths orbit around the Sun. This means that its distance from the Earth varies by about 42,000km. The tides it raises at its closest (or perigee) are noticeably higher than those it produces when furthest away (at its apogee). Although gravity changes with the square of the distance between two objects, the tides one raises on another go with the cube of the distance, so this effect is bigger than you might expect.
In addition, although the Moons orbit around the Earth is more or less aligned with the Earths track around the Sun, the match is not perfect. In fact they are at an angle of about 5° to one another. That is why there is not an eclipse of the Sun at every new moon – instead, it only happens when the Moon crosses the line the Sun follows in the sky at exactly the moment when the Sun happens to be there. The result for our purposes is that sometimes the tidal bulges raised by the Sun and Moon are better-aligned than at others. Summary: during a January spring tide during an eclipse with the Moon at perigee, stand well back from the beach. But a neap tide in July, with the Moon at its furthest north or south of the Sun and at apogee, is going to be far less impressive.
Like a lot of things, tides are at their most interesting when you look at them in detail, not in this big-picture way. In practice, you need a big mass of water to make a respectable tide. Although they can be detected in water masses such as the Great Lakes, this really calls for an ocean rather than a sea. Most places on the Mediterranean coastline have a tidal range – the change in height of the sea between high and low tide – of less than a metre, even though the Mediterranean measures 3300km from west to east.
The biggest tides, of many metres, are raised in the Atlantic, Indian and Pacific Oceans. Things really become interesting when they arrive at the coastline. Because of inertia, the tidal bulges in the oceans never point directly at the place in the sky where the Moon or Sun can be seen. Instead they lag behind them. But when the sea reaches the land, the bulges lag even further behind. A large area of shallows near the land means that many cubic kilometres of seawater that were running along happily in the open ocean pile up and rub along the sea floor, losing energy and slowing up because of friction. And the lag gets even worse when a large landmass is in the way. Thus a high tide in the Scilly Isles, at the south-west corner of the British Isles, takes over five hours to get to the Orkney and Shetland Islands at the north-east. More intriguingly, it takes even longer to make the shorter journey to my home port of Liverpool. The reason is that to get there, the water has to make its way up the comparatively shallow Irish Sea.
Tides are complex but tide tables are accurate, partly because the position of the Sun and the Moon in the sky can be predicted accurately and partly because many years of data has been collected on the time lags in the system. A couple of centuries of tide data exist for major ports such as Brest in France.
However, another basic fact about tides is very simple. When lunar and solar gravitation push large volumes of water into small sea areas, the only way is up. The highest tides in the world are in the Bay of Fundy on the Atlantic coast of Canada and the Severn estuary in Britain. Look at a map and you will see that the two places have something in common. Both have coastlines shaped to force the full tidal push of the Atlantic into a steadily narrowing funnel. In the Bay of Fundy, the tides are especially high because the water is built up in height by a strange resonance effect. In the Severn, the waters at their highest can form the Severn Bore, a wave – ridden by reckless surfers – that runs up the evernarrowing river for miles, far from the sea, until it runs out of steam because of the friction of the river bed and banks. About eighty rivers around the world, from the Amazon to the Seine, have something like it.
Over time, all this friction that occurs as the oceans are dragged against the shores by the tides means that energy is steadily draining from the Earth-Moon system. One result is that the Earths rotation is steadily slowing down. In fact days are getting longer by about 1.4 milliseconds (thousandths of a second) every century. That may not sound like much, but it adds up. About 600 million years ago, there were over 420 days in a year, with each day 21 present-day hours in length.
At the same time, the slow reduction in the energy available for the Earth-Moon system affects the Moon as well. It is edging gradually further away (at an average rate of 2.17cm per year) because less energy is needed to sustain a slower, more distant orbit. When there were 400 days in a year there were also 16 lunar months, in other words 16 lunar orbits round the Earth. At that time, the Moon would have looked larger and the tides would have been higher. Just now, the Moon has retreated to the point where it is the same size in the sky as the Sun – half a degree across – which makes it possible for a solar eclipse to occur when the Moon blocks it out. Come back in 620 million years, when the Moon has got even further away, and total eclipses will be a thing of the past. A few hundred million years ago, by the same token, they must have been more common.
Living with the tides
Life on Earth has had plenty of time to adapt to tides and they are integrated into the activity of many living systems. The tidal layers on the beach provide a series of different habitats to which plants and animals have adapted themselves. They are – from the top down – the splash fringe level, where water only splashes the shore rather than covering it; the high tide level, covered for several hours a day; the mid-tide level, covered about half the time; the low tide level, uncovered for a few hours per tide; the low fringe level, whose denizens can stand short periods out of the water; and, well, the sea.
Animals in the tidal zone know that the fall of the ocean means the chance to hunt for food if you are a predator. If you are prey, it is time to burrow or swim out of harms way. Some forms of life have adapted specifically to conditions in the tide zone. The most startling example is the mangrove, a family of trees that can cope with salt water and to having their roots exposed at low tide.
Because of rising sea levels, scientists have been paying more attention to salt-resistant species. Sugar beet is derived from plants that grow on salt marshes and does not mind a salty environment. The same goes for barley. There is also a salt-resistant tomato that grows on beaches around the Pacific. The only problem is that it tastes terrible. And there are sea grasses, species of grass that can prosper in the ocean provided the water is shallow enough for the Sun to get to them.