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Authors: Richard Dawkins

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A sideways look at summer

Now that we understand orbits, we can go back to the question of why we have winter and summer. Some people, you’ll remember, wrongly think it is because we are closer to the sun in summer and further away in winter. That would be a good explanation if Earth had an orbit like Pluto’s. In fact Pluto’s winter and summer (both very much colder than anything we experience here) are caused in exactly that way.

The Earth’s orbit, however, is almost circular, so the planet’s closeness to the sun cannot be what causes the changing seasons. For what it is worth, the Earth is actually closest to the sun (perihelion) in January and furthest (aphelion) in July, but the elliptical orbit is so close to circular that it makes no noticeable difference.

Well then, what does cause the change from winter to summer? Something quite different. The Earth spins on an axis, and the axis is tilted. This tilting is the true reason why we have seasons. Let’s see how it works.

As I said before, we could think of the axis as an axle, a rod running right through the globe and sticking out at the North Pole and the South Pole. Now think of the orbit of the Earth around the sun as a much larger wheel, with its own axle, this time running through the sun, and sticking out at the sun’s ‘north pole’ and the sun’s ‘south pole’. Those two axles could have been exactly parallel to each other, so that the Earth did not have a ‘tilt’ – in which case the noonday sun would always seem to be directly overhead at the equator, and day and night would be of equal length everywhere. There would be no seasons. The equator would be perpetually hot,
and
it would become colder and colder the further you moved away from the equator and towards either of the poles. You could get cool by moving away from the equator, but not by waiting for winter because there would be no winter to wait for. No summer, no seasons of any kind.

In fact, however, the two axles are not parallel. The axle (axis) of the Earth’s own spinning is tilted relative to the axle (axis) of our orbit around the sun. The tilt is not particularly great – about 23.5 degrees. If it were 90 degrees (which is about the tilt of the planet Uranus) the North Pole would be pointing straight towards the sun at one time of year (which we can call the northern midsummer) and straight away from the sun at the northern midwinter. If Earth were like Uranus, in midsummer the sun would be overhead all the time at the North Pole (there’d be no night there), while it would be icy cold and dark at the South Pole, with no suggestion of day. And vice versa six months later.

Since our planet is actually tilted at only 23.5 degrees instead of 90 degrees, we are about a quarter of the way from the no seasons extreme of no tilt at all towards the Uranus extreme of near total tilt. This is enough to mean that, as on Uranus, the sun never sets at the Earth’s North Pole in midsummer. It is perpetual day; but, unlike on Uranus, the sun is not overhead. It seems to loop around the sky as the Earth rotates, but it never quite dips below the horizon. That is true throughout the Arctic Circle. If you stood right on the Arctic Circle, say on the north-west tip of Iceland, on midsummer day, you’d see the sun skim along the northern horizon at midnight, but never actually set. Then it would loop
around
to its highest position (not very high) at midday.

In northern Scotland, which is a little way outside the Arctic Circle, the midsummer sun dips below the horizon far enough to make a sort of night – but not a very dark night, because the sun is never very far below the horizon.

So, the tilt of the Earth’s axis explains why we have winter (when the bit of the planet where we are is tilted away from the sun) and summer (when it’s tilted towards the sun), and why we have short days in winter and long days in summer. But does that explain why it is so cold in winter and so hot in summer? Why does the sun feel hotter when it is directly overhead than when it is low, near the horizon? It’s the same sun, so shouldn’t it be equally hot no matter what the angle at which we see it? No.

You can forget the fact that we are slightly nearer the sun when tilted towards it. That’s an infinitesimal difference (only a few thousand miles) compared to the total distance from the sun (about 93 million miles), and still negligible compared to the difference between the sun’s distance at perihelion and the sun’s distance at aphelion (about 3 million miles). No, what matters is partly the angle at which the sun’s rays hit us, and partly the fact that the days are longer in summer and shorter in winter. It’s that
angle
that makes the sun feel hotter at midday than in the late afternoon, and it’s that angle that makes it more important to put on sunscreen at midday than in the late afternoon. It’s a combination of the angle and the day length that makes the plants grow more in summer than in winter, with all that follows from that.

So why does this angle make such a difference? Here’s
one
way to explain it. Imagine that you are sunbathing at midday in the middle of the summer, and the sun is high overhead. A particular square inch of skin in the middle of your back is being hit by photons (tiny particles of light) at a rate that you could count with a light meter. Now, if you sunbathe at midday in winter, when the sun is relatively low in the sky because of the Earth’s tilt, light reaches the Earth at a shallower, more ‘sideways’ angle: therefore a given number of photons are ‘shared out’ over a larger area of skin. This means that the original square inch of skin gets a smaller share of the available photons than it did at midsummer. What is true of your skin is also true of the leaves of plants, and that really matters because plants use sunlight to make their food.

Night and day, winter and summer: these are the great alternating rhythms that rule our lives, and the lives of all living creatures except perhaps those that live in the dark, cold depths of the sea. Another set of rhythms that are not so important for us but matter greatly to other creatures, such as those that live on seashores, are the rhythms imposed by the orbiting moon, acting mostly through the tides. Lunar cycles are also the subject of ancient and disturbing myths – of werewolves and vampires, for example. But I must reluctantly leave this subject now and move on to the sun itself.

6

W
HAT IS
THE SUN?

 

THE SUN IS
so dazzlingly bright, so comforting in cold climates, so mercilessly scorching in hot ones, it is no wonder many peoples have worshipped it as a god. Sun worship often goes together with moon worship, and the sun and the moon are frequently regarded as being of opposite sex. The Tiv tribe of Nigeria and other parts of west Africa believe the sun is the son of their high god Awondo, and the moon is Awondo’s daughter. The Barotse tribe of south-east Africa think the sun is the moon’s husband rather than her brother. Myths often treat the sun as male and the moon as female, but it can be the other way around. In the Japanese Shinto religion the sun is the goddess Amaterasu, and the moon is her brother Ogetsuno.

Those great civilizations that flourished in South and Central America before the Spaniards arrived in the sixteenth century worshipped the sun. The Inca of the Andes believed that the sun and the moon were their ancestors. The Aztecs of Mexico shared many of their gods with older civilizations in the area, such as the Maya. Several of these gods had a connection with the sun, or in some cases were the sun. The Aztec ‘Myth of the Five Suns’ held that there had been four worlds
before
the present one, each with its own sun. The earlier four worlds were destroyed, one after the other, by catastrophes, often engineered by the gods. The first sun was the god called Black Tezcatlipoca; he fought with his brother, Quetzalcoatl, who knocked him out of the sky with his club. After a period of darkness, with no sun, Quetzalcoatl became the second sun. In his anger, Tezcatlipoca turned all the people into monkeys, whereupon Quetzalcoatl blew all the monkeys away, and then resigned as the second sun.

The god Tlaloc then became the third sun. Annoyed when Tezcatlipoca stole his wife Xochiquetzal, he sulked and refused to allow any rain to fall, so there was a terrible drought. The people begged and begged for rain, and Tlaloc became so fed up with their begging that he sent down a rain of fire instead. This burned up the world, and the gods had to start all over again.

The fourth sun was Tlaloc’s new wife, Chalchiuhtlicue. She started out well, but then Tezcatlipoca so upset her that she cried tears of blood for 52 years without stopping. This completely flooded the world, and yet again the gods had to start from scratch. Isn’t it strange, by the way, how exactly myths specify little details? How did the Aztecs decide that she cried for 52 years, not 51 or 53?

The fifth sun, which the Aztecs believed is the present one that we still see in the sky, was the god Tonatiuh, sometimes known as Huitzilopochtli. His mother, Coatlicue, gave birth to him after being accidentally impregnated by a bundle of feathers. This might sound odd, but such things would have seemed quite normal to people brought up with
traditional
myths (another Aztec goddess was impregnated by a gourd, which is the dried skin of a fruit like a pumpkin). Coatlicue’s 400 sons were so enraged to find their mother pregnant yet again that they tried to behead her. However, in the nick of time she gave birth to Huitzilopochtli. He was born fully armed and lost no time in killing all of his 400 half-brothers, except a few who escaped ‘to the south’. Huitzilopochtli then assumed his duties as the fifth sun.

The Aztecs believed that they had to sacrifice human victims to appease the sun god, otherwise he would not rise in the east each morning. Apparently it didn’t occur to them to try the experiment of not making sacrifices, to see whether the sun might, just possibly, rise anyway. The sacrifices themselves were famously gruesome. By the end of the Aztecs’ heyday, when the Spaniards arrived (bringing their own brand of gruesomeness), the sun cult had escalated to a gory climax. It is estimated that between 20,000 and 80,000 humans were sacrificed for the rededication of the Great Temple of Tenochititlan in 1487. Various gifts could be offered to appease the sun god, but what he really liked was human blood, and still-beating human hearts. One of the main purposes of warfare was to collect lots of prisoners of war so that they could be sacrificed, usually by having their hearts cut out. The ceremony normally took place on high ground (to be closer to the sun), for example on top of one of the magnificent pyramids for which the Aztecs, Maya and Inca are famous. Four priests would hold the victim down over the altar, while a fifth priest wielded the knife. He worked as fast as possible to cut the heart out so that it was still
beating
when held up to the sun. Meanwhile the heartless and bloody corpse would roll down the slopes of the hill or pyramid to the bottom, where it would be collected up by the old men and then dismembered, often to be eaten in ritual meals.

We also associate pyramids with another ancient civilization, that of Egypt. The ancient Egyptians, too, were sun-worshippers. One of the greatest of their gods was the sun god Ra.

An Egyptian legend regarded the curve of the sky as the body of the goddess Nut, arched over the Earth. Every night the goddess swallowed the sun, and then the following morning she gave birth to him again.

Various peoples, including the ancient Greeks and the Norsemen, had legends about the sun being a chariot driven across the sky. The Greek sun god was called Helios, and he has given his name to various scientific terms associated with the sun, as we saw in Chapter 5.

In other myths, the sun is not a god but one of the first creations of a god. In the creation myth of the Hebrew tribe of the Middle Eastern desert, the tribal god YHWH created light on the first of his six days of creation – but then, surprisingly, he didn’t create the sun until the fourth day! ‘And God made two great lights: the greater light to rule the day, and the lesser light to rule the night: he made the stars also.’ Where the light came from on the first day, before the sun and stars existed, we are not told.

It is time to turn to reality, and the true nature of the sun, as borne out by scientific evidence.

What is the sun, really?

The sun is a star. It’s no different from lots of other stars, except that we happen to be near it so it looks much bigger and brighter than the others. For the same reason, the sun, unlike any other star, feels hot, damages our eyes if we look straight at it, and burns our skin red if we stay out in it too long. It is not just a
little
bit nearer than any other star; it is
vastly
nearer. It is hard to grasp how far away the stars are, how big space is. Actually, it’s more than hard, it’s almost impossible. There’s a lovely book called
Earthsearch
by John Cassidy, which makes an attempt to grasp it, using a scale model.

  1. Go out into a big field with a football and plonk it down to represent the sun.
  2. Then walk 25 metres away and drop a peppercorn to represent the Earth’s size and its distance from the sun.
  3. The moon, to the same scale, would be a pinhead, and it would be only 5 centimetres away from the peppercorn.
  4. But the nearest other star, Proxima Centauri, to the same scale, would be another (slightly smaller) football located about … wait for it … six and a half thousand kilometres away!

There may or may not be planets orbiting Proxima Centauri, but there certainly are planets orbiting other stars, maybe most stars. And the distance between each star and its planets is usually small compared to the distance between the stars themselves.

How stars work

The difference between a star (like the sun) and a planet (like Mars or Jupiter) is that stars are bright and hot, and we see them by their own light, whereas planets are relatively cold and we see them only by reflected light from a nearby star, which they are orbiting. And that difference, in turn, results from the difference in size. Here’s how.

The larger any object is, the stronger the gravitational pull towards its centre. Everything pulls everything by gravity. Even you and I exert a gravitational pull on each other. But the pull is too weak to notice unless at least one of the bodies concerned is large. The Earth is large, so we feel a strong pull towards it, and when we drop something it falls ‘downwards’ – that is, towards the centre of the Earth.

A star is much larger than a planet like Earth, so its gravitational pull is much stronger. The middle of a large star is under huge pressure because a gigantic gravitational force is pulling all the stuff in the star towards the centre. And the greater the pressure inside a star, the hotter it gets. When the temperature gets really high – much hotter than you or I can possibly imagine – the star starts to behave like a sort of slow-acting hydrogen bomb, giving out huge quantities of heat and light, and we see it shining brightly in the night sky. The intense heat tends to make the star swell up like a balloon, but at the same time gravity pulls it back in again. There is a balance between the outward push of the heat and the inward pull of gravity. The star acts as its own thermostat. The hotter it gets, the more it swells; and the bigger it gets, the
less
concentrated the mass of matter in the centre becomes, so it cools down a bit. This means it starts to shrink again, and that heats it up again, and so on. That sounds as though the star bounces in and out like a heart beating, but it isn’t like that. Instead, it settles into an intermediate size, which keeps the star at just the right temperature to stay that way.

I began by saying that the sun is just a star like many others, but actually there are lots of different kinds of stars, and they come in a great range of sizes. Our sun is not very big, as stars go. It is slightly bigger than Proxima Centauri, but much smaller than lots of other stars.

What is the largest star we know? That depends on how you measure them. The star that measures the greatest distance across is called VY Canis Majoris. From side to side (diameter), it is 2,000 times the size of the sun. And the sun’s diameter is 100 times that of the Earth. However, VY Canis Majoris is so wispy and light that, despite its huge size, its mass is only about 30 times that of the sun, instead of the billions of times it would be if its material were equally dense. Others, such as the Pistol Star, and more recently discovered stars such as Eta Carinae and R136a1 (not a very catchy name!), are 100 times as massive as the sun, or even more. And the sun is more than 300,000 times the mass of the Earth, which means that the mass of Eta Carinae is 30 million times that of the Earth.

If a giant star like R136a1 has planets, they must be very very far away from it, or they would be instantly burned to vapour. Its gravity is so huge (because of its vast mass) that its planets could indeed be a very long way away and still be held
in
orbit around it. If there is such a planet, and anybody lives on it, R136a1 would probably look about as big to them as our sun looks to us, because although it is much larger, it would also be much further away – just the right distance away, in fact, and just the right apparent size to sustain life, otherwise life wouldn’t be there!

The life story of a star

Actually, however, it is unlikely that there are any planets orbiting R136a1, let alone any life on them. The reason is that extremely large stars have a very short life. R136a1 is probably only about a million years old, which is less than a thousandth of the age of the sun so far: not enough time for life to evolve.

The sun is a smaller, more ‘mainstream’ star: the kind of star that has a life story lasting billions of years (not just millions), during which it proceeds through a series of drawn-out stages, rather like a child growing up, becoming an adult, passing through middle age, eventually getting old and dying. Mainstream stars mostly consist of hydrogen, the simplest of all the elements. The ‘slow-acting hydrogen bomb’ in the interior of a star converts hydrogen to helium, the second simplest element (something else named after the Greek sun god Helios), releasing a massive amount of energy in the form of heat, light and other kinds of radiation. You remember we said that the size of a star is a balance between the outward push of heat and the inward pull of gravity? Well, this balance stays roughly the same, keeping the star simmering away for several billions of years, until it starts to run out
of
fuel. What usually happens then is that the star collapses into itself under the unrestrained influence of gravity – at which point all hell breaks loose (if it’s possible to imagine anything more hellish than the interior of a star already is).

The life story of a star is too long for astronomers to see more than a tiny snapshot of it. Fortunately, as they scan the skies with their telescopes, astronomers can find a range of stars, each at a different stage of its development: some ‘infant’ stars caught in the act of being formed from clouds of gas and dust, as our sun was four and a half billion years ago; plenty of ‘middle-aged’ stars like our sun; and some old and dying stars, which give a foretaste of what will happen to our sun in another few billion years’ time. Astronomers have built up a rich ‘zoo’ of stars, of all different sizes and stages in their life cycles. Each member of the ‘zoo’ shows what others used to be like, or will be like.

An ordinary star like our sun eventually runs out of hydrogen and, as I’ve just described, starts ‘burning’ helium instead (I’ve put that in quotation marks because it isn’t really burning but doing something much hotter). At this stage it is called a ‘red giant’. The sun will become a red giant in about five billion years’ time, which means it is pretty much in the middle of its life cycle at the moment. Long before then, our poor little planet will have become much too hot to live on. In two billion years the sun will be 15 per cent brighter than it is now, which means that the Earth will be like Venus is today. Nobody could live on Venus: the temperature there is over 400 degrees Celsius. But two billion years is a pretty long time, and humans will almost certainly be extinct long before then, so
that
there will be nobody left to fry. Or maybe our technology will have advanced to the point where we can actually move the Earth out to a more comfortable orbit. Later, when the helium, too, runs out, the sun will mostly disappear in a cloud of dust and debris, leaving a tiny core called a white dwarf, which will cool and fade.

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