The Second Book of General Ignorance (13 page)
Read The Second Book of General Ignorance Online
Authors: John Lloyd,John Mitchinson
‘When it has disappeared below the horizon’ is the wrong answer.
The sun has already set when its lower edge touches the horizon.
As the setting sun falls in the sky, its light passes through the atmosphere at an increasingly shallow angle and is bent more and more as the amount of air it has to pass through increases. At the end of the process, the light is bent so much that we can still apparently see the sun even though it’s physically below the horizon. By coincidence, the degree of bending is almost equal to the width of the sun – so when we see the lower rim of the sun kiss the horizon, the whole of it has in fact completely disappeared.
What we’re looking at is a mirage. The bending of the light also has the effect of reducing the apparent distance between the top and bottom of the sun. This can cause the sun to appear oval.
When sunlight travels through the atmosphere, green light is bent very slightly more than red light – as when passing through a prism. This means that the top of the setting sun has a very thin green rim – too thin to be seen by the naked eye. Very occasionally, when atmospheric conditions are right, this green rim can be artificially magnified and it shines for a second or so just as the sun disappears from view. This phenomenon is known as the ‘green flash’ and is considered a good omen by sailors.
Another common mirage is the one you see on a road in summer. Hot tarmac heats the air above it, producing a sharp shift in its density, which causes light to bend. You think you see water; what you’re actually seeing is a reflection of the sky. The brain tells you it’s water, because water also reflects the sky.
Desert mirages are the same: the thirsty adventurer only ever ‘sees’ water.
Any other images of the type associated with mirages in cartoons and films (palm trees, ice-cream vans, dancing girls, etc.) are just figments of a heat-addled imagination.
STEPHEN
Light from the setting sun passes through our atmo
sphere at a shallow angle; it is gradually bent as the air density,
i.e.
the pressure, increases. Not dissimilar to the image of your
legs when you sit in a swimming pool. Our brains cannot
accept that light is bent. The effect is to artificially raise the sun
in the last few minutes of its decline, through the thickness of
the atmosphere at that shallow angle there. And by coincidence,
the amount of bending is pretty much equal to the diameter of
the sun, so it’s exactly, exactly as it is there, that it’s actually
disappeared.PHILL JUPITUS
I hate this show.
‘Everyone knows’ that the wispy cirrus clouds are the highest – but they aren’t.
Clear midsummer evenings can occasionally reveal one of the loveliest and least understood phenomena of the night sky. Noctilucent (‘night-shining’) clouds are silvery blue streaks that form so high up in
the atmosphere they catch the sun’s light, even at night. At over 80 kilometres (50 miles) in altitude, they are seven times higher than the highest cirrus clouds.
The word atmosphere is Greek for ‘globe of vapour’. Earth’s atmosphere is a succession of layers of gas, stretching about 100 kilometres (62 miles) into space. We live in the troposphere (
tropos
is Greek for ‘change’), which is warm and moist and is where all the clouds (except the noctilucent ones) form. At 11 kilometres (7 miles) up, the stratosphere starts (
stratum
is Latin for ‘covering’): it contains the protective ozone layer. The outermost layer is the mesosphere, somewhat confusingly called the ‘middle sphere’ because it’s between the other, inner layers and space. It starts nearly 5 kilometres (about 3 miles) up and is 32 kilometres (20 miles) thick. It’s too high for most aircraft and too low for space flight, and it’s nicknamed the ‘ignorosphere’ because we know so little about it.
Noctilucent clouds form right on the boundary of the mesosphere and space. Clouds need water vapour and dust particles to form and the mesosphere is so dry and cold (about –123 °C) it was first thought that noctilucent clouds must be made of something other than water vapour. Now we know they are made of tiny ice crystals – a fiftieth of the width of a human hair – but we still don’t understand how they form.
Another thing we don’t know about them is whether they have always existed or not. No one had ever reported seeing them until 1885 when they were first named by Otto Jesse, a German cloud enthusiast. This was just two years after the eruption of Krakatoa and at a time when the industrial age was at its peak. It seems that this was the first time dust had ever got high enough for clouds to form in the mesosphere.
Today, the mesosphere is getting cooler still, as a result of increased carbon dioxide (CO
2
) emissions. At the same time,
ironically, carbon dioxide is busy heating up the troposphere.
CO
2
naturally absorbs heat. In the thin air of the mesosphere, it simply sucks it up. But, in the troposphere, nearer the Earth’s surface, where the gases are more densely packed, CO
2
collides continually with other substances (such as water vapour). This releases heat and causes global temperatures to rise and is known as the ‘greenhouse effect’.
Over the past three decades, the number of noctilucent clouds has more than doubled, which has led some scientists to liken them to miner’s canaries: their eerie beauty warning of the dangers of climate change to come.
A
lot
.
A popular unit of measurement for cloud-weight seems to be the elephant. According to the National Center for Atmospheric Research in Boulder, Colorado, an average cumulus cloud weighs about 100 elephants, while a big storm cloud tips the scales at 200,000 elephants.
This is nothing compared to a hurricane. If you extracted the water from a cubic metre of hurricane, weighed it and then multiplied it by the number of cubic metres in the whole hurricane cloud, you would find that a single hurricane weighs
40 million
elephants. That’s twenty-six times more elephants than exist on the planet.
Which raises an obvious question: how can something that weighs as much as even
one
elephant float in the sky? The answer is that the weight is distributed across a vast number of tiny water droplets and ice crystals spread over a very large area. The biggest droplets are only 0.2 millimetre (less than
0.008 inch) across: you’d need 2 billion of them to make a teaspoon of water. Clouds form on top of updraughts of warm air. The rising air is stronger than the downward pressure of the water droplets, and so clouds float. When the air cools, and sinks, it begins to rain.
In order to rain, the water in the clouds has to freeze before it falls. If the air temperature is low enough, it will fall as snow or hail; if not, the frozen drops melt on their way down. One puzzle is why there is so much rain in temperate climates like Britain, where clouds rarely get cold enough to freeze pure water. Catalysts like soot and dust help, providing nuclei around which ice can form, but there isn’t enough pollution of that kind to create all the rain.
The answer seems to be airborne microbes. Certain kinds of bacteria are first-class ‘ice nucleators’, to the extent that they have the magical ability to
make
water freeze. Adding
Pseudomonas syringae
, for example, to water, makes it freeze almost instantly, even at relatively warm temperatures of 5–6 °C.
The rain they ‘seed’ carries the bacteria to earth where they use their ice-making powers to mush up plant cells, including many crops, so they can feed on them. Air currents then sweep them back up into the atmosphere again, causing more rain.
If this theory is right, the implications are enormous: merely growing the kind of crops that these ice-making bacteria like could wipe out droughts forever.
It’s not half.
Because the Moon takes exactly the same amount of time to revolve around its own axis as it does to orbit the Earth, we only ever see one face of it.
But the Moon’s motion is not quite regular. As it goes round, it shifts backwards and forwards and side to side, revealing rather more of itself than half. This is known as ‘libration’, from the Latin
librare
, ‘to swing’, after the balancing movements of a pair of scales, or
libra
.
Galileo Galilei (1564–1642) discovered it in 1637, and it comes in three forms.
Latitudinal libration
is caused by the fact that the Moon is slightly tilted on its axis. This means that from a fixed point on the Earth’s surface the Moon appears to rock first towards and then away from us as it passes by, allowing us to glimpse a little more of its top and bottom in turn.
Longitudinal libration
, or side-to-side motion, results from the fact that Moon travels round the Earth at a slightly uneven speed. It always
rotates
at the same rate but, because it’s travelling round the Earth in an ellipse rather than a circle, it’s going faster when it’s closer to the Earth and slower when it’s further away. We can see more of its trailing edge when it’s going away from us, and more of its leading edge when it’s coming towards us.
Finally, there’s
diurnal
(‘daily’)
libration
. Because the Earth is also rotating on its axis, at different times of day we’re looking at the moon from a different angle. This allows us to see a bit round the back of the Moon’s western edge as it rises, and a bit more round the back of its eastern edge as it sets.
The net result is that in any one month (each twenty-eight-day orbit of the Moon) we see 59 per cent of the Moon’s
surface. The Soviet spacecraft Luna 3 took the first pictures of the ‘dark’ side of the Moon in 1959.
The fact that the Moon always shows the same face to the Earth is known as ‘tidal locking’. Many of the 169 known moons in the solar system are synchronised in this way: including both the moons of Mars, the five inner moons of Saturn and the four largest of Jupiter’s moons, known as the ‘Galilean satellites’ after Galileo who also discovered them in 1610.
Earth has a similar relationship with Venus. Despite spinning in the opposite direction to Earth, when Venus is closest to us (every 583 days) it always presents the same face. No one knows why. Astronomical bodies become tidally locked when they are relatively close to each other: Venus never gets nearer to us than 38 million kilometres (24 million miles). So it might just be chance.
STEPHEN
There is this strange thing called libration, which is like
vibration beginning with an ‘l’. It was a thing that was noted by
quite a few of the early astronomers …ROB BRYDON
Can I say, sorry Stephen, but that’s not an
acceptable way of defining a word: ‘Libration, it’s like vibration
but beginning with an l.’
In space, no one can hear you scream, but that’s not to say that there’s no noise there.
There are gases in space, which allow sound waves to travel, but interstellar gas is much less dense than Earth’s atmosphere. Whereas air has 30 billion, billion atoms per cubic
centimetre, deep space averages fewer than two.
If you were standing at the edge of an interstellar gas cloud and a sound came through it towards you, only a few atoms a second would hit your eardrums – too little for you to hear anything. An extremely sensitive microphone might do better, but humans are effectively deaf in space. Our ears aren’t up to it.
Even if you were standing next to an exploding supernova, the gases from the explosion would expand so rapidly that their density would decrease very fast and you’d hear very little.
Sound doesn’t travel well on Mars, either: its atmosphere is only 1 per cent as dense as ours. On Earth, a scream can travel a kilometre (
⅔
of a mile) before being absorbed by the air; on Mars, it would be inaudible at a distance of 15 metres (50 feet).
Black holes generate sound. There’s one in the Perseus cluster of galaxies, 250 million light years away. The signal was detected in 2003 in the form of X-rays (which will happily travel anywhere) by NASA’s Chandra X-ray Observatory satellite.
No one will ever hear it, though. It’s 57 octaves lower than middle C: over a million billion times deeper than the limits of human hearing.
It’s the deepest note ever detected from any object anywhere in the universe and it makes a noise in the pitch of B flat – the same as a vuvuzela.