FOR some reason, many of the odder beliefs that have been around for ages
seem to have assumed the status of self-evident truths. Take the idea that water
swirls down the plughole in a different direction on the other hemisphere of the
world. Or there鈥檚 the notion that the flight of bees defies all known laws of
aerodynamics.
I was reminded of these scientific old wives鈥� tales when, in the space of 24
hours, I heard two people claim that a particular building 鈥渨as the only one on
Earth visible from space鈥�. A British businessman was trying to impress a German
tourist by claiming that London鈥檚 Millennium Dome was the one in question. The
second person, a television news reporter, asserted that the passenger terminal
at Hong Kong鈥檚 new airport held the honour. It set me thinking. Are either of
these buildings visible from space? And what do people actually mean when they
say 鈥渧isible from space鈥�?
Space starts about 30 kilometres from your doorstep and
extends, essentially, to infinity. So, presumably, when people say 鈥渧isible
from space鈥� they don鈥檛 mean visible from Jupiter or the Large Magellanic Cloud.
They mean visible from nearby space鈥攕ay, the altitude of a typical space
shuttle mission, which is about 250 kilometres.
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And what do people mean by 鈥渧isible鈥�? Visible by whom or by what? If we鈥檙e
talking about spy satellites, they can see almost anything on the ground. In
fact, it鈥檚 not difficult to deduce what they can see because it is determined
not by technology but by the atmosphere. Looking at the ground from space is
like looking down at the pattern of tiles on the bottom of a swimming pool.
Turbulence smears the appearance of things. In the atmosphere, turbulence makes
it impossible to see anything smaller than 1/3600th of a degree across, or an
arcsecond鈥攁nd half that in favourable conditions. This kind of optical
resolution can be achieved even by a telescope with a mirror a few tens of
centimetres across. At a height of 250 kilometres, such a telescope could see
phone boxes, maybe even individual people. So when people say 鈥渧isible from
space鈥�, they mean visible from nearby space and with the naked eye.
The resolution of the human eye depends on the size of the pupil, which
changes from day to night. But, say, for the sake of argument, that it鈥檚 day and
that 鈥渆arthlight鈥� shrinks the pupil to a millimetre in diameter. In this case,
the resolution of the eye is about 2 arcminutes鈥攇ood enough to see a penny
from a distance of about 20 metres. From the altitude of the space shuttle
orbit, this resolution would allow an astronaut to see surface structures as
small as 150 metres across鈥攁bout the size of a big sports stadium.
Actually, when the atmosphere is still and the Earth not too dazzling, buildings
smaller than 100 metres across ought to be visible, always provided that they
show sufficient contrast with their surroundings.
The Millennium Dome is 365 metres across鈥攁 shiny white flying saucer
which has come to rest in the mud of London鈥檚 Docklands. So we can be sure it is
visible from space. That also goes for the passenger terminal at the new Hong
Kong International Airport, which is 1.27 kilometres long. But since a host of
other buildings must satisfy the visibility criteria, the fact that you can see
both these buildings from space isn鈥檛 really much of a claim to fame. It shows a
degree of temporal chauvinism. After all, the base of the Great Pyramid at Giza,
built by King Khufu around 2500 BC, is 230 metres square, and the Mayans and
Aztecs built pyramid-shaped temples as big as 300 metres across.
So Khufu could, perhaps, have claimed that his pyramid was the only building
on Earth that was visible from space鈥攁t least until his son King Khafre
built his own pyramid at Giza some years later. But everyone who鈥檚 made the
claim since has been talking through their hat!