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IN 1962, during his first orbit around the Earth, the American astronaut
John Glenn was treated to an unusual welcome. His eastward flight took him
into night over western Australia, and the inhabitants of Perth switched
on all their lights to greet the lone astronaut passing 260 kilometres overhead.
He sent a message down to mission control. ‘The lights show up very well.
Thank everybody for turning them on, will you?’

Glenn’s experience showed that artificial lights on the dark side of
the Earth can betray the existence of intelligent life on our planet much
more readily than artefacts such as the Great Wall of China in daytime.
Some scientists had suspected this as far back as the end of the past century.
When the British physicist Lord Kelvin saw the brilliant new electric lighting
of New York, he remarked that it was the most marvellously lit city on Earth,
so bright that it would be visible to Martians. A letter to the Pall Mall
Gazette at about the same time suggested that we could send signals to Mars
by dimming the lights of London in an obviously artificial way.

It has taken the almost three decades from the time of Glenn’s flight,
however, to build up a view of the whole Earth at night. Part of the problem
is that very few satellites are designed to investigate the Earth’s dark
side. Most satellites that survey the Earth are looking for weather patterns,
signs of military activity or geographical features that indicate oil and
mineral deposits. These naturally show up better on the side of the Earth
that is lit by the Sun. Clouds provide information for weather forecasts
but they are a menace for the would-be mappers of light. They can blot out
whole countries on any particular night. And researchers have to distinguish
artificial lights from natural illumination, including the aurorae around
the Earth’s magnetic poles and lightning strokes, which average 100 a second
over the Earth.

Two American scientists have, however, taken up the K K challenge of
examining artificial lighting around our planet. Fifteen years ago, Thomas
Croft of an American company, Technology for Communications International,
began to study images taken by the Defense Meteorological Satellites. These
regularly photograph the dark side of the Earth . In 1978, Croft published
the first analysis of various countries as seen at night-time, by selecting
photographs taken when they were free from cloud, lightning and aurorae.

Now, Woodruff Sullivan of the University of Washington has drawn up
an image of the whole world at night. Sullivan is a radio astronomer, who
is interested in the signals that we are inadvertently broadcasting to the
Universe. Ten years ago, he worked out the pattern of radio transmissions
from the world’s most powerful transmitters: military radar and television
stations. More recently, he has turned his attention to the artificial light
that escapes from the Earth.

Sullivan selected 40 cloud-free pictures taken by the Defense Meteorological
Satellites in the period 1974 to 1984, and used a computer at the Hansen
Planetarium in Salt Lake City, Utah, to combine them into the single picture
shown here. All the sources of light in this image are artificial, with
the exception of an aurora superimposed for artistic effect at the top left.
Sullivan estimates that the picture shows a million individual fires, and
the output of a million million watts of electrically-produced light.

The most brilliant single sources of light, however, are the flares
of burning gas at oilfields. These provide a brilliant fringe to the Gulf,
and large pools of light in North Africa and Siberia. The bright patch of
light between Australia and Asia marks gas flares in the oilfields on the
Indonesian island of Sulawesi. These flares burn off about 3 per cent of
the world’s total production of hydrocarbons.

City lights, in general, reflect the relative affluence of different
regions of the world. ‘Contrast the lustre,’ Sullivan says, ‘of North America,
Europe and Japan, where one-quarter of the world’s people use about three-quarters
of the world’s electricity, with the darkness of Asia, Africa and South
America.’ According to Sullivan, each person in the US uses 75 times as
much electricity as a person in India, and a Japanese 30 times as much as
an Indian. In the Caribbean, Puerto Rico shows this contrast vividly. Politically
and economically part of the US, it stands out more brilliantly than all
the other islands put together.

Even in the developed world, there are differences that reflect national
policies. Compare the darkness of France, where street-lighting is kept
to a minimum, to West Germany, the Low Countries and Britain, which light
up not only cities and towns but also major roads and motorways. The amount
of light picked up by the satellites reveals how much electricity is wasted
in illuminating the sky, a form of pollution that is ruining astronomers’
view of the distant Universe (see ‘Save our skies’, ¿ìè¶ÌÊÓƵ, 11 February).

Brilliant beads of light are also strung along some of the world’s important
lines of communication. The lights of Eygpt follow the Nile, while a semicircle
of lights in China marks the course of the Yellow River and its accompanying
railway. The darkness of the eastern Soviet Union is punctuated by a string
of towns and industrial development along the course of the Trans-Siberian
Railway. In the west of the country, all roads evidently lead to Moscow,
which sits like a spider in the centre of a web of converging lights.

Not all the electrical lighting is connected with large cities or transport
routes. Enclosed within the curve of lights that marks the islands of Japan
is a triangle of brilliant illuminations that lies in the middle of the
Sea of Japan. When Croft first found these lights on the satellite pictures,
he thought they were lightning flashes, but there had been no storm that
night. Instead, Croft discovered, the lights marked the position of a fleet
of Japanese boats that were fishing for squid. The boats use electric lights
to attract squid to the surface. According to Croft, a boat can have 50
lamps, each with a power of 3500 watts, so a fleet may produce 200 million
watts of light.

The multitude of dim lights scattered throughout the tropics represents
a much more primitive way of obtaining food. These are controlled fires,
lit to clear fresh ground for cultivation. Sullivan says that deliberate
fires account for almost all of the land burnt every year, 5 per cent of
the Earth’s land area. Only one-hundredth of this destruction is a result
of wild fires.

In the Sahel (to the south of the Sahara Desert in Africa), people burn
the savanna to destroy old grasses that are too tough for grazing, allowing
young shoots to grow in their place. The blaze also destroys insects and
other pests.

Fires in East Africa and Indochina are the results of another kind of
agriculture, slash-and-burn. Here, farmers clear new regions of arable land
from the forest, burning the trees that they remove. After a couple of years,
the new land loses its fertility, and the farmer has to clear more forest.
The practice is much more widespread than this image indicates. The satellite
photographs used here of the Congo and Amazon regions were taken during
the rainy season, the wrong time of year to show the extensive slash-and-burn
that is destroying their rainforests, and adding to the greenhouse effect.

These interpretations touch on only some of the myriad kinds of light
that form constellation Earth. Sullivan admits ‘in working on this image,
I have frequently strayed outside my area of expertise, and I welcome any
comments, criticisms and corrections to either the image or the interpretation’.

Sullivan hopes that this image will, by showing ‘how mankind reveals
itself at night-time’, add to people’s awareness of the Earth as a planet.
Referring to his studies of both the artificial lights and radio emission
from the Earth, Sullivan concludes sadly ‘at a time when mankind desperately
needs a regular nightly vision, we have wrapped the Earth in an electromagnetic
´Ú´Ç²µâ€™.

YOU CAN obtain a poster (800 X 590 millimetres) of this image, The Earth
at Night, from the Hansen Planetarium, 1098 South 200 West, Salt Lake City,
Utah 84101, US. The cost is $6.00, but enquire about handling and shipping
charges (telephone (801) 538 2242). British readers can obtain the poster
from the Armagh Planetarium, College Hill, Armagh BT61 9DB, for Pounds sterling
4.50 (including postage and packing).

¿ìè¶ÌÊÓƵ will forward any serious comments or criticisms about
this image or its interpretation to Sullivan.

* * *

Satellites keep a weather-eye on the Earth’s night face

SINCE 1972, the US Air Force has been operating satellites to provide
short-range forecasts of cloud cover. They orbits that take them north-south
over the Earth’s poles. Some of the satellites always orbit above the line
that separates day from night, so passing over regions that are experiencing
sunrise or sunset. Others an orbit at right angles, alternately passing
over regions that face the Sun directly (at noon on the ground below) and
regions that face directly away (where it is midnight). Satellites in the
second kind of orbit provided the images of the Earth at night that have
been used to make the picture on the previous page.

Each satellite has a small telescope that looks down towards the Earth.
It can detect a 50-watt light bulb on the Earth, some 800 kilometres below.
The telescope scans rapidly east-west, every 0.4 second, as the satellite
carries it north-south, and the sequence of lines builds up a picture in
a similar way to the lines that make the picture in a television set. The
east-west scan is 3000 kilometres wide, and the information is sent back
to Earth as the brightness in individual regions, which are known as ‘pixels’.
Each pixel is 3 kilometres square.

On Earth, the signal is converted into a photograph by scanning a laser
back and forth across a strip of photograph film. The brightness of the
laser beam varies according to the signals from the satellite to produce
the image.