Welcome to Earth

The picture is made up of thousands of satellite images

By BBC News Online science editor Dr David Whitehouse

This is what scientists are calling the most detailed colour image ever made of the entire Earth.

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A typical view of our planet

Composite satellite images showing the cloud-free Earth have been made before, but the American space agency's (Nasa) latest image beats all others in terms of accuracy and the amount of data that went into it.

For a year, thousands of images and satellite measurements were knitted together until every square kilometre of the globe was covered.

Climatic regions are well seen, as are tropical forests and grasslands. City lights, encroaching on the world's dark regions, are also easily detected. Care was taken to render the colours accurately so that the final map represents the Earth's actual tint and hue.

Cloud free

Although data from many satellites was used, much of the information for the map came from Nasa's Moderate Resolution Imaging Spectoradiometer (Modis) on the Terra satellite that is in a 700-kilometre (435 miles) orbit above the Earth.

Scientists say that Modis is a versatile sensor able to observe a variety of terrestrial, oceanic and atmospheric features of the Earth.

For the land and coastal regions, data were collected between June and September 2001. They were processed to remove clouds.

The polar regions were also surveyed by Modis, as well as by the National Oceanic and Atmospheric Administration's AVHRR sensor onboard a polar orbiting satellite. The Advanced Very High Resolution Radiometer is particularly suited to gathering polar data.

Bright lights, big city

The image of Earth's city lights was created with data from the US Defense Meteorological Satellite Program Operational Linescan System (OLS).

Trans-Siberian railroad

Originally designed to view clouds by moonlight, the OLS is also used to map the locations of permanent lights on the Earth's surface.

It shows that the brightest areas of the Earth are the most urbanised, but not necessarily the most populated.

Even without an underlying map, the outlines of many continents would still be visible. The United States interstate highway system appears as a latticework.

Blue and white

In Russia, the Trans-Siberian railroad is a thin line stretching from Moscow through the centre of Asia to Vladivostok. The Nile River, from the Aswan Dam to the Mediterranean Sea, is another bright thread through an otherwise dark region.

Antarctica is entirely dark. The interior jungles of Africa and South America are mostly dark, but lights are beginning to appear there.

Finally, Nasa scientists, having spent so much effort in removing the clouds from the images, decided to put them back to create a typical view of our planet. To do this they collected two days' worth of global cloud images and a third day of thermal infra-red imagery over the poles.

The result is a stunningly beautiful blue and white planet like no other known world. Welcome to Earth.

Last one out, turn off the light