DK Science: Planet Earth
The rocky ball that forms our world is one of nine planets in the Solar System. Earth is a sphere, with a slight bulge in the middle at the Equator, and a diameter of 12,756 km (7,926 miles). It hurtles at speeds of 105,000 kph (65,000 mph) during its orbit around the Sun, turning on its AXIS once every 24 hours. This journey takes a year to complete. The Earth is the only planet that is known to support life, in a zone called the BIOSPHERE.
The atmosphere is a layer of gas surrounding the Earth that is some 700 km (400 miles) thick. It is made up of nitrogen (78 per cent) and oxygen (21 per cent), plus traces of other gases. Tiny droplets of water vapour form the clouds we see.
Oceans cover 70.8 per cent of the Earth’s surface, to an average depth of 3.5 km (2 miles). The hydrosphere (watery zone) also includes freshwater rivers and lakes, but these make up less than 1 per cent of Earth’s water.
Dry land occupies 29.2 per cent of the Earth’s surface, where the lithosphere (rocky crust) rises above sea level to form seven continents and countless smaller islands. Land can be categorised into biomes – major habitats such as forests, grasslands, and deserts.
The cryosphere (frozen zone) includes snow and glaciers on high mountains, sea ice, and the huge ice caps that cover the landmasses of Greenland and the Antarctic. In the past, during long cold eras called ice ages, ice covered much more of Earth’s surface than it does today.
Meteorology, the study of Earth’s atmosphere, is one of the Earth sciences. Earth scientists study Earth’s physical characteristics, from raindrops to rivers and the rocks beneath our feet. Other branches of study include geology (rocks), hydrology, (oceans and freshwater), and ecology (living things and the environment).
Satellite images allow scientists to monitor everything from ocean currents to minerals hidden below ground. Techniques such as radar and sonar have transformed our understanding of our planet. Some Earth scientists also spend time in the field, which means working outdoors, collecting data and samples from clouds, cliffs, craters, volcanic lava, and deep-buried ice.
The biosphere is the part of Earth that contains what is needed for living things. This zone extends from the ocean floor to top of the troposphere (lower atmosphere). Tiny organisms can survive deep in the Earth’s crust, but most forms of life are found from a few hundred metres below sea level to about 1,000 m (3,300 ft) above sea level.
Ozone is a gas spread thinly through the atmosphere. It filters harmful ultraviolet (UV) rays from sunlight, while allowing visible light (the light we can see) to pass through. Other gases in the atmosphere trap the Sun’s heat when it is reflected from the Earth’s surface, providing additional warmth for living things.
Environmental scientist James Lovelock argues that the planet can be seen as a complete living organism, which he names Gaia, after the Greek goddess of Earth. Gaia theory states that Earth itself balances conditions to suit living things in the biosphere. This includes regulating the composition of the atmosphere the chemistry of the oceans, and ground surface temperature.
The ground beneath our feet may seem still, but in fact the Earth is spinning like a top as it orbits the Sun. The Earth takes 24 hours to rotate about its axis, an imaginary line running from the North Pole to the South Pole through the centre of the Earth. The Earth’s axis is not at a right-angle to the path of its orbit, but tilts at an angle of 23.5°. The angle between each region of Earth and the Sun’s rays alters through the year, producing seasonal changes in temperature and day length. These are most noticeable in regions next to the poles, which are most distant from the Equator.
As Earth turns about its axis, one half is bathed in sunlight and experiences day, while the other half is plunged into darkness and has night. The Earth always rotates eastward, so the Sun and stars appear to rise in the east and set in the west. The tilt of the planet means that at any time, one hemisphere (half of the Earth, as divided by the Equator) leans toward the Sun and experiences summer, while the other leans away and has winter.