Looking into space
For thousands of years, humans have told stories to explain the lights in the night sky. Even the earliest stargazers recognized that many of the objects that they could see behaved differently from each other, but it was in ancient Greece – from around the 6th century BCE – that astronomy began to be systematized.
Greek astronomers drew up formal lists of constellations, developed a scale of magnitude to describe the brightness of stars, and made attempts to model the paths of the planets. Following the invention of the telescope in 1608, physical differences between the various objects began to become more apparent, leading to an explosion in scientific knowledge.
The Universe is full of objects, large and small. Many of the closest bodies in our Solar System – asteroids, planets, and moons – are made visible by reflected sunlight.
They move against a seemingly fixed background of more distant objects: luminous stars, glowing nebulae, and remote galaxies.
How the Solar System formed
Our Solar System emerged from a collapsing disc of material that was in orbit around the newborn Sun some 4.6 billion years ago. Mid-sized bodies called planetesimals gradually formed, and eventually developed into today’s planets
The inner planets
The four planets at the centre of our Solar System – Mercury, Venus, Earth, and Mars – follow orbits that are relatively close to the Sun, separated from the much larger outer planets by smaller rocky bodies in the Asteroid Belt.
These worlds formed in a warm, ice-free region of the young Solar System, so their dominant materials are rocks and metals with high melting points.
How rocks form Rocks are large-scale accumulations of different mineral grains. They form in various ways, as a variety of different chemical and physical processes act on the raw materials erupted onto Earth’s surface from pockets of hot magma.
The transformation of rocks by forces such as new eruptions, heat, pressure, erosion, and chemical weathering (the reaction of minerals with materials in their surroundings) is an ongoing and endless process.
Earth is a water planet with 71 per cent of the surface covered in a saltwater ocean to an average depth of 3,700 m (2.3 miles). The ocean makes up more than 97 per cent of all water on Earth’s surface
Ocean currents, waves, and tides
Ocean water is in constant motion. At the surface, the water rises and falls due to waves created by the wind. Meanwhile along the coast, the water surges up and down the shoreline according to a regular pattern due to the tide – a longer-period rise and fall of the ocean caused by the Moon’s pull of gravity.
On the largest scale, the oceans are set in motion by currents flowing at a variety of depths, which are slowly but continuously mixing the water and profoundly affecting the pattern of Earth’s climates.
The carbon cycle
Carbon is an essential ingredient in all types of life. Every living thing takes in carbon in various forms from the environment and gives it out again. Together with physical processes this creates the carbon cycle. The carbon cycle is naturally balanced, but human activities are upsetting that balance.
Carbon in the environment
Compounds of carbon are found in the air, water, soils, and rocks. The fastest components of the cycle are plants taking carbon dioxide (CO2) from the air and water by photosynthesis and all kinds of life breathing out carbon dioxide, or releasing it when rotting.
The age of dinosaurs
Ruling reptiles The Mesozoic Era (252–66 MYA, million years ago), was dominated by large reptiles. Dinosaurs represent just one of many groups.
Reptiles in the sea were not closely related to dinosaurs and included turtles, plesiosaurs, crocodilians, and even giant lizards. The winged reptiles were relatives of dinosaurs called pterosaurs. On land, dinosaurs were joined by numerous other reptiles, including the ancestors of today’s lizards and crocodiles.
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