The Junior Colour Encyclopedia of Prehistoric Life [antikvár]

IN THE BEGINNINGThe Creation of EarthNo one knows for certain how the Universe and the Solar System began. Over the years many scientists have suggested theories. Today, however, we beheve that the Universe began about 18,000 million years ago with an enormous explosion, known as the Big Bang. After the explosion matter was hurled out into space. There it condensed into galaxies. Inside the galaxies, stars began to form.As long ago as 1775 Immanuel Kant, a German philosopher, suggested that the Solar System was formed from a spinning disc of dust and gas. The centre of the disc became a star, the Sun and the outer parts became the planets. Other scientists had similar ideas. But in 1916 Sir James Jeans, an English astronomer, put forward a completely different theory. He suggested that another star passed close to the Sun and drew off a cigar-shaped filament of material. This broke up into the planets. Yet another theory was put forward by the Russian scientist Otto Schmidt in 1944. He suggested that the Sun passed through a cloud of gas and dust and gathered up a disc of material that later became the planets.Today scientists believe that the ideas of Immanuel Kant were correct. According to the modem Cold Gas Disc Theory, the Sun formed at the centre of a whirling disc of dust and gas. As gravity pulled the particles together, vast amounts of energy were released and the Sun began to heat up. Eventually it became so hot that hydrogen atoms fused together to form helium. This reaction is called thermonuclear fusion. It gives out an enormous amount of heat and light. The Sun began to shine brightly about 5,000 million years ago and will continue to bum for at least another 5,000 million years, before it starts to cool down.According to Sir James Jeans' theory (the tidal theory) of how the planets formed, a passing star dragged out a tongue of hot gas from the Sun's surface. This broke up and the pieces cooled and condensed into the planets. If this theory were true, then the sun and planets would be made of the same chemicals. But in fact their composition is very dilfcrent. Scientists now believe that such a tongue of gas would have drifted out into space rather than form planets.From earliest times people have had many fanciful ideas about the Earth, planets and stars. This idea of the Earth carried by three elephants standing on the back of a tortoise comcs from an ancient Indian tribe.