The Earth The Earth is unlike every other planet in the Solar System in a number of different ways. • It is the only planet that has an atmosphere containing 21 percent oxygen. • It is the only planet that has liquid water on its surface. • It is the only planet in the solar system that has life. • The Earth is the only inner planet (Mercury, Venus, Earth and Mars) to have one large satellite, the Moon. Mars has two very tiny moons. Mercury and Venus have none. • The Earth is fragile. Its surface is split into plates (tectonic plates) which float on a rocky mantle – the layer between the surface of the earth, its crust, and its hot liquid core. The inside of the Earth is active and earthquakes, volcanoes and mountain building takes place along the boundaries of the tectonic plates. • As a result of the Earth’s geological activity (the volcanoes and earthquakes) the surface of the Earth has far fewer craters than the surface of planets such as Mars, Venus and Mercury or the surface of the Moon. The craters have sunk down or been worn away by wind and rain over millions of years. • When viewed from outer space much of the Earth’s surface cannot be seen because of clouds of water vapour. The water vapour makes the Earth, when seen from outside, into a brilliant shining orb, as you can see in Figure 1. Figure 1. Earth taken from the Apollo 17 spacecraft in 1972 • The Earth is the third planet from the Sun and comes between the planet Venus and the planet Mars. Figure 2. The planets of the Solar System. • The Earth takes 365¼ days to complete its orbit round the Sun. The Earth’s year is therefore 365 days long but the ¼ days are added up and every fourth year has one extra day, on the 29th of February. This fourth year is called a Leap Year (366 days) and is always a year which can be divided exactly by 4 – 2000, 2004, 2008, 2012, 2016. • The planets closer to the Sun, Mercury and Venus, have shorter years than the Earth. The planets further away from the Sun have longer years; Pluto takes 249 of our years to make one orbit of the Sun. • As the Earth orbits round the Sun it turns on its axis, rotating right round in 24 hours. The side of the Earth that faces the Sun has daytime and the side of the Earth that is turned away from the Sun has night-time. When it is daytime in Britain, it is night-time on the opposite side of the Earth in New Zealand. • As the Earth orbits round the Sun it tilts very slightly and so gives us the seasons. When the Earth has tilted so that the northern half of the Earth is a little away from the Sun, the northern hemisphere (meaning half of the Earth’s sphere) has winter. • At this time the southern hemisphere is tilted very slightly towards the Sun and the southern hemisphere has summer. Winter in Britain means summer in New Zealand. Closer to the Equator there is much less difference between summer and winter. • The Earth is 93 million miles, or 150 million kilometres from the Sun. • We can also say that the Earth is 1 AU from the Sun. Astronomers (people who study the planets and stars) use a measurement called an Astronomical Unit, AU. Earth’s measurements are taken as the standard for this system, so Earth is 1 AU from the Sun, Venus is 0.72 AU from the Sun and Mars is 1.52 AU from the Sun. Pluto, furthest out, is 39.5 AU from the Sun. How many million miles from the Sun is Pluto? • The Earth’s diameter, the distance round its middle at the Equator, is 7928 miles, or 12760 kilometres. • The Earth is not an exact sphere; the diameter going round the North and South Poles is slightly less than the diameter round the Equator. The Polar diameter is 7891 miles, or 12700 kilometres. (Try measuring the different diameters of an orange and you will see how they can differ). • The Earth is larger than Mercury, Venus and Mars, the planets closest to it. On astronomy maps it often looks smaller than Venus because Venus is shown with a thick layer of sulphuric acid which lies 40 miles above the • The Earth differs from all the other planets because it has such a wide diversity of life and intelligent beings. This has only been possible because of the Earth’s atmosphere which has protected the Earth and allowed life to flourish.
Posted on: Thu, 13 Mar 2014 11:27:27 +0000
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