From the perspective we get on Earth, our planet appears to be big and sturdy with an endless ocean of air. From space, astronauts often get the impression that the Earth is small with a thin, fragile layer of atmosphere. For a space traveller, the distinguishing Earth features are the blue oceans, brown and green land masses and white clouds set against a black background. Many dream of travelling in space and viewing the wonders of the universe. In reality all of us are space travellers. Our spaceship is the planet Earth, travelling at a speed of 108,000 kilometres (67,000 miles) per hour.
Earth is the 3rd planet from the Sun at a distance of about 150 million kilometres (93.2 million miles). It takes 365.256 days for the Earth to travel around the Sun and 23.9345 hours for the Earth to rotate a complete revolution. It has a diameter of 12,756 kilometres (7,973 miles), only a few hundred kilometres larger than that of Venus. But what most distinguishes our world from Venus from any distance is Earth's satellite, the Moon, where our planetary twin in space has none. Earth's atmosphere is composed of 78 percent nitrogen, 21 percent oxygen and 1 percent other constituents. Earth is the only planet in the solar system known to harbour complex life-forms. Our planet's rapid spin and molten nickel-iron core give rise to an extensive magnetic field, which, along with the atmosphere, shields us from nearly all of the harmful radiation coming from the Sun and other stars. Earth's atmosphere protects us from meteors, most of which burn up before they can strike the surface.
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