User:BoumerzougAdel/TWA/Earth/2



 The Earth is the third planet from the Sun. It is one of the four terrestrial planets in our Solar System. This means most of its mass is solid. The other three are Mercury, Venus, and Mars. The Earth is also called the Blue Planet, "Planet Earth", and "Terra".

The Earth is home to millions of species of plants and animals, including humans. Earth is the only planet in the galaxy known to support life. Earth has many places that are suitable for humans to live in, although some areas of the planet can are dangerous or uninhabitable.

Science shows that Earth formed around 4.5 billion years ago. The organisms that live on Earth have completely changed its air (atmosphere); this is called a biosphere.

71% of Earth's surface is covered in salt water oceans. Earth is the only place in the Solar System where liquid water is known to exist at present. The other 29% is made of rocky land in the shape of continents and islands.

Earth interacts with other objects in the Solar System, particularly the Sun and the Moon. The Earth orbits the Sun roughly once every 365.25 days. One spin is called a 'day' and one orbit around the Sun is called a 'year'. This is why there are 365 days in a year, but a leap day added to the calendar once every 4 years.

== Shape and structure ==

Hot air rises. As it rises it gets colder again and falls. This creates convection currents. When hot air meets cold air different weather effects happen. Convection currents are the cause of almost all weather on Earth. When it gets hot on the surface water evaporates and becomes steam or water vapour. This hot water vapour rises. As it rises it gets colder. When it gets cold enough it turns back into water again. This causes the clouds and rain. It is called the water cycle.

== Orbit and rotation ==

The Earth takes about 24 hours to complete one day and about 365 days to complete a year. Actually, the Earth take 365.24 days to revolve around the sun. After every four years, an extra day is added, and the year has 366 days. This is a leap year. The Earth is, on average, 150 million kilometers away from the Sun, and moving at a speed of 30 kilometers a second or 108,000 miles an hour. The Moon orbits the Earth at an average distance of 250,000 miles. It is tidally locked to Earth, which means it always has the same side facing the Earth. It takes roughly one month to complete one orbit. The Earth is part of the Solar System and orbits the sun along with thousands of small objects and eight planets. The Sun, and therefore the Solar System, are currently traveling through the Orion Arm of the Milky Way Galaxy and will be for approximately the next 10,000 years.