User:Pterryx/TWA/Earth/2



 The  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 is also called the Blue Planet, "Planet ", and "Terra".

The is home to millions of species of plants and animals, including humans. is the only planet in the galaxy known to support life. 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 formed around 4.5 billion years ago. The organisms that live on have completely changed its air (atmosphere); this is called a biosphere.

71% of 's surface is covered in salt water oceans. 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.

interacts with other objects in the Solar System, particularly the Sun and the Moon. The 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
is a terrestrial planet. This means it is made up of solid rock unlike a gas giant such as Jupiter. It is the largest out of the four terrestrial planets in mass and diameter. The 's shape is an oblate spheroid. This means it is basically a sphere but it bulges around the middle. The circumference of the is about 40,000 kilometers; the average width of the  is about 12,700 kilometers. The highest point on is the peak of Mount Everest at 8,848 meters above sea level. The lowest natural point is the bottom of the Mariana Trench at 10,911 meters below sea level. Because of the bulge at the middle or the equator, the farthest point from the 's center is the top of Mount Chimborazo in Ecuador.

Inside, the is similar to the other terrestrial planets. It has an outer, solid rock layer called the crust. Everything that lives on is on top of the crust. Below that is a layer of thick, semi-liquid rock called the mantle. Under that is a thin liquid layer called the outer core and then the solid iron inner core. The thickness of the crust changes. On land the average is between 30–50 kilometers thick. Under the oceans in some places it is only 6 kilometers thick.

Tectonic plates
The is the only terrestrial planet with active plate tectonics. Due to plate tectonics the 's crust basically floats on the thick liquid rock of the mantle below. The crust is split up into parts called plates. These plates interact as they move about causing quakes and creating volcanoes and mountain ranges. The place where plates meet are called plate boundaries. There are three types of plate boundary: constructive, destructive and transform.

Surface
The changes greatly from place to place. Over 70% of the surface is covered by water. The underwater surface has many of the same features as the above sea with volcanoes, mountains and trenches or canyons. The 30% not covered by water is mostly forests, deserts plains, mountains and plateaus. Human civilization has led to increasing urbanisation — the growth of cities. Many things can change the surface of the. Plate tectonics is main cause of change but there are others such as erosion from wind and rain, erosion by the oceans or meteorite impacts. There are three main types of rock that make up the 's surface: Igneous rock is made when magma or lava from the mantle reaches the surface and cools. As it gets colder it turns into rock or solidifies. Sedimentary rock is made from sediment, like sand or small bits of other rock, that has been crushed and packed tightly together. Metamorphic rock which is made when either of the other two types are changed by high or low temperatures and pressures.

Atmosphere
A planet's atmosphere is a layer of different gases surrounding it. It is kept there by gravity. The 's atmosphere is made of about 78% nitrogen, 21% oxygen and small amounts of other gases. This mixture is often called air. Further up there is a layer of ozone gas called the Ozone layer. Ozone absorbs ultraviolet radiation from the Sun. Ultraviolet radiation is dangerous to people, so without the Ozone layer life would not be possible. The atmosphere also protects the from crashes with meteors and small asteroids. This is because they burn up due to all the friction as they fly through it. It also helps to keep warm. Some gases including carbon dioxide and methane act like a blanket around the, they trap heat under them, keeping the warm.

== Weather, climate, and water cycle == 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. 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 takes about 24 hours to complete one day and about 365 days to complete a year. Actually, the 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 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 at an average distance of 250,000 miles. It is tidally locked to, which means it always has the same side facing the. It takes roughly one month to complete one orbit. The 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.