Earth is the third planet from the Sun - on which the human race lives.
Earth 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 Planet Earth, Blue Planet and Terra.
Earth is home to millions of species of plants and animals, including humans. Earth is the only place in the universe where life has been confirmed to exist. Science shows that the Earth formed around 4.5 billion years ago. The things that live on Earth have completely changed its air or atmosphere. This is called a biosphere.
About 71% of Earth's surface is covered in salt water oceans. Earth is the only place in the universe where liquid water is known to exist. 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 or goes around 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 we have 365 days in a year. Earth has only one moon, known as the Moon.
Most scientists think that the Earth and the other planets formed about 4.5 billion years ago. They were made of the leftover gas from the nebula that made the Sun. Some scientists think that the Moon might have been made from part of a small planet which is sometimes called Theia. Scientists believe that it crashed into Earth a part broke off — becoming the Moon.
Condensing water vapour or steam, comets and asteroids hiting the Earth made the oceans. Within a billion years (that is at about 3.8 billion years ago) the first life evolved. Some life developed photosynthesis which lets plants make food from the Sun's light and water. This released a lot of oxygen into the atmosphere or air, making the Earth's surface suitable for life. This oxygen also formed the ozone layer which protects the Earth from bad ultraviolet radiation from the Sun. This protection made it possible for things to move from the deep ocean to the surface. Long ago almost all land was in one place. This is called a supercontinent. The earliest known supercontinent was called Rodina. Scientists think that soon after this there was a time when the Earth was almost entirely covered by thick ice sheets called glaciers. This is called Snowball Earth theory.
Earth 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. Earth also has the strongest gravitational and magnetic field.
The Earth's shape is an oblate spheroid. This means it is basically a sphere but it bulges around the middle. The circumference of the Earth is about 40,000 kilometers; the average width of the Earth is about 12,700 km. The highest point on Earth is the peak of Mount Everest at 8,848 m 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 Earth's center is the top of Mount Chimborazo in Ecuador.
Inside, the Earth is similar to the other terrestrial planets. It has an outer, solid rock layer called the crust. Everything that lives on Earth is on top of the crust. Below that is a layer of thick, 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. The inside of the Earth is very hot, the temperature of the outer core may be as high as 7,000 degrees Celsius.
The Earth is the only terrestrial planet with active plate tectonics. Due to plate tectonics the Earth'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 earthquakes 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.
- At a constructive plate boundary two plates move away from each other, and hot magma (liquid rock) is pushed upwards through the cracks. These kinds of boundaries make ocean rifts, undersea volcanoes or ridges. An example of this kind of plate boundary is the island of Hawaii.
- At a destructive plate boundary, two plates move towards each other. This forms islands, volcanoes and high mountain ranges. The Himalayas were made by this process.
- At a transform plate boundary, two plates move parallel to each other. As they move the grind against each other. This kind of plate boundary causes earthquakes.
The Earth changes greatly from place to place. Over 70% of the Earth surface is covered by water. The underwater surface has many of the same features as the above sea with volcanos, mountains and trenches or canyons. The 30% not covered by water is mostly forests, deserts, plains, mountains and plateaux. Human civilisation has led to increasing urbanisation — the growth of cities.
Many things can change the surface of the Earth. 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 Earth'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.
All the water on Earth, on land or in the atmosphere, is part of the hydrosphere. No other planet, that humans know of has a hydrosphere. About 97.5% of all water is salt water. About half the fresh water is currently ice. The oceans absorb or soak up carbon dioxide, a gas that adds to the greenhouse effect and global warming.
A planet's atmosphere is a layer of different gases surrounding it. It is kept there by gravity. The Earth's atmosphere is made of roughly 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 earth 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 Earth warm. Some gases including carbon dioxide and methane act like a blanket around the Earth, they trap heat under them, keeping the Earth warm. This is called the natural greenhouse effect. When humans build factories and power plants to make electricity, combustion is involved. Combustion lets out a lot of carbon dioxide. This carbon dioxide goes up into the atmosphere and traps more heat. This is called global warming.
Hot air rises. As it rises it gets colder again and falls. This is called 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 gets cold enough it turns back into water again. This causes the clouds and rain. It is called the water cycle.
The Earth takes about 24 hours to complete one day and 365 days to complete a year. The Earth is, on average, 150 million miles away from the Sun, and moving at a speed of 30 kilometers a second or 108,000 miles an hour.
The Earth rotates at a perfect speed: it is not too fast or too slow. If earth rotated too quickly, the winds would be so strong that there would be great hurricanes everywhere. Survival (staying alive) would be very hard indeed! However, if the earth rotated too slowly - this would give us longer days and longer nights - the temperature changes would be extremely hard for life on Earth. This is because the longer the nights are, the colder they get; the longer the days, the hotter they get. If Earth rotated slowly, then, we would have freezing nights and boiling days.
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.
Earth has the perfect amount of mass. If the Earth was smaller, it would have less gravity. This would mean the earth wouldn't pull on us so hard. Running and jumping would be easy. Falling wouldn't hurt. This may sound fun, but in real life it would be quite dangerous. Since gravity keeps things on the ground, without enough gravity, cars and people could blow right off the ground and into space! The earth would not be able to pull rain to the ground, so the water cycle would not work, and there would be droughts and hunger all over the world. It's a good thing Earth doesn't have less gravity, or less mass.
It's also a good thing that the earth doesn't have more mass. If it did, gravity would be much too strong. It would be hard just to walk around. We would get tired very easily. It would be a lot of work to go upstairs. Also, many dangerous chemicals that are dangerous to breathe would not float into space. A freshly painted house often has a bad smell. If the earth had more gravity, that smell might never go away! Because of the earth's perfect mass, many dangerous chemicals rise up out of the atmosphere and into space, leaving us with clean, safe air to breathe.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)