The Earth is the third planet from the Sun and is the only planet in the solar system that we know of that sustains life. Our planet Earth is located about 93 million miles from the Sun and receives enough sunshine to support life. The Earth spins like a top as it orbits the Sun. One complete spin on its axis takes one day or about 24 hours while one complete orbit around the Sun takes about one year or 365 days. Since the Earth is tilted to one side as it travels around the Sun, for part of the year the northern half has summer and gets more direct rays of Sunlight while the southern half has winter. The southern half is then tilted toward the Sun as Earth continues its orbit which allows the southern half to have summer and the northern half to have winter. More than 70 percent of the surface area of the Earth is covered by water. It weighs about 6,600 million trillion tons. Its mass is what creates a force of gravity which pulls everything to the planet's center. The Earth was formed about 4.5 billion years ago and over those years it has changed. At first its air was poisonous and there was no life. It is thought that from a gas mixture of methane, ammonia and carbon dioxide and water vapor in the atmosphere the first organic molecules began the first building blocks of life on Earth when sparked by lightening and the radiation from the Sun. The first one-celled plants and animals were said to have formed about three billion years ago.
Just as then, the Earth continues to change. The Earth's surface is constantly affected by water and wind, while its interior is altered below the surface from volcanoes and earthquakes. The Earth is covered by a solid 22 mile layer of rock known as the crust. It is made up of granite and basalt. This is where the continents and oceans lie. This crust is cracked into huge pieces we call plates that meet, among other places, along the rim of the Pacific Ocean. Where the plates meet is where volcanoes and earthquakes are prominent. The crust is in constant change by pressures from within the Earth. The mantle lies beneath the crust and is a 1,800 mile thick layer of thick, hot, dense rock. It will bend under steady force but can also break if hit hard. The core is about 2,170 miles in radius and is made up of iron and nickel. The Earth's protective blanket of air is called the atmosphere which consists of many layers. Although you can not feel it, the Earth's atmosphere is very heavy. It weighs about 5,000 trillion tons. The atmosphere extends upward for several hundred miles and as you go higher the air becomes less dense. When you fly in an airplane or even in a space craft you must bring the air you need to breathe. The atmosphere surrounds the entire planet and is made up of mostly nitrogen and oxygen along with a small amount of carbon dioxide, dust particles and water. The atmosphere is what protects us from getting to much Sun during the day yet keeps in enough heat during the night so we do not freeze. We live in the layer of air known as the troposphere which is the lowest layer of the atmosphere where we have oxygen and most weather phenomena take place.