Planet Earth is a dense, rocky ball about 7,920 miles (12,750 km) across. It is one of nine planets circling our local star, the Sun. Earth is the only planet we know of that can support life. From space, its surface looks blue and cool, but inside it is so hot that rock can melt.
The Sun began to form around 5 billion years ago out of a cloud of whirling dust and gas in space. As it formed, the Sun’s gravity gradually pulled the dust and gas into lumps, which became the planets. At first, Earth was a ball of molten rock. Its surface rock slowly began to cool and harden around 4 billion years ago.
Table 16. EARTH’S FORMATION
|5.0 billion years ago||The solar system begins to form from gas and dust swirling in space.|
|4.6 billion years ago||Earth begins to form as a ball of molten rock.|
|4.5 billion years ago||Volcanoes erupt gas and steam to form the oceans, and molten rock.|
|4.2 billion years ago||Earth’s surface cools and the hard outer crust forms.|
|3.6 billion years ago||The first continents form; life begins on Earth.|
Our planet is made of four main layers. The outer layer is called the crust. Below is the mantle, which is solid near the top and molten below. Temperatures become hotter near Earth’s center. The outer core is a mass of molten rock. The temperature of the inner core is over 9,000°F (5,000°C).
The crust is made up of oceanic crust, below the oceans, and continental crust, which carries Earth’s land. It is Earth’s thinnest layer, at only 4–43 miles (6–70 km) thick. The mantle, at 1,800 miles (2,900 km) deep, is the thickest layer. The outer core is around 1,240 miles (2,000 km) thick. The inner core at the center of Earth is about 1,702 miles (2,740 km) across.
Earth’s atmosphere is a layer of gases about 430 miles (700 km) thick. Without the atmosphere, there could be no life on Earth. It protects us from harmful rays in sunlight and prevents our planet from becoming too hot or too cold.
Earth is just the right distance from the Sun so that temperatures are neither too hot nor too cold for living things to survive. Earth’s atmosphere and oceans also help to control temperatures. And Earth has air and water- both vital for life.
Earth takes one year, or 365.242 days, to orbit, or travel, around the Sun. At the same time, it spins on its axis (an imaginary line joining the North and South poles). As it spins, Earth is tilted on its axis at an angle of 23.5°. This tilt produces the SEASONS.
Seasons are regular weather patterns in different places on Earth’s surface caused by the planet’s tilt. At any time, one hemisphere (half of Earth above or below the equator) leans toward the Sun, exposing it to more of the Sun’s light and heat and bringing warm summer days. The other half tilts away and has winter.