Earth

Earth is the third planet from the Sun and the only astronomical object known to harbor life. This special planet is an ocean world, covered mostly by liquid surface water. The water makes up about 70.8% of Earth's crust, with the rest being land. Earth's surface changes slowly because of moving tectonic plates, which can create mountains, volcanoes, and sometimes cause earthquakes.

Earth has a special layer of air called an atmosphere. It is made mostly of nitrogen and oxygen, with some water vapor that forms clouds. This atmosphere helps protect the planet and keeps the temperature right for water to stay liquid. Earth is rounded like a ball, with a distance around it of about 40,000 kilometers.

Earth orbits it the Sun, making one full trip in about 365.25 days. It also spins around its own center, which takes slightly less than a day. Because Earth's axis is tilted, we have different seasons during the year. A natural satellite, the Moon, orbits Earth and helps control our tides.

Life on Earth began very early, and humans have lived here for a long time. We depend on Earth's biosphere and its natural resources. Our actions are changing the planet in ways that can harm people and nature.

Etymology

The word Earth comes from old languages. In Old English, it was written as eorðe. Early people sometimes thought of Earth as a goddess, like the mother of Thor.

Later, people began using a capital letter for Earth when talking about our planet. Some people still write it in lowercase, as earth. In science and stories, you might also see the name Terra, used in languages like Italian and Portuguese.

Natural history

Main articles: History of Earth and Timeline of natural history

Earth formed about 4.5 billion years ago from a cloud of dust and gas around the young Sun. It grew by pulling in more material over millions of years. Early Earth had no air or water. Volcanic activity released gases and water vapor, which formed the air and oceans we have today.

Over time, Earth's surface cooled and hardened. The movement of its plates created the continents we see now. These continents have come together and split apart many times. In the far future, the Sun will grow very large and hot, changing Earth forever.

Bulk properties

Further information: Geophysics

Earth is round like an egg because of how it spins. It is the fifth largest planet in our Solar System and the largest of the rocky planets. Its average diameter is about 12,742 kilometers, but it bulges a little in the middle because of its spin.

Inside Earth, there are several layers. The outer layer is a solid crust. Below the crust is a thick, soft layer called the mantle. Deep inside is a hot, liquid outer core and a solid inner core. Earth is made mostly of iron, oxygen, silicon, and magnesium. Heat inside Earth helps create volcanoes and earthquakes.

Earth also has a magnetic field. This field protects us from harmful solar winds. It changes shape slowly over time.

Surface environment

Further information: Planetary surface, Land cover, Land, and Pedosphere

Earth's surface is where the air meets the solid ground and oceans. It covers about 510 million square kilometers. Earth has two halves called the Northern and Southern, split by latitude. It also has two sides called the Eastern and Western, split by longitude.

Most of Earth's surface is covered by water—about 70.8%. This makes Earth a water world. The water is split into five big parts: the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. The land makes up about 29.2% of Earth's surface. This land has places like Africa-Eurasia, America, Antarctica, and Australia, plus many islands.

Earth's land and ocean floors form the top layer of Earth's crust. The land has many different shapes, such as mountains, deserts, and flat areas. The ocean floor also has different shapes, including deep valleys and underwater mountains.

Tectonic plates

Main article: Plate tectonics

Earth's outer layer, the lithosphere, is split into pieces called tectonic plates. These plates move around. When they push together, pull apart, or slide past each other, they can cause earthquakes and volcanic activity.

Hydrosphere

Main article: Hydrosphere

Further information: Ocean, Sea, and Cryosphere

Earth's hydrosphere is all of its water. Most of this water is in the big ocean, covering about 361.8 million square kilometers. The ocean is about 3,682 meters deep on average. About 97.5% of Earth's water is salty, and the rest is fresh water, mostly frozen in ice caps and glaciers.

Atmosphere

Main article: Atmosphere of Earth

Further information: Peplosphere

Earth's atmosphere has a pressure of about 101.325 kPa at sea level. It is mostly made of nitrogen (78.084%) and oxygen (20.946%), with small amounts of other gases. Water vapor makes up a tiny part of the air.

The atmosphere has several layers. The troposphere is the bottom layer, where most weather happens. Above it are the stratosphere, mesosphere, and thermosphere. The air gets thinner as you go up and finally fades into space. The ozone layer in the stratosphere protects Earth from harmful sunlight.

Weather and climate

Main articles: Weather and Climate

Earth's weather and climate are shaped by the Sun's energy, the planet's spin, and ocean currents. The atmosphere moves heat around, making different weather patterns. Climate changes depending on how far you are from the poles, with tropical, subtropical, temperate, and polar areas. Being near oceans, atmospheric circulation, and topography also change local climates.

The water cycle is important for life on Earth. Water evaporates, rises, cools, and falls as precipitation. This cycle helps shape the land and supports living things. Climate classifications, like the Köppen system, describe the world's different climate zones using temperature and precipitation.

Orbit and rotation

Main article: Axial tilt § Earth

Earth spins around once every 24 hours. We call this a day. Earth also moves around the Sun. This takes about 365 days and we call this a year. Earth is tilted on its axis. Because of this tilt, different parts of Earth get more sunlight at different times. This causes seasons like summer and winter. When one half of Earth gets more sunlight, it is summer there. The other half has winter. This tilt also makes days longer in summer and shorter in winter.

Gravitational domain and influence

Further information: Satellite system (astronomy)

Gravitational field

Main article: Gravity of Earth

Earth's gravity is what keeps everything on the ground and gives objects their weight. Near the surface, things fall at about 9.8 meters per second squared. Earth’s gravity can reach out about 1.5 million kilometers. At this distance, Earth’s pull is stronger than the Sun’s.

Moon

Main articles: Moon, Lunar theory, and Orbit of the Moon

The Moon is Earth’s natural satellite and is quite large compared to the planet it orbits. It is about a quarter of Earth’s diameter. The Moon and Earth orbit a common point every 27.32 days. Because of the Moon’s distance from Earth, it appears almost the same size as the Sun in the sky, which allows for solar eclipses. The Moon’s gravity causes tides in Earth’s oceans. The Moon always shows the same face to Earth because its rotation matches its orbit. The Moon slowly moves away from Earth, getting about 38 millimeters farther each year.

Asteroids and artificial satellites

Main articles: Near-Earth object and Claimed moons of Earth

Earth has some asteroids that share its orbit, called quasi-satellites and trojans. There are also many human-made satellites orbiting Earth. As of September 2021, there were 4,550 active satellites and many more pieces of space debris. The International Space Station is the largest artificial satellite orbiting Earth.

Life on Earth

Main articles: Biosphere and History of life

Earth is the only place we know where life can exist. Life began in Earth's water about 4 billion years ago. Earth gives us liquid water, which helps living things grow and stay alive. Plants and animals take nutrients from water, soil, and air, and these nutrients move between different kinds of living things.

Life on Earth has changed the planet. It has created many different environments, called biomes, such as forests and oceans. These biomes have plants and animals that are similar to each other. Life has also changed Earth's air and surface over time. Humans have also changed Earth through things like farming and cities.

Origin of life and evolution

Main articles: Abiogenesis, Earliest known life forms, and History of life

Chemical reactions created the first living molecules a very long time ago. Later, all living things came from one common ancestor. Some living things learned to use the Sun's energy through photosynthesis, which filled the air with oxygen and created a protective layer high in the sky. Over time, simple cells joined together to form more complex ones, and then many cells worked together to create the many kinds of life we see today.

Challenges for life on Earth

Extreme weather, like big storms, can affect life in many places. Earth also has natural events such as earthquakes, volcanic eruptions, and floods. Human actions, like pollution and cutting down forests, have also changed Earth. These changes include warming the planet, which causes ice to melt and sea levels to rise.

Earth and humans

Main article: Human geography

See also: World

Humans came from earlier primates in Africa about 300,000 years ago. They have since spread all over Earth. With farming, they began to live on land. By the 1900s, humans had reached Antarctica, the last continent. The number of humans has grown fast since the 1800s. It reached eight billion in the 2020s, with more people expected in sub-Saharan Africa.

Most people live in south to east Asia, and 90% live in the Northern Hemisphere. Humans now live mostly in urban areas, with many in cities. Some humans have lived for short times in space stations and on the Moon.

Main articles: Natural resource and Land use

Earth gives us things we need. Non-renewable resources like fossil fuels take millions of years to form. Humans also use minerals, which can hurt the environment. Plants and animals give us food, wood, medicine, and clean air. Land and water are important for growing food and building homes.

Main articles: Human impact on the environment and Climate change

Humans have changed Earth's environment. Burning fossil fuels has added greenhouse gases, making Earth warmer, called global warming. This has made glaciers melt and raised sea levels. Scientists say there are limits to how much humans can change Earth without causing problems.

Different cultures have many ideas about Earth. Some see it as a mother or a god. Pictures of Earth from space, like Earthrise and The Blue Marble, have helped people see how beautiful and fragile Earth is. Over time, people have learned that Earth is a round planet, not flat, and that it is very old, billions of years old.