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, with liquid surface water covering most of its surface. The water makes up about 70.8% of Earth's crust, while the remaining part is land. Earth's surface is shaped by slowly moving tectonic plates, which create mountains, volcanoes, and sometimes cause earthquakes.

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

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

Life on Earth began very early in the planet's history, and humans have lived here for about 300,000 years. We rely on Earth's biosphere and natural resources, but our actions are changing the planet in ways that can harm both people and nature.

Etymology

The word Earth comes from old languages. In Old English, it was written as eorðe, similar to words in other Germanic languages. Early people often thought of Earth as a goddess, like the Norse mother of Thor.

Over time, people started writing Earth with a capital letter when talking about our planet, especially when mentioning it with other planets. Some still use the lowercase earth. In science and stories, you might also see the name Terra, especially 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. During its early time, Earth had no air or water, but volcanic activity brought out gases and water vapor that 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 has a rounded shape, 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 at the middle because of its spin.

Inside Earth, there are several layers. The outer layer is a solid crust, which is a bit thinner under the oceans and thicker under the continents. Below the crust is a very 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 comes from when the planet was formed and from radioactive elements, which helps drive movements in the mantle and create volcanoes and earthquakes.

Earth also has a magnetic field, generated deep inside the planet. This field stretches out into space and protects us from harmful solar winds. It changes shape slowly over time and can even flip direction, though this happens very rarely.

Surface environment

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

Earth's surface is where the atmosphere meets the solid Earth and its oceans. It covers about 510 million square kilometers. Earth is divided into two hemispheres: the Northern and Southern by latitude, and the Eastern and Western by longitude.

Most of Earth's surface is covered by ocean water—about 70.8%. This vast ocean makes Earth a water world. The ocean is divided into five main parts: the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. The remaining land area makes up about 29.2% of Earth's surface. This land includes continents like Africa-Eurasia, America, Antarctica, and Australia, along with many islands.

Earth's land and ocean floors form the top layer of Earth's crust. The land has many different features, such as mountains, deserts, and plains. The ocean floor also has varied terrain, including deep trenches and underwater mountains.

Tectonic plates

Main article: Plate tectonics

Earth's outer layer, the lithosphere, is divided into tectonic plates. These plates move relative to each other. At convergent boundaries, plates come together; at divergent boundaries, they move apart; and at transform boundaries, they slide past each other. These movements can cause earthquakes and volcanic activity.

Hydrosphere

Main article: Hydrosphere

Further information: Ocean, Sea, and Cryosphere

Earth's hydrosphere includes all of its water. Most of this water is in the global ocean, which covers about 361.8 million square kilometers. The ocean's average depth is about 3,682 meters. About 97.5% of Earth's water is salty, while the rest is fresh water, mostly locked in ice caps and glaciers.

Atmosphere

Main article: Atmosphere of Earth

Further information: Peplosphere

Earth's atmosphere has an average pressure of about 101.325 kPa at sea level. It is made mostly of nitrogen (78.084%) and oxygen (20.946%), with small amounts of other gases. Water vapor makes up between 0.01% and 4% of the atmosphere.

The atmosphere has several layers. The troposphere is the lowest layer, where most weather occurs. Above it are the stratosphere, mesosphere, and thermosphere. The upper atmosphere gradually thins out into space. The ozone layer in the stratosphere protects Earth from harmful ultraviolet light.

Weather and climate

Main articles: Weather and Climate

Earth's weather and climate are influenced by the Sun's energy, the planet's rotation, and ocean currents. The atmosphere redistributes heat, creating weather patterns. The climate varies by latitude, with tropical, subtropical, temperate, and polar regions. Proximity to oceans, atmospheric circulation, and topography also affect local climates.

The water cycle is important for life on Earth. Water evaporates from surfaces, rises, cools, and falls as precipitation. This cycle shapes landscapes and supports ecosystems. Climate classifications, like the Köppen system, help describe the world's different climate zones based on temperature and precipitation patterns.

Orbit and rotation

Main article: Axial tilt § Earth

Earth spins around once every 24 hours, which is called a day. It also moves around the Sun, which takes about 365 days and is called a year. Because Earth is tilted on its axis, different parts get more sunlight at different times, causing seasons like summer and winter. When one half of Earth gets more sunlight, it experiences summer, while the other half has winter. This tilt also leads to longer days in summer and shorter days 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, which is the distance where 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 about four billion years 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 originated from earlier primates in Eastern Africa around 300,000 years ago and have since spread across Earth. With the development of agriculture, they began settling on land. By the 20th century, humans had explored and settled Antarctica, the last continent. The human population has grown quickly since the 19th century, reaching eight billion in the 2020s, with most growth expected in sub-Saharan Africa.

Most people live in south to eastern Asia, and 90% live in the Northern Hemisphere. Humans have moved increasingly to urban areas, with most living in cities by the 21st century. Beyond Earth, some humans have lived temporarily in space stations and on the Moon.

Main articles: Natural resource and Land use

Earth provides resources that humans use. Non-renewable resources like fossil fuels take millions of years to form. Humans also use minerals from the ground, which can harm the environment. Earth's 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

Human actions have changed Earth's environment. Burning fossil fuels has increased greenhouse gases, leading to warmer temperatures, known as global warming. This has caused glaciers to melt and sea levels to rise. Scientists have identified limits to how much humans can change Earth without causing harm.

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 Earth's beauty and fragility. Over time, people have learned that Earth is a round planet, not flat, and that it is very old, billions of years old.