Earth

Earth is the third planet of the solar system. It is the only planet known to have life on it. The Earth formed around 4.5 billion years ago. It is one of four rocky planets on the inside of the Solar System. The other three are Mercury, Venus, and Mars.

The large mass of the Sun keeps the Earth in orbit, just as the mass of Earth keeps the moon moving around it. Earth also turns around in space, so that different parts face the Sun at different times. Earth goes around the Sun once (one year) for every 365​1⁄4 times it turns around (one day).

Earth is the only planet in the Solar System that has a large amount of liquid water on its surface. About 74% of the surface of Earth is covered by liquid or frozen water. Because of this, people sometimes call it the blue planet.

Because of its water, Earth is home to millions of species of plants and animals which require water to survive. The things that live on Earth have changed its surface greatly. For example, early cyanobacteria changed the air and gave it oxygen. The living part of Earth's surface is called the "biosphere".

Earth is rocky. It is the largest of the rocky planets moving around the Sun by mass and by size. It is much smaller than the gas giants such as Jupiter.

Overall, Earth is made of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%). The 1.2% left over is made of many different kinds of other chemicals. Chemicals that are very uncommon (such as gold and platinum) can be very valuable.

The structure of Earth changes from the inside to the outside. The center of earth (Earth's core) is mostly iron (88.8%), nickel (5.8%), sulfur (4.5%), and less than 1% other things. The Earth's crust is largely oxygen (47%). Oxygen is normally a gas but it can join with other chemicals to make compounds like water and rocks. 99.22% of rocks have oxygen in them. The most common oxygen-having rocks are silica (made with silicon), alumina (made with aluminium), rust (made with iron), lime (made with calcium), magnesia (made with magnesium), potash (made with potassium), and sodium oxide, and there are others as well.

Earth's shape is a spheroid: not quite a sphere because it is slightly squashed on the top and bottom. The shape is called an oblate spheroid. As Earth spins around itself, centrifugal force forces the equator out a little and pulls the poles in a little. The equator, around the middle of Earth's surface, is about 40,075 kilometers or 24,900 miles long.

The highest mountain above sea level—the well-known Mount Everest (which is 8,848 metres or 29,029 feet above sea level)—is not actually the one that is the farthest away from the center of the Earth. Instead, the sleeping volcano Mount Chimborazo in Ecuador is; it is only 6,263 metres or 20,548 feet above sea level but it is almost at the equator. Because of this, Mount Chimborazo is 6,384 kilometres or 3,967 miles from the center of the Earth, while Mount Everest is 2 kilometres or 1.2 miles closer to it. Similarly, the lowest point below sea level that we are conscious of is the Challenger Deep in the Mariana Trench in the Pacific Ocean. It is about 10,971 metres or 35,994 feet below sea level, but, again, there are probably places at the bottom of the Arctic Ocean that are nearer to the center of the Earth.

The deepest hole ever dug is only about 12.3 kilometers or 7.6 miles. We know something about the inside of the Earth, though, because we can learn things from earthquakes and the times when volcanoes erupt. We are able to see how quickly the shock waves move through Earth in different places.

The inside of Earth is very different from the outside. Almost all of Earth's liquid water is in the seas or close to the surface. The surface also has a lot of oxygen, which comes from plants. Small and simple kinds of life can live far under the surface, but animals and plants only live on the surface or in the seas. The rocks on the surface of Earth (Earth's crust) are well known. They are thicker where there is land, between 30 to 50 km or 19 to 31 mi thick. Under the seas they are sometimes only 6 km or 3.7 mi thick. There are three groups of rocks that make up most of the Earth's crust. Some rock is made when the hot liquid rock comes from inside the earth (igneous rocks); another type of rock is made when sediment is laid down, usually under the sea (sedimentary rocks); and a third kind of rock is made when the other two are changed by very high temperature or pressure (metamorphic rocks). A very few rocks also fall out of the sky (meteorites).

Below the crust is warm and almost-liquid rock that is always moving around (the Earth's mantle). Then, there is a thin liquid layer of heated rock (the outer core). This is very hot: 7,000 °C or 13,000 °F or 7,300 K. The middle of the inside of the Earth would be liquid as well but all the weight of the rock above it pushes it back into being solid. This solid middle part (the inner core) is almost all iron. This is what makes the Earth magnetic.

Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds). Because Earth's solar day is now slightly longer than it was during the 19th century due to tidal deceleration, each day varies between 0 and 2 ms longer than the mean solar day.

Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.0989 seconds of mean solar time (UT1), or 23h 56m 4.0989s. Earth's rotation period relative to the precessing or moving mean March equinox (when the Sun is at 90° on the equator), is 86,164.0905 seconds of mean solar time (UT1) (23h 56m 4.0905s). Thus the sidereal day is shorter than the stellar day by about 8.4 ms.

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or the Moon every two minutes; from Earth's surface, the apparent sizes of the Sun and the Moon are approximately the same.

Earth orbits the Sun at an average distance of about 150 million km (93 million mi) every 365.2564 mean solar days, or one sidereal year. This gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of Earth averages about 29.78 km/s (107,200 km/h; 66,600 mph), which is fast enough to travel a distance equal to Earth's diameter, about 12,742 km (7,918 mi), in seven minutes, and the distance to the Moon, 384,000 km (239,000 mi), in about 3.5 hours.

The Moon and Earth orbit a common barycenter every 27.32 days relative to the background stars. When combined with the Earth-Moon system's common orbit around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon, and their axial rotations are all counterclockwise. Viewed from a vantage point above the Sun and Earth's north poles, Earth orbits in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.44 degrees from the perpendicular to the Earth-Sun plane (the ecliptic), and the Earth-Moon plane is tilted up to ±5.1 degrees against the Earth-Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.

The Hill sphere, or the sphere of gravitational influence, of Earth is about 1.5 million km (930,000 mi) in radius. This is the maximum distance at which Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.[148] Earth, along with the Solar System, is situated in the Milky Way and orbits about 28,000 light-years from its center. It is about 20 light-years above the galactic plane in the Orion Arm.