Earth

Earth is the third planet from the Sun and is the largest of the four terrestrial planets. The Earth is the only planet in our solar system not to be named after a Greek or Roman deity. The Earth was formed approximately 4.54 billion years ago and is the only known planet to support life.

About 29.2% of Earth's surface is land consisting of continents and islands while the remaining 70.8% is covered with water, mostly by oceans, seas, gulfs, and other salt-water bodies, but also by lakes, rivers, and other fresh water, which together constitute the hydrosphere. Much of Earth's polar regions are covered in ice. Earth's outer layer is divided into several rigid tectonic plates that migrate across the surface over many millions of years, while its interior remains active with a solid iron inner core, a liquid outer core that generates Earth's magnetic field, and a convective mantle that drives plate tectonics. The Earth has a moon named The Moon, its distance is 384,000 km and the size is 3,474 km.

Earth's atmosphere consists mostly of nitrogen and oxygen. More solar energy is received by equatorial regions than polar regions, and is redistributed by atmospheric and ocean circulation. Greenhouse gases also play an important role in regulating the surface temperature. See Earth.

Size and orbit
The shape of Earth is nearly spherical. With a size of 12,742 kilometres, it is the largest terrestrial planet in the Solar System. Therefore, the orbital period of Earth is 365.25 days (or commonly referred as 1 Earth year) and rotational period is 23 hours 58 minutes 4 seconds (not exactly 24 hours). The mass of Earth is 5.98 x 10^24 kg, according to internal structure of Earth.[1]

Structure
The earth is made up of three different layers: the crust, the mantle, the outer core and the inner core. Earth's crust is a thin shell on the outside of Earth, accounting for less than 1% of Earth's volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The mantle is a layer of silicate rock between the crust and the outer core, and thus makes up 67% of the mass of Earth. It has a thickness of 2,900 kilometres (1,800 mi) making up about 84% of Earth's volume. Earth's outer core is a fluid layer about 2,400 km (1,500 mi) thick and composed of mostly liquid iron and nickel that lies above Earth's solid inner core and below its mantle. Earth's inner core is the innermost geologic layer of the planet Earth, along with the hottest, and densest layer of solid ball of iron. The temperature in the inner core is about 5,200° Celsius (9,392° Fahrenheit), and is far above the melting point of iron and hotter than Sun's surface temperature.

Gravity and magnetic field
The gravity of Earth is the acceleration that is imparted to objects due to the distribution of mass within Earth. Near Earth's surface, gravitational acceleration is approximately 9.8 m/s2 (32 ft/s2). Local differences in topography, geology, and deeper tectonic structure cause local and broad, regional differences in Earth's gravitational field, known as gravity anomalies. Furthermore,The main part of Earth's magnetic field is generated in the core, the site of a dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The reversal occured 730,000 years ago.

Atmosphere
Main article: Earth's atmosphere

The atmospheric pressure at Earth's sea level averages 101.325 kPa (14.696 psi), with a scale height of about 8.5 km (5.3 mi). Earth's atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, and 0.1 percent other gases. Trace amounts of carbon dioxide, methane, water vapor, and neon are some of the other gases that make up the remaining 0.1 percent. The atmosphere is divided into five different layers, based on temperature.

Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 Gya, forming the primarily nitrogen–oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent conversion of atmospheric O2 to O3. The ozone layer blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature. This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Water vapor, carbon dioxide, methane, nitrous oxide, and ozone are the primary greenhouse gases in the atmosphere.

Weather and climate
See also: Sky

Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km (6.8 mi) of the surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower-density air then rises and is replaced by cooler, higher-density air. The result is atmospheric circulation that drives the weather and climate through redistribution of thermal energy. The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°. Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions. The amount of solar energy reaching the Earth's surface decreases with increasing latitude. At higher latitudes, the sunlight reaches the surface at lower angles, and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C (0.7 °F) per degree of latitude from the equator. Earth's surface can be subdivided into specific latitudinal belts of the climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.

Further factors that affect a location's climates are its proximity to oceans, the oceanic and atmospheric circulation, and topology. Places close to oceans typically have colder summers and warmer winters, due to the fact that oceans can store large amounts of heat. The wind transports the cold or the heat of the ocean to the land. Atmospheric circulation also plays an important role:NULL. Finally, temperatures decrease with height causing mountainous areas to be colder than low-lying areas. Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and falls to the surface as precipitation. Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This water cycle is a vital mechanism for supporting life on land and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topographic features, and temperature differences determine the average precipitation that falls in each region.

The commonly used Köppen climate classification system has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes. The Köppen system rates regions based on observed temperature and precipitation. Surface air temperature can rise to around 55 °C (131 °F) in hot deserts, such as Death Valley, and can fall as low as −89 °C (−128 °F) in Antarctica.

Moon
The Moon is a relatively large, terrestrial, planet-like natural satellite, with a diameter of 3,475 km. It is the largest moon in the Solar System relative to the size of its planet, and with a average distance from earth in 384,000 km, although Charon is larger relative to the dwarf planet Pluto. The natural satellites of other planets are also referred to as "moons", after Earth's. The most widely accepted theory of the Moon's origin, the giant-impact hypothesis, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements and the fact that its composition is nearly identical to that of Earth's crust.

The gravitational attraction between Earth and the Moon causes tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases. Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm/a (1.5 in/year). Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 3.45 cm/yr, add up to significant changes.