The Earth

Earth (or the Earth) is the third from the sun, and the fifth-largest of the eight planets in the solar system. It is also the largest, most massive, and densest of the Solar System's four (or ) planets. It is sometimes referred to as the Blue Planet,or Terra.
Home to millions of life forms, including humans, Earth is the only place in the where life is known to exist. The planet formed ago, and on its surface within a billion years. Since then, Earth's has significantly altered and other conditions on the planet, enabling the proliferation of as well as the formation of the which, together with , blocks harmful radiation, permitting life on land. The physical properties of the Earth, as well as its geological history and orbit, allowed life to persist during this period. The world is expected to continue supporting life for another 1.5 billion years, after which the rising luminosity of the Sun will eliminate the biosphere.
Earth's is divided into several rigid segments, or , that gradually migrate across the surface over periods of . About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet's surface. Earth's interior remains active, with a thick layer of relatively solid , a liquid that generates a magnetic field, and a solid iron .
Earth interacts with other objects in , including the Sun and the . At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a , which is equal to 365.26 .The Earth's axis of rotation is 23.4° away from the to its , producing seasonal variations on the planet's surface with a period of one (365.24 solar days). Earth's only known , the Moon, which began orbiting it about 4.53 billion years ago, provides ocean , stabilizes the axial tilt and gradually slows the planet's rotation. Between approximately 4.1 and 3.8 billion years ago, impacts during the caused significant changes to the surface environment.
Both the resources of the planet, as well as the products of the biosphere, contribute resources that are used to support a global human population. The inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade and military action. Human cultures have developed many views of the planet, including personification as a deity, a belief in a or in , and a modern perspective of the world as an integrated environment that requires stewardship.
== Chronology ==
Scientists have been able to reconstruct detailed information about the planet's past. The earliest dated Solar System material is dated to 4.5672 ± 0.0006 billion years ago,and by 4.54 billion years ago (within an uncertainty of 1%)the Earth and the other planets in the Solar System formed out of the —a disk-shaped mass of dust and gas left over from the formation of the Sun. This assembly of the Earth through accretion was largely completed within 10–20 million years.Initially , the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed shortly thereafter, 4.53 billion years ago,most likely as the result of a Mars-sized object (sometimes called ) with about 10% of the Earth's mass impacting the Earth in a glancing blow. Some of this object's mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the Moon.
Outgassing and activity produced the primordial atmosphere. Condensing , augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects . The newly-formed Sun was only 70% of its present , yet evidence shows that the early oceans remained liquid—a contradiction dubbed the . A combination of and higher levels of served to raise the Earth's surface temperature, preventing the oceans from freezing over
Two major models have been proposed for the rate of continental growth:steady growth to the present-day and rapid growth early in Earth history. Current research shows that the second option is most likely, with rapid initial growth of continental crustfollowed by a long-term steady continental area.On lasting hundreds of millions of years, the surface continually reshaped itself as continents formed and broke up. The continents migrated across the surface, occasionally combining to form a . Roughly 750 million years ago (), one of the earliest known supercontinents, , began to break apart. The continents later recombined to form , 600–540 Ma, then finally , which broke apart 180 Ma.
=== Evolution of life ===
At present, Earth provides the only example of an environment that has given rise to the of life. Highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the existed.The development of allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed in a layer of (a form of [O]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the called . True multicellular organisms formed as cells within became increasingly specialized. Aided by the absorption of harmful by the , life colonized the surface of Earth.
Since the 1960s, it has been hypothesized that severe action between 750 and 580 Ma, during the , covered much of the planet in a sheet of ice. This hypothesis has been termed "", and is of particular interest because it preceded the , when multicellular life forms began to proliferate.
Following the Cambrian explosion, about 535 Ma, there have been five . The was 65 Ma, when a meteorite collision probably triggered the extinction of the (non-avian) and other large reptiles, but spared small animals such as , which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several million years ago, an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright. This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,affecting both the nature and quantity of other life forms.
The present pattern of began about 40 Ma and then intensified during the about 3 Ma. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last ice age ended 10,000 years ago.
=== Future ===
The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1  (1.1 billion years) and by 40% over the next 3.5 Gyr.Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the possible loss of the planet's oceans.
The Earth's increasing surface temperature will accelerate the , reducing its concentration to lethal levels for plants (10 for ) in an estimated 900 million years. The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years. After another billion years all surface water will have disappeared and the mean global temperature will reach 70 °C(158 °F). The Earth is expected to be effectively habitable for about another 500 million years, although this may be extended up to if the nitrogen is removed from the atmosphere.Even if the Sun were eternal and stable, the continued internal cooling of the Earth would result in a loss of much of its CO due to reduced , and 35% of the water in the oceans would descend to the due to reduced steam venting from mid-ocean ridges.
The Sun, as part of its , will become a in about 5 Gyr. Models predict that the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000 km). Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from the Sun when the star reaches it maximum radius. Therefore, the planet is expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life will be destroyed because of the Sun's increased luminosity.owever, a more recent simulation indicates that Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be destroyed.
== Composition and structure ==
Earth is a terrestrial planet, meaning that it is a rocky body, rather than a like . It is the largest of the four solar terrestrial planets, both in terms of size and mass. Of these four planets, Earth also has the highest density, the highest , the strongest magnetic field, and fastest rotation. It also is the only terrestrial planet with active 
=== Shape ===
The shape of the Earth is very close to that of an , a sphere squished along the orientation from pole to pole such that there is a around the . This bulge results from the of the Earth, and causes the diameter at the equator to be 43 km larger than the to pole diameter The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/, as the was originally defined as 1/10,000,000 of the distance from the equator to the through , France.
Local deviates from this idealized spheroid, though on a global scale, these deviations are very small: Earth has a of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in . The largest local deviations in the rocky surface of the Earth are (8,848 m above local sea level) and the (10,911 m below local sea level). Because of the equatorial bulge, the feature farthest from the center of the Earth is actually in .
Chemical Composition of the Crust
Compound 	Formula	Composition	Composition
Compound 	Formula	Continental	Oceanic
silica 	SiO 	60.2% 	48.6%
alumina 	AlO 	15.2% 	16.5%
lime 	CaO 	5.5% 	12.3%
magnesia 	MgO 	3.1% 	6.8%
iron(II) oxide 	FeO 	3.8% 	6.2%
sodium oxide 	NaO 	3.0% 	2.6%
potassium oxide 	KO 	2.8% 	0.4%
iron(III) oxide 	FeO 	2.5% 	2.3%
water 	HO 	1.4% 	1.1%
carbon dioxide 	CO 	1.2% 	1.4%
titanium dioxide 	TiO 	0.7% 	1.4%
phosphorus pentoxide	PO 	0.2% 	0.3%
Total 	Total 	99.6% 	99.9%
=== Chemical composition ===
The mass of the Earth is approximately 5.98 × 10 kg. It is composed mostly of (32.1%), oxygen (30.1%), (15.1%), (13.9%), (2.9%), (1.8%), (1.5%), and (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to , the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.
The geochemist calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.
=== Internal structure ===
The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their or physical () properties. The outer layer of the Earth is a chemically distinct solid , which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the , and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents. The crust and the cold, rigid, top of the are collectively known as the , and it is of the lithosphere that the are comprised. Beneath the lithosphere is the , a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 kilometers below the surface, spanning a that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid lies above a solid .The inner core may rotate at a slightly higher than the remainder of the planet, advancing by 0.1–0.5° per year.
Geologic layers of the Earth
Earth cutaway from core to exosphere. Not to scale.	Depth 	Component Layer 	Density
Earth cutaway from core to exosphere. Not to scale.	0–60 	Lithosphere 	—
Earth cutaway from core to exosphere. Not to scale.	0–35 	... Crust 	2.2–2.9
Earth cutaway from core to exosphere. Not to scale.	35–60 	... Upper mantle 	3.4–4.4
Earth cutaway from core to exosphere. Not to scale.	35–2890 	Mantle 	3.4–5.6
Earth cutaway from core to exosphere. Not to scale.	100–700 	... Asthenosphere	—
Earth cutaway from core to exosphere. Not to scale.	2890–5100	Outer core 	9.9–12.2
Earth cutaway from core to exosphere. Not to scale.	5100–6378	Inner core 	12.8–13.1
=== Heat ===
Earth's comes from a combination of (about 20%) and heat produced through (80%). The major heat-producing isotopes in the Earth are , , , and . At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 . Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. This extra heat production, twice present-day at approximately 3 billion years ago would have increased temperature gradients within the Earth, increasing the rates of and , and allowing the production of igneous rocks such as that are not formed today.
Present-day major heat-producing isotopes
Isotope	Heat release	Half-life 	Mean mantle concentration	Heat release
U 	9.46 × 10-5 	4.47 × 109 	30.8 × 10-9 	2.91 × 10-12
U 	5.69 × 10-4 	7.04 × 108 	0.22 × 10-9 	1.25 × 10-13
Th 	2.64 × 10-5 	1.40 × 1010	124 × 10-9 	3.27 × 10-12
K 	2.92 × 10-5 	1.25 × 109 	36.9 × 10-9 	1.08 × 10-12
Total heat loss from the earth is A portion of the core's thermal energy is transported toward the crust by ; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce and .More of the heat in the Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, majority of which occurs in the oceans due to the crust there being much thinner than that of the continents.
=== Tectonic plates ===
Earth's main plates[69]
 	
Plate name 	Area
African Plate[note 11]	78.0
Antarctic Plate 	60.9
Australian Plate 	47.2
Eurasian Plate 	67.8
North American Plate 	75.9
South American Plate 	43.6
Pacific Plate 	103.3
The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called . These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: , at which two plates come together, , at which two plates are pulled apart, and , in which two plates slide past one another laterally. , volcanic activity, , and formation can occur along these plate boundariesThe tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates,and their motion is strongly coupled with patterns convection inside the .
As the tectonic plates migrate across the planet, the ocean floor is under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates . The combination of these processes continually recycles the back into the mantle. Because of this recycling, most of the ocean floor is less than 100 million years in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 million years.By comparison, the oldest dated continental crust is 4030 million years old.
Other notable plates include the , the , the , the off the west coast of and the in the southern . The Australian Plate actually fused with Indian Plate between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the advancing at a rate of 75 mm/yrand the Pacific Plate moving 52–69 mm/yr. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr.
=== Surface ===
The Earth's varies greatly from place to place. About 70.8%of the surface is covered by water, with much of the below sea level. The submerged surface has mountainous features, including a globe-spanning system, as well as undersea ,, , and . The remaining 29.2% not covered by water consists of , , , , and other .
The planetary surface undergoes reshaping over geological time periods due to the effects of . The surface features built up or deformed through plate tectonics are subject to steady from , thermal cycles, and chemical effects. , , the build-up of , and large meteorite impactsalso act to reshape the landscape.The consists of lower density material such as the and . Less common is , a denser volcanic rock that is the primary constituent of the ocean floors is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust. The third form of rock material found on Earth is , which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include , the , , , and .Common carbonate minerals include (found in ), and .
The is the outermost layer of the Earth that is composed of and subject to . It exists at the interface of the , atmosphere, and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3 × 10 km² of cropland and 3.4 × 10 km² of pastureland.
The elevation of the land surface of the Earth varies from the low point of −418 m at the  to a 2005-estimated maximum altitude of 8,848 m at the top of . The mean height of land above sea level is 840 m.
=== Hydrosphere ===
The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the Solar System. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is of the in the with a depth of −10,911.4 m. The average depth of the oceans is 3,800 m, more than four times the average height of the continents.
The mass of the oceans is approximately 1.35 × 10 , or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386 × 10 km. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice
About 3.5% of the total mass of the oceans consists of . Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.Sea water has an important influence on the world's climate, with the oceans acting as a large . Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the .
=== Atmosphere ===
The on the surface of the Earth averages 101.325 , with a of about 8.5 km. It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the varies with , ranging between 8 km at the poles to 17 km at the equator, with some variation due to weather and seasonal factors.
Earth's biosphere has significantly altered its . evolved 2.7 billion years ago, the primarily nitrogen-oxygen that exists today. This change enabled the proliferation of as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks , permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small to burn up before they strike the surface, and moderating temperature. This last phenomenon is known as the : trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Carbon dioxide, water vapor, methane and ozone are the primary in the Earth's atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C and life would likely not exist.
==== Weather and climate ====
The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the . Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, ad is replaced by cooler, higher density air. The result is that drives the weather and climate through redistribution of heat energy.
The primary atmospheric circulation bands consist of the  in the equatorial region below 30° latitude and the in the mid-latitudes between 30° and 60°.Ocean currents are also important factors in determining climate, particularly the  that distributes heat energy from the equatorial oceans to the polar regions.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 settles to the surface as . Most of the water is then transported back to lower elevations by river systems, usually returning to the oceans or being deposited into . This 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. , topological features and temperature differences determine the average precipitation that falls in each region.
The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the (or equatorial), , and climates.Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly used system (as modified by 's student Rudolph Geiger) has five broad groups (humid tropics, , humid middle latitudes, and cold polar), which are further divided into more specific subtypes
==== Upper atmosphere ====
Above the troposphere, the atmosphere is usually divided into the , , and . Each of these layers has a different , defining the rate of change in temperature with height. Beyond these, the thins out into the . This is where the Earth's magnetic fields interact with the .An important part of the atmosphere for life on Earth is the ozone layer, a component of the stratosphere that partially shields the surface from ultraviolet light. The , defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.
Due to thermal energy, some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can from the planet's gravity. This results in a slow but steady . Because unfixed has a low molecular weight, it can achieve more readily and it leaks into outer space at a greater rate than other gasses The leakage of hydrogen into space is a contributing factor in pushing the Earth from an initially state to its current one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.Hence the ability of hydrogen to escape from the Earth's atmosphere may have influenced the nature of life that developed on the planetIn the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.
=== Magnetic field ===
The is shaped roughly as a , with the poles currently located proximate to the planet's geographic poles. According to , the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.
The field forms the , which deflects particles in the . The sunward edge of the is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the , a pair of concentric, -shaped regions of energetic . When the enters the Earth's atmosphere at the magnetic poles, it forms the .
== Orbit and rotation ==
=== Rotation ===
Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time. Each of these seconds is slightly longer than an second because Earth's solar day is now slightly longer than it was during the 19th century due to .
Earth's rotation period relative to the , called its stellar day by the (IERS), is of mean solar time (UT1), or Earth's rotation period relative to the or moving mean vernal , misnamed its sidereal day, is of mean solar time (UT1) . Thus the sidereal day is shorter than the stellar day by about 8.4 ms. The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005 and 1962–2005.
Apart from within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the Earth's sky is to the west at a rate of 15°/h = 15'/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes; the apparent sizes of the Sun and the Moon are approximately the same.
=== Orbit ===
Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one . From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, or a Sun or Moon diameter every 12 hours. Because of this motion, on average it takes 24 hours—a —for Earth to complete a full rotation about its axis so that the Sun returns to the . The orbital speed of the Earth averages about 30 km/s (108,000 km/h), which is fast enough to cover the planet's diameter (about 12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.
The Moon revolves with the Earth around a common every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common revolution around the Sun, the period of the , from new moon to new moon, is 29.53 days. Viewed from the , the motion of Earth, the Moon and their axial rotations are all . Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth appears to revolve in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's some 23.5 degrees from the perpendicular to the Earth–Sun plane, and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between and .
The , or sphere of influence, of the Earth is about 1.5 Gm (or 1,500,000 ) in radius. This is maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.Earth, along with the Solar System, is situated in the , orbiting about 28,000  from the center of the galaxy. It is currently about 20 light years above the galaxy's in the .
=== Axial tilt and seasons ===
Because of the axial tilt of the Earth, the amount of sunlight reaching any given point on the surface varies over the course of the year. This results in change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. Above the , an extreme case is reached where there is no daylight at all for part of the year—a . In the southern hemisphere the situation is exactly reversed, with the oriented opposite the direction of the North Pole.By astronomical convention, the four seasons are determined by the —the point in the orbit of maximum axial tilt toward or away from the Sun—and the , when the direction of the tilt and the direction to the Sun are perpendicular. Winter solstice occurs on about December 21, summer solstice is near June 21, spring equinox is around March 20 and autumnal equinox is about September 23.
The angle of the Earth's tilt is relatively stable over long periods of time. However, the tilt does undergo ; a slight, irregular motion with a main period of 18.6 years. The orientation (rather than the angle) of the Earth's axis also changes over time, around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a . Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth's equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This has multiple, cyclical components, which collectively are termed . In addition to an annual component to this motion, there is a 14-month cycle called the . The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.
In modern times, Earth's occurs around January 3, and the around July 4. However, these dates change over time due to and other orbital factors, which follow cyclical patterns known as . The changing Earth-Sun distance results in an increase of about 6.9% in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach
Category:Locations