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115-399: The true horizon is a theoretical line, which can only be observed to any degree of accuracy when it lies along a relatively smooth surface such as that of Earth's oceans . At many locations, this line is obscured by terrain , and on Earth it can also be obscured by life forms such as trees and/or human constructs such as buildings. The resulting intersection of such obstructions with the sky

230-410: A halocline . If a zone undergoes a strong, vertical chemistry gradient with depth, it contains a chemocline . Temperature and salinity control ocean water density. Colder and saltier water is denser, and this density plays a crucial role in regulating the global water circulation within the ocean. The halocline often coincides with the thermocline, and the combination produces a pronounced pycnocline ,

345-489: A mid-ocean ridge , which creates a long mountain range beneath the ocean. Together they form the global mid-oceanic ridge system that features the longest mountain range in the world. The longest continuous mountain range is 65,000 km (40,000 mi). This underwater mountain range is several times longer than the longest continental mountain range – the Andes . Oceanographers state that less than 20% of

460-489: A parallel of latitude is given by p = N cos ⁡ ( φ ) {\displaystyle p=N\cos(\varphi )} . The Earth's meridional radius of curvature at the equator equals the meridian's semi-latus rectum : The Earth's prime-vertical radius of curvature at the equator equals the equatorial radius, N e = a . The Earth's polar radius of curvature (either meridional or prime-vertical) is: The principal curvatures are

575-497: A boat ( h B =1.7   m) can just see the tops of trees on a nearby shore ( h L =10   m), the trees are probably about D BL =16 km away. Referring to the figure at the right, and using the approximation above , the top of the lighthouse will be visible to a lookout in a crow's nest at the top of a mast of the boat if where D BL is in kilometres and h B and h L are in metres. As another example, suppose an observer, whose eyes are two metres above

690-412: A boundary between less dense surface water and dense deep water. Earth radius Earth radius (denoted as R 🜨 or R E ) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid (an oblate ellipsoid ), the radius ranges from a maximum ( equatorial radius , denoted a ) of nearly 6,378 km (3,963 mi) to

805-475: A constant terrestrial radius; if the choice between equatorial or polar radii is not explicit, the equatorial radius is to be assumed, as recommended by the International Astronomical Union (IAU). Earth's rotation , internal density variations, and external tidal forces cause its shape to deviate systematically from a perfect sphere. Local topography increases the variance, resulting in

920-447: A distant object is visible above the horizon. Suppose an observer's eye is 10 metres above sea level, and he is watching a ship that is 20 km away. His horizon is: kilometres from him, which comes to about 11.3 kilometres away. The ship is a further 8.7 km away. The height of a point on the ship that is just visible to the observer is given by: which comes to almost exactly six metres. The observer can therefore see that part of

1035-579: A gentle breeze on a pond causes ripples to form. A stronger gust blowing over the ocean causes larger waves as the moving air pushes against the raised ridges of water. The waves reach their maximum height when the rate at which they are travelling nearly matches the speed of the wind. In open water, when the wind blows continuously as happens in the Southern Hemisphere in the Roaring Forties , long, organized masses of water called swell roll across

1150-414: A meter over a nearly 12-hour period (see Earth tide ). Given local and transient influences on surface height, the values defined below are based on a "general purpose" model, refined as globally precisely as possible within 5 m (16 ft) of reference ellipsoid height, and to within 100 m (330 ft) of mean sea level (neglecting geoid height). Additionally, the radius can be estimated from

1265-400: A minimum ( polar radius , denoted b ) of nearly 6,357 km (3,950 mi). A globally-average value is usually considered to be 6,371 kilometres (3,959 mi) with a 0.3% variability (±10 km) for the following reasons. The International Union of Geodesy and Geophysics (IUGG) provides three reference values: the mean radius ( R 1 ) of three radii measured at two equator points and

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1380-451: A more precise value for its polar radius is needed. The geocentric radius is the distance from the Earth's center to a point on the spheroid surface at geodetic latitude φ , given by the formula: where a and b are, respectively, the equatorial radius and the polar radius. The extrema geocentric radii on the ellipsoid coincide with the equatorial and polar radii. They are vertices of

1495-449: A pole; the authalic radius , which is the radius of a sphere with the same surface area ( R 2 ); and the volumetric radius , which is the radius of a sphere having the same volume as the ellipsoid ( R 3 ). All three values are about 6,371 kilometres (3,959 mi). Other ways to define and measure the Earth's radius involve either the spheroid's radius of curvature or the actual topography . A few definitions yield values outside

1610-403: A result, the photic zone is the most biodiverse and the source of the food supply which sustains most of the ocean ecosystem . Ocean photosynthesis also produces half of the oxygen in the Earth's atmosphere. Light can only penetrate a few hundred more meters; the rest of the deeper ocean is cold and dark (these zones are called mesopelagic and aphotic zones). The continental shelf is where

1725-416: A right triangle, with the sum of the radius and the height as the hypotenuse. With referring to the second figure at the right leads to the following: The exact formula above can be expanded as: where R is the radius of the Earth ( R and h must be in the same units). For example, if a satellite is at a height of 2000 km, the distance to the horizon is 5,430 kilometres (3,370 mi); neglecting

1840-496: A shallow area and this, coupled with a low pressure system, can raise the surface of the ocean dramatically above a typical high tide. The average depth of the oceans is about 4 km. More precisely the average depth is 3,688 meters (12,100 ft). Nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep. "Deep ocean," which is anything below 200 meters (660 ft), covers about 66% of Earth's surface. This figure does not include seas not connected to

1955-466: A sphere in many ways. This section describes the common ways. The various radii derived here use the notation and dimensions noted above for the Earth as derived from the WGS-84 ellipsoid; namely, A sphere being a gross approximation of the spheroid, which itself is an approximation of the geoid, units are given here in kilometers rather than the millimeter resolution appropriate for geodesy. In geophysics,

2070-540: A sphere. The word horizon derives from the Greek ὁρίζων κύκλος ( horízōn kýklos ) 'separating circle', where ὁρίζων is from the verb ὁρίζω ( horízō ) 'to divide, to separate', which in turn derives from ὅρος ( hóros ) 'boundary, landmark'. Historically, the distance to the visible horizon has long been vital to survival and successful navigation, especially at sea, because it determined an observer's maximum range of vision and thus of communication , with all

2185-445: A spherical model as "the radius of the earth" . When considering the Earth's real surface, on the other hand, it is uncommon to refer to a "radius", since there is generally no practical need. Rather, elevation above or below sea level is useful. Regardless of the model, any of these geocentric radii falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km (3,950 to 3,963 mi). Hence,

2300-424: A surface of profound complexity. Our descriptions of Earth's surface must be simpler than reality in order to be tractable. Hence, we create models to approximate characteristics of Earth's surface, generally relying on the simplest model that suits the need. Each of the models in common use involve some notion of the geometric radius . Strictly speaking, spheres are the only solids to have radii, but broader uses of

2415-403: A wave-cut platform develops at the foot of the cliff and this has a protective effect, reducing further wave-erosion. Material worn from the margins of the land eventually ends up in the sea. Here it is subject to attrition as currents flowing parallel to the coast scour out channels and transport sand and pebbles away from their place of origin. Sediment carried to the sea by rivers settles on

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2530-406: A zone undergoes dramatic changes in temperature with depth, it contains a thermocline , a distinct boundary between warmer surface water and colder deep water. In tropical regions, the thermocline is typically deeper compared to higher latitudes. Unlike polar waters , where solar energy input is limited, temperature stratification is less pronounced, and a distinct thermocline is often absent. This

2645-439: Is a bay , a small bay with a narrow inlet is a cove and a large bay may be referred to as a gulf . Coastlines are influenced by several factors including the strength of the waves arriving on the shore, the gradient of the land margin, the composition and hardness of the coastal rock, the inclination of the off-shore slope and the changes of the level of the land due to local uplift or submergence. Normally, waves roll towards

2760-738: Is about −2 °C (28 °F). In all parts of the ocean, deep ocean temperatures range between −2 °C (28 °F) and 5 °C (41 °F). Constant circulation of water in the ocean creates ocean currents . Those currents are caused by forces operating on the water, such as temperature and salinity differences, atmospheric circulation (wind), and the Coriolis effect . Tides create tidal currents, while wind and waves cause surface currents. The Gulf Stream , Kuroshio Current , Agulhas Current and Antarctic Circumpolar Current are all major ocean currents. Such currents transport massive amounts of water, gases, pollutants and heat to different parts of

2875-444: Is an important reference point for oceanography and geography, particularly as mean sea level . The ocean surface has globally little, but measurable topography , depending on the ocean's volumes. The ocean surface is a crucial interface for oceanic and atmospheric processes. Allowing interchange of particles, enriching the air and water, as well as grounds by some particles becoming sediments . This interchange has fertilized life in

2990-409: Is called the visible horizon . On Earth, when looking at a sea from a shore, the part of the sea closest to the horizon is called the offing . The true horizon surrounds the observer and it is typically assumed to be a circle, drawn on the surface of a perfectly spherical model of the relevant celestial body, i.e., a small circle of the local osculating sphere . With respect to Earth, the center of

3105-411: Is customarily divided into five principal oceans – listed below in descending order of area and volume: The ocean fills Earth's oceanic basins . Earth's oceanic basins cover different geologic provinces of Earth's oceanic crust as well as continental crust . As such it covers mainly Earth's structural basins , but also continental shelfs . In mid-ocean, magma is constantly being thrust through

3220-399: Is due to the fact that surface waters in polar latitudes are nearly as cold as deeper waters. Below the thermocline, water everywhere in the ocean is very cold, ranging from −1 °C to 3 °C. Because this deep and cold layer contains the bulk of ocean water, the average temperature of the world ocean is 3.9 °C. If a zone undergoes dramatic changes in salinity with depth, it contains

3335-452: Is irregular, unevenly dominating the Earth's surface . This leads to the distinction of the Earth's surface into a water and land hemisphere , as well as the division of the ocean into different oceans. Seawater covers about 361,000,000 km (139,000,000 sq mi) and the ocean's furthest pole of inaccessibility , known as " Point Nemo ", in a region known as spacecraft cemetery of

3450-423: Is more directly comparable to the geographical distance on a map. It can be formulated in terms of γ in radians , then Solving for s gives The distance s can also be expressed in terms of the line-of-sight distance d ; from the second figure at the right, substituting for γ and rearranging gives The distances d and s are nearly the same when the height of the object is negligible compared to

3565-555: Is possible to combine the principal radii of curvature above in a non-directional manner. The Earth's Gaussian radius of curvature at latitude φ is: Where K is the Gaussian curvature , K = κ 1 κ 2 = det B det A {\displaystyle K=\kappa _{1}\,\kappa _{2}={\frac {\det \,B}{\det \,A}}} . The Earth's mean radius of curvature at latitude φ is: The Earth can be modeled as

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3680-454: Is produced and magma is forced up creating underwater mountains, some of which may form chains of volcanic islands near to deep trenches. Near some of the boundaries between the land and sea, the slightly denser oceanic plates slide beneath the continental plates and more subduction trenches are formed. As they grate together, the continental plates are deformed and buckle causing mountain building and seismic activity. Every ocean basin has

3795-407: Is pushed across the surface of the ocean by the wind, but this represents a transfer of energy and not horizontal movement of water. As waves approach land and move into shallow water , they change their behavior. If approaching at an angle, waves may bend ( refraction ) or wrap around rocks and headlands ( diffraction ). When the wave reaches a point where its deepest oscillations of the water contact

3910-423: Is reflected back out of the water. Red light is most easily absorbed and thus does not reach great depths, usually to less than 50 meters (164 ft). Blue light, in comparison, can penetrate up to 200 meters (656 ft). Second, water molecules and very tiny particles in ocean water preferentially scatter blue light more than light of other colors. Blue light scattering by water and tiny particles happens even in

4025-467: Is sometimes referred to as the World Ocean, global ocean or the great ocean . The concept of a continuous body of water with relatively unrestricted exchange between its components is critical in oceanography . The word ocean comes from the figure in classical antiquity , Oceanus ( / oʊ ˈ s iː ə n ə s / ; Ancient Greek : Ὠκεανός Ōkeanós , pronounced [ɔːkeanós] ),

4140-403: Is the angular dip of the horizon. It is related to the horizon zenith angle z {\displaystyle z} by: For a non-negative height h {\displaystyle h} , the angle z {\displaystyle z} is always ≥ 90°. To compute the greatest distance D BL at which an observer B can see the top of an object L above the horizon, simply add

4255-405: Is the primary component of Earth's hydrosphere and is thereby essential to life on Earth. The ocean influences climate and weather patterns, the carbon cycle , and the water cycle by acting as a huge heat reservoir . Ocean scientists split the ocean into vertical and horizontal zones based on physical and biological conditions. The pelagic zone is the open ocean's water column from

4370-681: The Global Positioning System gained importance, true global models were developed which, while not as accurate for regional work, best approximate the Earth as a whole. The following radii are derived from the World Geodetic System 1984 ( WGS-84 ) reference ellipsoid . It is an idealized surface, and the Earth measurements used to calculate it have an uncertainty of ±2 m in both the equatorial and polar dimensions. Additional discrepancies caused by topographical variation at specific locations can be significant. When identifying

4485-670: The North Sea or the Red Sea . There is no sharp distinction between seas and oceans, though generally seas are smaller, and are often partly (as marginal seas ) or wholly (as inland seas ) bordered by land. The contemporary concept of the World Ocean was coined in the early 20th century by the Russian oceanographer Yuly Shokalsky to refer to the continuous ocean that covers and encircles most of Earth. The global, interconnected body of salt water

4600-472: The South Pacific Ocean , at 48°52.6′S 123°23.6′W  /  48.8767°S 123.3933°W  / -48.8767; -123.3933  ( Point Nemo ) . This point is roughly 2,688 km (1,670 mi) from the nearest land. There are different customs to subdivide the ocean and are adjourned by smaller bodies of water such as, seas , gulfs , bays , bights , and straits . The ocean

4715-578: The Thames Barrier is designed to protect London from a storm surge, while the failure of the dykes and levees around New Orleans during Hurricane Katrina created a humanitarian crisis in the United States. Most of the ocean is blue in color, but in some places the ocean is blue-green, green, or even yellow to brown. Blue ocean color is a result of several factors. First, water preferentially absorbs red light, which means that blue light remains and

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4830-406: The coastline and structure of the world ocean. A global ocean has existed in one form or another on Earth for eons. Since its formation the ocean has taken many conditions and shapes with many past ocean divisions and potentially at times covering the whole globe. During colder climatic periods, more ice caps and glaciers form, and enough of the global water supply accumulates as ice to lessen

4945-403: The equatorial radius a is larger than the polar radius b by approximately aq . The oblateness constant q is given by where ω is the angular frequency , G is the gravitational constant , and M is the mass of the planet. For the Earth ⁠ 1 / q ⁠ ≈ 289 , which is close to the measured inverse flattening ⁠ 1 / f ⁠ ≈ 298.257 . Additionally,

5060-591: The ocean floor , they begin to slow down. This pulls the crests closer together and increases the waves' height , which is called wave shoaling . When the ratio of the wave's height to the water depth increases above a certain limit, it " breaks ", toppling over in a mass of foaming water. This rushes in a sheet up the beach before retreating into the ocean under the influence of gravity. Earthquakes , volcanic eruptions or other major geological disturbances can set off waves that can lead to tsunamis in coastal areas which can be very dangerous. The ocean's surface

5175-616: The second fundamental form for a surface (Equation (123) in ): e, f, and g are elements of the shape tensor: n = N | N | {\displaystyle n={\frac {N}{|N|}}} is the unit normal to the surface at r {\displaystyle r} , and because ∂ r ∂ φ {\displaystyle {\frac {\partial r}{\partial \varphi }}} and ∂ r ∂ λ {\displaystyle {\frac {\partial r}{\partial \lambda }}} are tangents to

5290-476: The Earth deviates from a perfect sphere by only a third of a percent, which supports the spherical model in most contexts and justifies the term "radius of the Earth". While specific values differ, the concepts in this article generalize to any major planet . Rotation of a planet causes it to approximate an oblate ellipsoid /spheroid with a bulge at the equator and flattening at the North and South Poles , so that

5405-442: The Earth's biosphere . Oceanic evaporation , as a phase of the water cycle, is the source of most rainfall (about 90%), causing a global cloud cover of 67% and a consistent oceanic cloud cover of 72%. Ocean temperatures affect climate and wind patterns that affect life on land. One of the most dramatic forms of weather occurs over the oceans: tropical cyclones (also called "typhoons" and "hurricanes" depending upon where

5520-483: The Earth's surface for hundreds of kilometres. Opposite conditions occur, for example, in deserts, where the surface is very hot, so hot, low-density air is below cooler air. This causes light to be refracted upward, causing mirage effects that make the concept of the horizon somewhat meaningless. Calculated values for the effects of refraction under unusual conditions are therefore only approximate. Nevertheless, attempts have been made to calculate them more accurately than

5635-538: The Earth's surface is about where h is height above sea level and R is the Earth radius . The expression can be simplified as: where the constant equals k = 3.57 km/m = 1.22 mi/ft . In this equation, Earth's surface is assumed to be perfectly spherical, with R equal to about 6,371 kilometres (3,959 mi). Assuming no atmospheric refraction and a spherical Earth with radius R=6,371 kilometres (3,959 mi): On terrestrial planets and other solid celestial bodies with negligible atmospheric effects,

5750-452: The Moon are 20x stronger than the Moon's tidal forces on the Earth.) The primary effect of lunar tidal forces is to bulge Earth matter towards the near and far sides of the Earth, relative to the moon. The "perpendicular" sides, from which the Moon appears in line with the local horizon, experience "tidal troughs". Since it takes nearly 25 hours for the Earth to rotate under the Moon (accounting for

5865-403: The Moon's 28 day orbit around Earth), tides thus cycle over a course of 12.5 hours. However, the rocky continents pose obstacles for the tidal bulges, so the timing of tidal maxima may not actually align with the Moon in most localities on Earth, as the oceans are forced to "dodge" the continents. Timing and magnitude of tides vary widely across the Earth as a result of the continents. Thus, knowing

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5980-468: The Moon's gravity, oceanic tides are also substantially modulated by the Sun's tidal forces, by the rotation of the Earth, and by the shape of the rocky continents blocking oceanic water flow. (Tidal forces vary more with distance than the "base" force of gravity: the Moon's tidal forces on Earth are more than double the Sun's, despite the latter's much stronger gravitational force on Earth. Earth's tidal forces upon

6095-400: The Moon's position does not allow a local to predict tide timings, instead requiring precomputed tide tables which account for the continents and the Sun, among others. During each tidal cycle, at any given place the tidal waters rise to maximum height, high tide, before ebbing away again to the minimum level, low tide. As the water recedes, it gradually reveals the foreshore , also known as

6210-643: The Okeanos is represented with a dragon-tail on some early Greek vases. Scientists believe that a sizable quantity of water would have been in the material that formed Earth. Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation. This is called atmospheric escape . During planetary formation , Earth possibly had magma oceans . Subsequently, outgassing , volcanic activity and meteorite impacts , produced an early atmosphere of carbon dioxide , nitrogen and water vapor , according to current theories. The gases and

6325-706: The World Ocean, such as the Caspian Sea . The deepest region of the ocean is at the Mariana Trench , located in the Pacific Ocean near the Northern Mariana Islands . The maximum depth has been estimated to be 10,971 meters (35,994 ft). The British naval vessel Challenger II surveyed the trench in 1951 and named the deepest part of the trench the " Challenger Deep ". In 1960, the Trieste successfully reached

6440-433: The air above it, a cold, dense layer of air forms close to the surface, causing light to be refracted downward as it travels, and therefore, to some extent, to go around the curvature of the Earth. The reverse happens if the ground is hotter than the air above it, as often happens in deserts, producing mirages . As an approximate compensation for refraction, surveyors measuring distances longer than 100 meters subtract 14% from

6555-408: The aircraft. Pilots can also retain their spatial orientation by referring to the horizon. In many contexts, especially perspective drawing , the curvature of the Earth is disregarded and the horizon is considered the theoretical line to which points on any horizontal plane converge (when projected onto the picture plane) as their distance from the observer increases. For observers near sea level,

6670-421: The amounts in other parts of the water cycle. The reverse is true during warm periods. During the last ice age, glaciers covered almost one-third of Earth's land mass with the result being that the oceans were about 122 m (400 ft) lower than today. During the last global "warm spell," about 125,000 years ago, the seas were about 5.5 m (18 ft) higher than they are now. About three million years ago

6785-464: The appearance of the horizon. Usually, the density of the air just above the surface of the Earth is greater than its density at greater altitudes. This makes its refractive index greater near the surface than at higher altitudes, which causes light that is travelling roughly horizontally to be refracted downward. This makes the actual distance to the horizon greater than the distance calculated with geometrical formulas. With standard atmospheric conditions,

6900-479: The atmosphere are thought to have accumulated over millions of years. After Earth's surface had significantly cooled, the water vapor over time would have condensed, forming Earth's first oceans. The early oceans might have been significantly hotter than today and appeared green due to high iron content. Geological evidence helps constrain the time frame for liquid water existing on Earth. A sample of pillow basalt (a type of rock formed during an underwater eruption)

7015-407: The bottom of the trench, manned by a crew of two men. Oceanographers classify the ocean into vertical and horizontal zones based on physical and biological conditions. The pelagic zone consists of the water column of the open ocean, and can be divided into further regions categorized by light abundance and by depth. The ocean zones can be grouped by light penetration into (from top to bottom):

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7130-408: The bulge at the equator shows slow variations. The bulge had been decreasing, but since 1998 the bulge has increased, possibly due to redistribution of ocean mass via currents. The variation in density and crustal thickness causes gravity to vary across the surface and in time, so that the mean sea level differs from the ellipsoid. This difference is the geoid height , positive above or outside

7245-444: The calculated curvature error and ensure lines of sight are at least 1.5 metres from the ground, to reduce random errors created by refraction. If the Earth were an airless world like the Moon, the above calculations would be accurate. However, Earth has an atmosphere of air , whose density and refractive index vary considerably depending on the temperature and pressure. This makes the air refract light to varying extents, affecting

7360-403: The curvature of the Earth at a point. Like a torus , the curvature at a point will be greatest (tightest) in one direction (north–south on Earth) and smallest (flattest) perpendicularly (east–west). The corresponding radius of curvature depends on the location and direction of measurement from that point. A consequence is that a distance to the true horizon at the equator is slightly shorter in

7475-422: The difference between this geometrical horizon (which assumes a perfectly flat, infinite ground plane) and the true horizon (which assumes a spherical Earth surface) is imperceptible to the unaided eye. However, for someone on a 1,000 m (3,300 ft) hill looking out across the sea, the true horizon will be about a degree below a horizontal line. In astronomy, the horizon is the horizontal plane through

7590-419: The difference is about 8%. This changes the factor of 3.57, in the metric formulas used above, to about 3.86. For instance, if an observer is standing on seashore, with eyes 1.70 m above sea level, according to the simple geometrical formulas given above the horizon should be 4.7 km away. Actually, atmospheric refraction allows the observer to see 300 metres farther, moving the true horizon 5 km away from

7705-501: The distance to the horizon can easily be calculated. The tangent-secant theorem states that Make the following substitutions: with d, D, and h all measured in the same units. The formula now becomes or where R is the radius of the Earth. The same equation can also be derived using the Pythagorean theorem . At the horizon, the line of sight is a tangent to the Earth and is also perpendicular to Earth's radius. This sets up

7820-464: The distance to the horizon for a "standard observer" varies as the square root of the planet's radius. Thus, the horizon on Mercury is 62% as far away from the observer as it is on Earth, on Mars the figure is 73%, on the Moon the figure is 52%, on Mimas the figure is 18%, and so on. If the Earth is assumed to be a featureless sphere (rather than an oblate spheroid ) with no atmospheric refraction, then

7935-406: The distance to the horizon is If d is in nautical miles , and h in feet, the constant factor is about 1.06, which is close enough to 1 that it is often ignored, giving: These formulas may be used when h is much smaller than the radius of the Earth (6371 km or 3959 mi), including all views from any mountaintops, airplanes, or high-altitude balloons. With the constants as given, both

8050-440: The distances to the horizon from each of the two points: For example, for an observer B with a height of h B =1.70 m standing on the ground, the horizon is D B =4.65 km away. For a tower with a height of h L =100 m, the horizon distance is D L =35.7 km. Thus an observer on a beach can see the top of the tower as long as it is not more than D BL =40.35 km away. Conversely, if an observer on

8165-501: The elder of the Titans in classical Greek mythology . Oceanus was believed by the ancient Greeks and Romans to be the divine personification of an enormous river encircling the world. The concept of Ōkeanós has an Indo-European connection. Greek Ōkeanós has been compared to the Vedic epithet ā-śáyāna-, predicated of the dragon Vṛtra-, who captured the cows/rivers. Related to this notion,

8280-496: The ellipse and also coincide with minimum and maximum radius of curvature. There are two principal radii of curvature : along the meridional and prime-vertical normal sections . In particular, the Earth's meridional radius of curvature (in the north–south direction) at φ is: where e {\displaystyle e} is the eccentricity of the earth. This is the radius that Eratosthenes measured in his arc measurement . If one point had appeared due east of

8395-483: The ellipsoid, negative below or inside. The geoid height variation is under 110 m (360 ft) on Earth. The geoid height can change abruptly due to earthquakes (such as the Sumatra-Andaman earthquake ) or reduction in ice masses (such as Greenland ). Not all deformations originate within the Earth. Gravitational attraction from the Moon or Sun can cause the Earth's surface at a given point to vary by tenths of

8510-418: The eyes of the observer. It is the fundamental plane of the horizontal coordinate system , the locus of points that have an altitude of zero degrees. While similar in ways to the geometrical horizon, in this context a horizon may be considered to be a plane in space, rather than a line on a picture plane. Ignoring the effect of atmospheric refraction , distance to the true horizon from an observer close to

8625-412: The formation of unusually high rogue waves . Most waves are less than 3 m (10 ft) high and it is not unusual for strong storms to double or triple that height. Rogue waves, however, have been documented at heights above 25 meters (82 ft). The top of a wave is known as the crest, the lowest point between waves is the trough and the distance between the crests is the wavelength. The wave

8740-462: The horizon is at a distance of about 4.8 kilometres (3 mi). When observed from very high standpoints, such as a space station , the horizon is much farther away and it encompasses a much larger area of Earth's surface. In this case, the horizon would no longer be a perfect circle, not even a plane curve such as an ellipse, especially when the observer is above the equator, as the Earth's surface can be better modeled as an oblate ellipsoid than as

8855-421: The interface between water and air is called swell – a term used in sailing , surfing and navigation . These motions profoundly affect ships on the surface of the ocean and the well-being of people on those ships who might suffer from sea sickness . Wind blowing over the surface of a body of water forms waves that are perpendicular to the direction of the wind. The friction between air and water caused by

8970-482: The intertidal zone. The difference in height between the high tide and low tide is known as the tidal range or tidal amplitude. When the sun and moon are aligned (full moon or new moon), the combined effect results in the higher "spring tides", while the sun and moon misaligning (half moons) result in lesser tidal ranges. In the open ocean tidal ranges are less than 1 meter, but in coastal areas these tidal ranges increase to more than 10 meters in some areas. Some of

9085-710: The largest tidal ranges in the world occur in the Bay of Fundy and Ungava Bay in Canada, reaching up to 16 meters. Other locations with record high tidal ranges include the Bristol Channel between England and Wales, Cook Inlet in Alaska, and the Río Gallegos in Argentina. Tides are not to be confused with storm surges , which can occur when high winds pile water up against the coast in

9200-443: The level ground, uses binoculars to look at a distant building which he knows to consist of thirty storeys , each 3.5 metres high. He counts the stories he can see and finds there are only ten. So twenty stories or 70 metres of the building are hidden from him by the curvature of the Earth. From this, he can calculate his distance from the building: which comes to about 35 kilometres. It is similarly possible to calculate how much of

9315-466: The majority of Earth's surface. It includes the Pacific , Atlantic , Indian , Southern/Antarctic , and Arctic oceans. As a general term, "the ocean" and "the sea" are often interchangeable. Strictly speaking, a "sea" is a body of water (generally a division of the world ocean) partly or fully enclosed by land. The word "sea" can also be used for many specific, much smaller bodies of seawater, such as

9430-403: The metric and imperial formulas are precise to within 1% (see the next section for how to obtain greater precision). If h is significant with respect to R , as with most satellites, then the approximation is no longer valid, and the exact formula is required. Another relationship involves the great-circle distance s along the arc over the curved surface of the Earth to the horizon; this

9545-424: The north–south direction than in the east–west direction. In summary, local variations in terrain prevent defining a single "precise" radius. One can only adopt an idealized model. Since the estimate by Eratosthenes , many models have been created. Historically, these models were based on regional topography, giving the best reference ellipsoid for the area under survey. As satellite remote sensing and especially

9660-439: The observer. This correction can be, and often is, applied as a fairly good approximation when atmospheric conditions are close to standard . When conditions are unusual, this approximation fails. Refraction is strongly affected by temperature gradients, which can vary considerably from day to day, especially over water. In extreme cases, usually in springtime, when warm air overlies cold water, refraction can allow light to follow

9775-420: The obvious consequences for safety and the transmission of information that this range implied. This importance lessened with the development of the radio and the telegraph , but even today, when flying an aircraft under visual flight rules , a technique called attitude flying is used to control the aircraft, where the pilot uses the visual relationship between the aircraft's nose and the horizon to control

9890-408: The ocean faces many environmental threats, such as marine pollution , overfishing , and the effects of climate change . Those effects include ocean warming , ocean acidification and sea level rise . The continental shelf and coastal waters are most affected by human activity. The terms "the ocean" or "the sea" used without specification refer to the interconnected body of salt water covering

10005-423: The ocean meets dry land. It is more shallow, with a depth of a few hundred meters or less. Human activity often has negative impacts on marine life within the continental shelf. Ocean temperatures depend on the amount of solar radiation reaching the ocean surface. In the tropics, surface temperatures can rise to over 30 °C (86 °F). Near the poles where sea ice forms, the temperature in equilibrium

10120-508: The ocean, on land and air. All these processes and components together make up ocean surface ecosystems . Tides are the regular rise and fall in water level experienced by oceans, primarily driven by the Moon 's gravitational tidal forces upon the Earth. Tidal forces affect all matter on Earth, but only fluids like the ocean demonstrate the effects on human timescales. (For example, tidal forces acting on rock may produce tidal locking between two planetary bodies.) Though primarily driven by

10235-469: The ocean. If the wind dies down, the wave formation is reduced, but already-formed waves continue to travel in their original direction until they meet land. The size of the waves depends on the fetch , the distance that the wind has blown over the water and the strength and duration of that wind. When waves meet others coming from different directions, interference between the two can produce broken, irregular seas. Constructive interference can lead to

10350-419: The oceans absorb CO 2 from the atmosphere , a higher concentration leads to ocean acidification (a drop in pH value ). The ocean provides many benefits to humans such as ecosystem services , access to seafood and other marine resources , and a means of transport . The ocean is known to be the habitat of over 230,000 species , but may hold considerably more – perhaps over two million species. Yet,

10465-425: The oceans could have been up to 50 m (165 ft) higher. The entire ocean, containing 97% of Earth's water, spans 70.8% of Earth 's surface, making it Earth's global ocean or world ocean . This makes Earth, along with its vibrant hydrosphere a "water world" or " ocean world ", particularly in Earth's early history when the ocean is thought to have possibly covered Earth completely. The ocean's shape

10580-434: The oceans have been mapped. The zone where land meets sea is known as the coast , and the part between the lowest spring tides and the upper limit reached by splashing waves is the shore . A beach is the accumulation of sand or shingle on the shore. A headland is a point of land jutting out into the sea and a larger promontory is known as a cape . The indentation of a coastline, especially between two headlands,

10695-533: The oceans may have always been on the Earth since the beginning of the planet's formation. In this model, atmospheric greenhouse gases kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity . The origin of Earth's oceans is unknown. Oceans are thought to have formed in the Hadean eon and may have been the cause for the emergence of life . Plate tectonics , post-glacial rebound , and sea level rise continually change

10810-400: The other, one finds the approximate curvature in the east–west direction. This Earth's prime-vertical radius of curvature , also called the Earth's transverse radius of curvature , is defined perpendicular ( orthogonal ) to M at geodetic latitude φ and is: N can also be interpreted geometrically as the normal distance from the ellipsoid surface to the polar axis. The radius of

10925-451: The photic zone, the mesopelagic zone and the aphotic deep ocean zone: The pelagic part of the aphotic zone can be further divided into vertical regions according to depth and temperature: Distinct boundaries between ocean surface waters and deep waters can be drawn based on the properties of the water. These boundaries are called thermoclines (temperature), haloclines (salinity), chemoclines (chemistry), and pycnoclines (density). If

11040-452: The position of an observable location, the use of more precise values for WGS-84 radii may not yield a corresponding improvement in accuracy . The value for the equatorial radius is defined to the nearest 0.1 m in WGS-84. The value for the polar radius in this section has been rounded to the nearest 0.1 m, which is expected to be adequate for most uses. Refer to the WGS-84 ellipsoid if

11155-418: The power of a storm wave impacting on the foot of a cliff has a shattering effect as air in cracks and crevices is compressed and then expands rapidly with release of pressure. At the same time, sand and pebbles have an erosive effect as they are thrown against the rocks. This tends to undercut the cliff, and normal weathering processes such as the action of frost follows, causing further destruction. Gradually,

11270-406: The radius (that is, h  ≪  R ). When the observer is elevated, the horizon zenith angle can be greater than 90°. The maximum visible zenith angle occurs when the ray is tangent to Earth's surface; from triangle OCG in the figure at right, where h {\displaystyle h} is the observer's height above the surface and γ {\displaystyle \gamma }

11385-439: The range between the polar radius and equatorial radius because they account for localized effects. A nominal Earth radius (denoted R E N {\displaystyle {\mathcal {R}}_{\mathrm {E} }^{\mathrm {N} }} ) is sometimes used as a unit of measurement in astronomy and geophysics , a conversion factor used when expressing planetary properties as multiples or fractions of

11500-581: The roots of Equation (125) in: where in the first fundamental form for a surface (Equation (112) in ): E, F, and G are elements of the metric tensor : r = [ r 1 , r 2 , r 3 ] T = [ x , y , z ] T {\displaystyle r=[r^{1},r^{2},r^{3}]^{T}=[x,y,z]^{T}} , w 1 = φ {\displaystyle w^{1}=\varphi } , w 2 = λ , {\displaystyle w^{2}=\lambda ,} in

11615-455: The seabed between adjoining plates to form mid-oceanic ridges and here convection currents within the mantle tend to drive the two plates apart. Parallel to these ridges and nearer the coasts, one oceanic plate may slide beneath another oceanic plate in a process known as subduction . Deep trenches are formed here and the process is accompanied by friction as the plates grind together. The movement proceeds in jerks which cause earthquakes, heat

11730-407: The seabed causing deltas to form in estuaries. All these materials move back and forth under the influence of waves, tides and currents. Dredging removes material and deepens channels but may have unexpected effects elsewhere on the coastline. Governments make efforts to prevent flooding of the land by the building of breakwaters , seawalls , dykes and levees and other sea defences. For instance,

11845-401: The second fundamental form gives the distance from r + d r {\displaystyle r+dr} to the plane tangent at r {\displaystyle r} . The Earth's azimuthal radius of curvature , along an Earth normal section at an azimuth (measured clockwise from north) α and at latitude φ , is derived from Euler's curvature formula as follows: It

11960-490: The second term in parentheses would give a distance of 5,048 kilometres (3,137 mi), a 7% error. If the observer is close to the surface of the Earth, then it is valid to disregard h in the term (2 R + h ) , and the formula becomes- Using kilometres for d and R , and metres for h , and taking the radius of the Earth as 6371 km, the distance to the horizon is Using imperial units , with d and R in statute miles (as commonly used on land), and h in feet,

12075-407: The ship that is more than six metres above the level of the water. The part of the ship that is below this height is hidden from him by the curvature of the Earth. In this situation, the ship is said to be hull-down . Due to atmospheric refraction the distance to the visible horizon is further than the distance based on a simple geometric calculation. If the ground (or water) surface is colder than

12190-401: The shore at the rate of six to eight per minute and these are known as constructive waves as they tend to move material up the beach and have little erosive effect. Storm waves arrive on shore in rapid succession and are known as destructive waves as the swash moves beach material seawards. Under their influence, the sand and shingle on the beach is ground together and abraded. Around high tide,

12305-474: The simple approximation described above. Earth%27s oceans The ocean is the body of salt water that covers approximately 70.8% of Earth . In English , the term ocean also refers to any of the large bodies of water into which the world ocean is conventionally divided. The following names describe five different areas of the ocean: Pacific , Atlantic , Indian , Antarctic/Southern , and Arctic . The ocean contains 97% of Earth's water and

12420-512: The surface to the ocean floor. The water column is further divided into zones based on depth and the amount of light present. The photic zone starts at the surface and is defined to be "the depth at which light intensity is only 1% of the surface value" (approximately 200 m in the open ocean). This is the zone where photosynthesis can occur. In this process plants and microscopic algae (free floating phytoplankton ) use light, water, carbon dioxide, and nutrients to produce organic matter. As

12535-402: The surface, is normal to the surface at r {\displaystyle r} . With F = f = 0 {\displaystyle F=f=0} for an oblate spheroid, the curvatures are and the principal radii of curvature are The first and second radii of curvature correspond, respectively, to the Earth's meridional and prime-vertical radii of curvature. Geometrically,

12650-456: The system forms). As the world's ocean is the principal component of Earth's hydrosphere , it is integral to life on Earth, forms part of the carbon cycle and water cycle , and – as a huge heat reservoir – influences climate and weather patterns. The motions of the ocean surface, known as undulations or wind waves , are the partial and alternate rising and falling of the ocean surface. The series of mechanical waves that propagate along

12765-442: The term radius are common in many fields, including those dealing with models of Earth. The following is a partial list of models of Earth's surface, ordered from exact to more approximate: In the case of the geoid and ellipsoids, the fixed distance from any point on the model to the specified center is called "a radius of the Earth" or "the radius of the Earth at that point" . It is also common to refer to any mean radius of

12880-463: The true horizon is below the observer and below sea level . Its radius or horizontal distance from the observer varies slightly from day to day due to atmospheric refraction , which is greatly affected by weather conditions. Also, the higher the observer's eyes are from sea level, the farther away the horizon is from the observer. For instance, in standard atmospheric conditions , for an observer with eye level above sea level by 1.8 metres (6 ft),

12995-434: The very clearest ocean water, and is similar to blue light scattering in the sky . Ocean water represents the largest body of water within the global water cycle (oceans contain 97% of Earth's water ). Evaporation from the ocean moves water into the atmosphere to later rain back down onto land and the ocean. Oceans have a significant effect on the biosphere . The ocean as a whole is thought to cover approximately 90% of

13110-476: The world, and from the surface into the deep ocean. All this has impacts on the global climate system . Ocean water contains dissolved gases, including oxygen , carbon dioxide and nitrogen . An exchange of these gases occurs at the ocean's surface. The solubility of these gases depends on the temperature and salinity of the water. The carbon dioxide concentration in the atmosphere is rising due to CO 2 emissions , mainly from fossil fuel combustion. As

13225-750: Was recovered from the Isua Greenstone Belt and provides evidence that water existed on Earth 3.8 billion years ago. In the Nuvvuagittuq Greenstone Belt , Quebec , Canada, rocks dated at 3.8 billion years old by one study and 4.28 billion years old by another show evidence of the presence of water at these ages. If oceans existed earlier than this, any geological evidence either has yet to be discovered, or has since been destroyed by geological processes like crustal recycling . However, in August 2020, researchers reported that sufficient water to fill

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