The equator (yellow line) at the December solstice. The direction of the Sun is to the left.Countries and territories that are intersected by the equator (red) or thePrime Meridian (blue), which intersect at "Null Island".
Inspatial (3D)geometry, as applied inastronomy, the equator of a rotatingspheroid (such as aplanet) is the parallel (circle of latitude) at which latitude is defined to be 0°. It is animaginary line on the spheroid, equidistant from itspoles, dividing it into northern and southern hemispheres. In other words, it is the intersection of the spheroid with theplaneperpendicular to its axis ofrotation and midway between itsgeographical poles.
On and near the equator (on Earth), noontimesunlight appears almost directly overhead (no more than about 23° from thezenith) every day, year-round. Consequently, the equator has a rather stable daytime temperature throughout the year. On theequinoxes (approximately 20 March and 23 September) thesubsolar point crosses Earth's equator at a shallow angle, sunlight shines perpendicular to Earth's axis of rotation, and all latitudes have nearly a 12-hour day and 12-hour night.[2]
The name is derived frommedieval Latin wordaequator, in the phrasecirculus aequator diei et noctis, meaning 'circle equalizing day and night', from theLatin wordaequare 'make equal'.[3]
In the cycle of Earth'sseasons, the equatorial plane runs through the Sun twice ayear: on theequinoxes inMarch andSeptember. To a person on Earth, the Sunappears to travel along the equator (or along the celestial equator) at these times.
Locations on the equator experience the shortestsunrises andsunsets because the Sun'sdaily path is nearly perpendicular to thehorizon for most of the year. The length ofdaylight (sunrise to sunset) is almost constant throughout the year; it is about 14 minutes longer than nighttime due toatmospheric refraction and the fact that sunrise begins (or sunset ends) as the upper limb, not the center, of the Sun's disk contacts the horizon.
Earthbulges slightly at the equator; its average diameter is 12,742 km (7,918 mi), but the diameter at the equator is about 43 km (27 mi) greater than at the poles.[1]
Sites near the equator, such as theGuiana Space Centre inKourou, French Guiana, are good locations forspaceports as they have the fastestrotational speed of any latitude, 460 m (1,509 ft)/sec. The addedvelocity reduces the fuel needed to launch spacecraft eastward (in the direction of Earth's rotation) to orbit, while simultaneously avoiding costly maneuvers to flatteninclination during missions such as theApollo Moon landings.[4]
The precise location of the equator is not truly fixed; the true equatorial plane is perpendicular to theEarth's rotation axis, whichdrifts about 9 metres (30 ft) during a year.
Geological samples show that the equator significantly changed positions between 48 and 12 million years ago, as sediment deposited by ocean thermal currents at the equator shifted. The deposits by thermal currents are determined by the axis of Earth, which determines solar coverage ofEarth's surface. Changes in Earth's axis can also be observed in the geographical layout of volcanic island chains, which are created by shifting hot spots under Earth's crust as the axis and crust move.[5] This is consistent with theIndian tectonic plate colliding with theEurasian tectonic plate, which is causing theHimalayan uplift.
The International Association of Geodesy (IAG) and the International Astronomical Union (IAU) use an equatorial radius of 6,378.1366 km (3,963.1903 mi) (codified as the IAU 2009 value).[6] This equatorial radius is also in the 2003 and 2010 IERS Conventions.[7] It is also the equatorial radius used for the IERS 2003 ellipsoid. If it were really circular, the length of the equator would then be exactly 2π times the radius, namely 40,075.0142 km (24,901.4594 mi). TheGRS 80 (Geodetic Reference System 1980) as approved and adopted by the IUGG at its Canberra, Australia meeting of 1979 has an equatorial radius of 6,378.137 km (3,963.191 mi). TheWGS 84 (World Geodetic System 1984) which is a standard for use in cartography, geodesy, and satellite navigation includingGPS, also has an equatorial radius of 6,378.137 km (3,963.191 mi). For both GRS 80 and WGS 84, this results in a length for the equator of 40,075.0167 km (24,901.4609 mi).
Thegeographical mile is defined as onearc-minute of the equator, so it has different values depending on which radius is assumed. For example, by WSG-84, the distance is 1,855.3248 metres (6,087.024 ft), while by IAU-2000, it is 1,855.3257 metres (6,087.027 ft). This is a difference of less than one millimetre (0.039 in) over the total distance (approximately 1.86 kilometres or 1.16 miles).
Earth is commonly modeled as asphere flattened 0.336% along its axis. This makes the equator 0.16% longer than ameridian (a great circle passing through the two poles). The IUGG standard meridian is, to the nearest millimetre, 40,007.862917 kilometres (24,859.733480 mi), one arc-minute of which is 1,852.216 metres (6,076.82 ft), explaining theSI standardization of thenautical mile as 1,852 metres (6,076 ft), more than 3 metres (9.8 ft) less than thegeographical mile.
Thesea-level surface of Earth (thegeoid) is irregular, so the actual length of the equator is not so easy to determine.Aviation Week and Space Technology on 9 October 1961 reported that measurements using theTransit IV-A satellite had shown the equatorial diameter from longitude 11° West to 169° East to be 300 metres (1,000 ft) greater than its diameter ninety degrees away.[8]
The equator passes over approximately 8714 km of land (21.7%) and 31,361 km of sea (78.3%).[10] It passes through the land of elevensovereign states.Indonesia is the country straddling the greatest length of the equatorial line across both land and sea. Starting at thePrime Meridian and heading eastwards, the equator passes through:
Amazonas,Roraima,Pará,Amapá (passing slightly south of the city center of the state capitalMacapá, and precisely at the Marco Zero monument and the Avenue Equatorial)
Diagram of the seasons, showing the situation at the December solstice. Regardless of the time of day (i.e.Earth's rotation on its axis), theNorth Pole will be dark, and theSouth Pole will be illuminated; see alsopolar night andpolar day. In addition to the density ofincident light, thedissipation of light in theatmosphere is greater when it falls at a shallow angle. Note that the equator is not under thesubsolar point during this time of year.
Seasons result from the tilt of Earth's axis away from a line perpendicular to the plane of its revolution around the Sun. Throughout the year, the Northern and Southern hemispheres are alternately turned either toward or away from the Sun, depending on Earth's position in its orbit. The hemisphere turned toward the Sun receives more sunlight and is in summer, while the other hemisphere receives less sun and is in winter (seesolstice).
At theequinoxes, Earth's axis is perpendicular to the Sun rather than tilted toward or away, meaning that day and night are both about 12 hours long across the whole of Earth.
Near the equator, this means the variation in the strength of solar radiation is different relative to the time of year than it is at higher latitudes: maximum solar radiation is received during the equinoxes, when a place at the equator is under thesubsolar point at high noon, and the intermediate seasons of spring and autumn occur at higher latitudes; and the minimum occurs duringboth solstices, when either pole is tilted towards or away from the sun, resulting in either summer or winter in both hemispheres. This also results in a corresponding movement of the equator away from the subsolar point, which is then situated over or near the relevanttropiccircle. Nevertheless, temperatures are high year-round due to the Earth'saxial tilt of 23.5° not being enough to create a low minimum middaydeclination to sufficiently weaken the Sun's rays even during the solstices. High year-round temperatures extend to about 25° north or south of the equator, although the moderate seasonal temperature difference is defined by the opposing solstices (as it is at higher latitudes) near the poleward limits of this range.
Near the equator, there is little temperature change throughout the year, though there may be dramatic differences in rainfall and humidity. The terms summer, autumn, winter and spring do not generally apply. Lowlands around the equator generally have atropical rainforest climate, also known as an equatorial climate, though cold ocean currents cause some regions to havetropical monsoon climates with adry season in the middle of the year, and theSomali Current generated by theAsian monsoon due to continental heating via the highTibetan Plateau causesGreater Somalia to have an arid climate despite its equatorial location.
Average annual temperatures in equatorial lowlands are around 31 °C (88 °F) during the afternoon and 23 °C (73 °F) around sunrise. Rainfall is very high away from cold ocean current upwelling zones, from 2,500 to 3,500 mm (100 to 140 in) per year. There are about 200 rainy days per year and average annual sunshine hours are around 2,000. Despite high year-round sea level temperatures, some higher altitudes such as theAndes andMount Kilimanjaro have glaciers. The highest point on the equator is at the elevation of 4,690 metres (15,387 ft), at0°0′0″N77°59′31″W / 0.00000°N 77.99194°W /0.00000; -77.99194 (highest point on the equator), found on the southern slopes ofVolcán Cayambe [summit 5,790 metres (18,996 ft)] inEcuador. This is slightly above thesnow line and is the only place on the equator where snow lies on the ground. At the equator, the snow line is around 1,000 metres (3,300 ft)lower than onMount Everest and as much as 2,000 metres (6,600 ft) lower than the highest snow line in the world, near theTropic of Capricorn onLlullaillaco.
There is a widespread maritime tradition of holding ceremonies to mark a sailor's first crossing of the equator. In the past, these ceremonies have been notorious for their brutality, especially in naval practice.[citation needed] Milder line-crossing ceremonies, typically featuringKing Neptune, are also held for passengers' entertainment on some civilian ocean liners and cruise ships.[citation needed]
^William Barnaby Faherty; Charles D. Benson (1978)."Moonport: A History of Apollo Launch Facilities and Operations". NASA History Series. p. Chapter 1.2: A Saturn Launch Site. NASA Special Publication-4204. Archived fromthe original on 15 September 2018. Retrieved8 May 2019.Equatorial launch sites offered certain advantages over facilities within the continental United States. A launching due east from a site on the equator could take advantage of the earth's maximum rotational velocity (460 m/s (1,510 ft/s)) to achieve orbital speed. The more frequent overhead passage of the orbiting vehicle above an equatorial base would facilitate tracking and communications. Most important, an equatorial launch site would avoid the costly dogleg technique, a prerequisite for placing rockets into equatorial orbit from sites such as Cape Canaveral, Florida (28 degrees north latitude). The necessary correction in the space vehicle's trajectory could be very expensive - engineers estimated that doglegging a Saturn vehicle into a low-altitude equatorial orbit from Cape Canaveral used enough extra propellant to reduce the payload by as much as 80%. In higher orbits, the penalty was less severe but still involved at least a 20% loss of payload.
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