Equator

An equator of a rotating spheroid (such as a planet) is its zeroth circle of latitude (parallel). It is the imaginary line on the spheroid's surface, equidistant from its poles, dividing it into northern and southern hemispheres. In other words, it is the intersection of the spheroid's surface with the plane perpendicular to its axis of rotation and midway between its geographical poles.

On Earth, the Equator is about 40,075 km (24,901 mi) long, of which 78.8% lies across water and 21.3% over land. Indonesia is the country straddling the greatest length of the equatorial line across both land and sea.

Line across the Earth
Equator in the map of the Earth
Equator and Prime Meridian
Nations or territories that touch the Equator (red) or the IERS Reference Meridian (blue)

Etymology

The name is derived from medieval Latin word aequator, in the phrase circulus aequator diei et noctis, meaning ‘circle equalizing day and night’, from the Latin word aequare meaning ‘make equal’.[1]

Overview

Top: A monument marking the Equator near the city of Pontianak, Indonesia
Bottom: Road sign marking the Equator near Nanyuki, Kenya

Equator monument
Equator sign kenya

The latitude of the Earth's equator is, by definition, 0° (zero degrees) of arc. The Equator is one of the five notable circles of latitude on Earth; the other four are both polar circles (the Arctic Circle and the Antarctic Circle) and both tropical circles (the Tropic of Cancer and the Tropic of Capricorn). The Equator is the only line of latitude which is also a great circle — that is, one whose plane passes through the center of the globe. The plane of Earth's equator, when projected outwards to the celestial sphere, defines the celestial equator.

In the cycle of Earth's seasons, the equatorial plane runs through the Sun twice per year: on the equinoxes in March and September. To a person on Earth, the Sun appears to travel above the Equator (or along the celestial equator) at these times. Light rays from the Sun's center are perpendicular to Earth's surface at the point of solar noon on the Equator.

Equator Sao Tome
The Equator marked as it crosses Ilhéu das Rolas, in São Tomé and Príncipe
Equator Line Monument, Macapá city, Brazil
The Marco Zero monument marking the Equator in Macapá, Brazil.

Locations on the Equator experience the shortest sunrises and sunsets because the Sun's daily path is nearly perpendicular to the horizon for most of the year. The length of daylight (sunrise to sunset) is almost constant throughout the year; it is about 14 minutes longer than nighttime due to atmospheric 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.

Earth bulges slightly at the Equator; the "average" diameter of Earth is 12,750 km (7,920 mi), but the diameter at the Equator is about 43 km (27 mi) greater than at the poles.[2]

Sites near the Equator, such as the Guiana Space Centre in Kourou, French Guiana, are good locations for spaceports as they have a faster rotational speed than other latitudes; the added velocity reduces the fuel needed to launch spacecraft. Since Earth rotates eastward, spacecraft must also be launched eastward (if not, to the southeast or northeast) to take advantage of this Earth-boost of speed.

Geodesy

Precise location

The precise location of the Equator is not truly fixed; the true equatorial plane is perpendicular to the Earth's spin axis, which drifts about 9 metres (30 ft) during a year. This effect must be accounted for in detailed geophysical measurements.

Exact length

The International Association of Geodesy (IAG) and the International Astronomical Union (IAU) have chosen to use an equatorial radius of 6,378.1366 kilometres (3,963.1903 mi) (codified as the IAU 2009 value).[3] This equatorial radius is also in the 2003 and 2010 IERS Conventions.[4] 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 kilometres (24,901.4594 mi). The GRS 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 kilometres (3,963.191 mi). The WGS 84 (World Geodetic System 1984) which is a standard for use in cartography, geodesy, and satellite navigation including GPS, also has an equatorial radius of 6,378.137 kilometres (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).

The geographical mile is defined as one arc-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).

The earth is commonly modeled as a sphere flattened 0.336% along its axis. This makes the Equator 0.16% longer than a meridian (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 the SI standardization of the nautical mile as 1,852 metres (6,076 ft), more than 3 metres (9.8 ft) less than the geographical mile.

The sea-level surface of the Earth (the geoid) 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 the Transit IV-A satellite had shown the equatorial "diameter" from longitude 11° West to 169° East to be 1,000 feet (300 m) greater than its "diameter" ninety degrees away.

Equatorial countries and territories

ECSundialGPS
GPS reading taken on the Equator close to the Quitsato Sundial, at Mitad del Mundo, Ecuador.[5]

The Equator passes through the land of 11 countries. Starting at the Prime Meridian and heading eastwards, the Equator passes through:

Co-ordinates Country, territory or sea Notes
0°N 0°E / 0°N 0°E Atlantic Ocean Gulf of Guinea
0°0′N 6°31′E / 0.000°N 6.517°E  São Tomé and Príncipe Passing through Pestana Equador resort on the Ilhéu das Rolas
0°0′N 9°21′E / 0.000°N 9.350°E  Gabon Passing 8.9 km (5.5 mi) south of Ayem, 10.6 km (6.6 mi) north of Mayene, Booue
0°0′N 13°56′E / 0.000°N 13.933°E  Republic of the Congo Passing through the town of Makoua.
0°0′N 17°46′E / 0.000°N 17.767°E  Democratic Republic of the Congo Passing 9 km (5.6 mi) south of central Butembo
0°0′N 29°43′E / 0.000°N 29.717°E  Uganda Passing 32 km (20 mi) south of central Kampala
0°0′N 32°22′E / 0.000°N 32.367°E Lake Victoria Passing through some islands of  Uganda in Mukono District and Namayingo District
0°0′N 34°0′E / 0.000°N 34.000°E  Kenya Passing 6 km (3.7 mi) north of central Kisumu
0°0′N 41°0′E / 0.000°N 41.000°E  Somalia Passing south of Jamame
0°0′N 42°53′E / 0.000°N 42.883°E Indian Ocean Passing between Huvadhu Atoll and Fuvahmulah of the  Maldives
0°0′N 98°12′E / 0.000°N 98.200°E  Indonesia The Batu Islands, Sumatra and the Lingga Islands
0°0′N 104°34′E / 0.000°N 104.567°E Karimata Strait
0°0′N 109°9′E / 0.000°N 109.150°E  Indonesia Borneo (passing through Pontianak)
0°0′N 117°30′E / 0.000°N 117.500°E Makassar Strait
0°0′N 119°40′E / 0.000°N 119.667°E  Indonesia Sulawesi (Celebes)
0°0′N 120°5′E / 0.000°N 120.083°E Gulf of Tomini
0°0′N 124°0′E / 0.000°N 124.000°E Molucca Sea
0°0′N 127°24′E / 0.000°N 127.400°E  Indonesia Kayoa and Halmahera islands
0°0′N 127°53′E / 0.000°N 127.883°E Halmahera Sea
0°0′N 129°20′E / 0.000°N 129.333°E  Indonesia Gebe and Kawe islands
0°0′N 129°21′E / 0.000°N 129.350°E Pacific Ocean Passing between Aranuka and Nonouti atolls,  Kiribati (at 0°0′N 173°40′E / 0.000°N 173.667°E)
0°0′N 80°6′W / 0.000°N 80.100°W  Ecuador Passing 24 km (15 mi) north of central Quito, near Mitad del Mundo, and precisely at the location of Catequilla, a pre-Columbian ruin. Also, Isabela Island in the Galápagos Islands
0°0′N 75°32′W / 0.000°N 75.533°W  Colombia Passing 4.3 km (2.7 mi) north of the border with Peru
0°0′N 70°3′W / 0.000°N 70.050°W  Brazil Amazonas, Roraima, Pará, Amapá (passing slightly south of the city center of the state capital Macapá, and precisely at the Marco Zero monument and the Avenue Equatorial)
0°0′N 49°21′W / 0.000°N 49.350°W Atlantic Ocean At the Perigoso Canal (sv) on the mouth of the Amazon River

Despite its name, no part of Equatorial Guinea lies on the Equator. However, its island of Annobón is 155 km (96 mi) south of the Equator, and the rest of the country lies to the north.

Equatorial seasons and climate

Seasons

Diagram of the seasons, depicting the situation at the December solstice. Regardless of the time of day (i.e. the Earth’s rotation on its axis), the North Pole will be dark, and the South Pole will be illuminated; see also arctic winter. In addition to the density of incident light, the dissipation of light in the atmosphere is greater when it falls at a shallow angle.

Seasons result from the tilt of the Earth's axis compared 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 (see solstice).

At the equinoxes, the 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 the Earth.

The Equator lies mostly on the three largest oceans: the Atlantic Ocean, the Indian Ocean, and the Pacific Ocean. 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 a tropical rainforest climate, also known as an equatorial climate, though cold currents cause some regions to have tropical monsoon climates with a dry season in the middle of the year, and the Somali Current generated by the Asian monsoon due to continental heating via the high Tibetan Plateau causes Greater 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 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 the Andes and Mount Kilimanjaro have glaciers. The highest point on the Equator is at the elevation of 4,690 metres (15,387 ft), at 0°0′0″N 77°59′31″W / 0.00000°N 77.99194°W, found on the southern slopes of Volcán Cayambe [summit 5,790 metres (18,996 ft)] in Ecuador. This is slightly above the snow 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 on Mount Everest and as much as 2,000 metres (6,600 ft) lower than the highest snow line in the world, near the Tropic of Capricorn on Llullaillaco.

Climate data for Macapá, Brazil in South America
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Average high °C (°F) 29.7
(85.5)
29.2
(84.6)
29.3
(84.7)
29.5
(85.1)
30.0
(86.0)
30.3
(86.5)
30.6
(87.1)
31.5
(88.7)
32.1
(89.8)
32.6
(90.7)
32.3
(90.1)
31.4
(88.5)
30.71
(87.28)
Daily mean °C (°F) 26.4
(79.5)
26.2
(79.2)
26.3
(79.3)
26.5
(79.7)
26.8
(80.2)
26.8
(80.2)
26.8
(80.2)
27.4
(81.3)
27.8
(82.0)
28.1
(82.6)
27.9
(82.2)
27.4
(81.3)
27.03
(80.65)
Average low °C (°F) 23.0
(73.4)
23.1
(73.6)
23.2
(73.8)
23.5
(74.3)
23.5
(74.3)
23.2
(73.8)
22.9
(73.2)
23.3
(73.9)
23.4
(74.1)
23.5
(74.3)
23.5
(74.3)
23.4
(74.1)
23.29
(73.92)
Average rainfall mm (inches) 299.6
(11.80)
347.0
(13.66)
407.2
(16.03)
384.3
(15.13)
351.5
(13.84)
220.1
(8.67)
184.8
(7.28)
98.00
(3.86)
42.60
(1.68)
35.50
(1.40)
58.40
(2.30)
142.5
(5.61)
2,571.5
(101.26)
Average rainy days (≥ 0.1 mm) 23 22 24 24 25 22 19 13 6 5 6 14 203
Mean monthly sunshine hours 148.8 113.1 108.5 114.0 151.9 189.0 226.3 272.8 273.0 282.1 252.0 204.6 2,336.1
Source: World Meteorological Organization (UN),[6] Hong Kong Observatory[7]
Climate data for Pontianak, Indonesia in Asia
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Average high °C (°F) 32.4
(90.3)
32.7
(90.9)
32.9
(91.2)
33.2
(91.8)
33.0
(91.4)
33.2
(91.8)
32.9
(91.2)
33.4
(92.1)
32.6
(90.7)
32.6
(90.7)
32.2
(90.0)
32.0
(89.6)
32.7
(90.9)
Daily mean °C (°F) 27.6
(81.7)
27.7
(81.9)
28.0
(82.4)
28.2
(82.8)
28.2
(82.8)
28.2
(82.8)
27.7
(81.9)
27.9
(82.2)
27.6
(81.7)
27.7
(81.9)
27.4
(81.3)
27.2
(81.0)
27.7
(81.9)
Average low °C (°F) 22.7
(72.9)
22.6
(72.7)
23.0
(73.4)
23.2
(73.8)
23.4
(74.1)
23.1
(73.6)
22.5
(72.5)
22.3
(72.1)
22.6
(72.7)
22.8
(73.0)
22.6
(72.7)
22.4
(72.3)
22.7
(72.9)
Average rainfall mm (inches) 260
(10.2)
215
(8.5)
254
(10.0)
292
(11.5)
256
(10.1)
212
(8.3)
201
(7.9)
180
(7.1)
295
(11.6)
329
(13.0)
400
(15.7)
302
(11.9)
3,196
(125.8)
Average rainy days (≥ 0.1 mm) 15 13 21 22 20 18 16 25 14 27 25 22 238
Source: World Meteorological Organization (UN)[8]
Climate data for Libreville, Gabon in Africa
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Average high °C (°F) 29.5
(85.1)
30.0
(86.0)
30.2
(86.4)
30.1
(86.2)
29.4
(84.9)
27.6
(81.7)
26.4
(79.5)
26.8
(80.2)
27.5
(81.5)
28.0
(82.4)
28.4
(83.1)
29.0
(84.2)
28.58
(83.44)
Daily mean °C (°F) 26.8
(80.2)
27.0
(80.6)
27.1
(80.8)
26.6
(79.9)
26.7
(80.1)
25.4
(77.7)
24.3
(75.7)
24.3
(75.7)
25.4
(77.7)
25.7
(78.3)
25.9
(78.6)
26.2
(79.2)
25.95
(78.71)
Average low °C (°F) 24.1
(75.4)
24.0
(75.2)
23.9
(75.0)
23.1
(73.6)
24.0
(75.2)
23.2
(73.8)
22.1
(71.8)
21.8
(71.2)
23.2
(73.8)
23.4
(74.1)
23.4
(74.1)
23.4
(74.1)
23.30
(73.94)
Average rainfall mm (inches) 250.3
(9.85)
243.1
(9.57)
363.2
(14.30)
339.0
(13.35)
247.3
(9.74)
54.10
(2.13)
6.600
(0.26)
13.70
(0.54)
104.0
(4.09)
427.2
(16.82)
490.0
(19.29)
303.2
(11.94)
2,841.7
(111.88)
Average rainy days (≥ 0.1 mm) 17.9 14.8 19.5 19.2 16.0 3.70 1.70 4.90 14.5 25.0 22.6 17.6 177.4
Mean monthly sunshine hours 176.7 182.7 176.7 177.0 158.1 132.0 117.8 89.90 96.00 111.6 135.0 167.4 1,720.9
Source: World Meteorological Organization (UN),[9] Hong Kong Observatory[10]

Crossing-the-Equator ceremonies

Certain navies, such as the Royal Navy, Russian Navy, and US Navy, have a tradition of holding ceremonies aboard ships to mark sailors' first crossing of the Equator. These ceremonial events have in the past been notorious for their brutality. Milder line-crossing ceremonies, typically featuring King Neptune, are also held for passengers' entertainment on some civilian ocean liners and cruise ships.

See also

References

  1. ^ "Definition of equator". OxfordDictionaries.com. Retrieved 5 May 2018.
  2. ^ "Equator". National Geographic - Education. Retrieved 29 May 2013.
  3. ^ The IAU 2009 system of astronomical constants:
  4. ^ IERS Conventions
  5. ^ Instituto Geográfico Militar de Ecuador (24 January 2005). "Memoria Técnica de la Determinación de la Latitud Cero" (in Spanish).
  6. ^ "Weather Information for Macapa".
  7. ^ Climatological Information for Macapa, Brazil - Hong Kong Observatory
  8. ^ "Weather Information for Pontianak".
  9. ^ "Weather Information for Libreville".
  10. ^ Climatological Information for Libreville, Gabon - Hong Kong Observatory

Sources

45th parallel north

The 45th parallel north is a circle of latitude that is 45 degrees north of Earth's equator. It crosses Europe, Asia, the Pacific Ocean, North America, and the Atlantic Ocean.

The 45th parallel north is often called the halfway point between the Equator and the North Pole, but the true halfway point is actually 16.2 km (10.1 mi) north of the 45th parallel because Earth is an oblate spheroid; that is, it bulges at the equator and is flattened at the poles.At this latitude, the Sun is visible for 15 hours 37 minutes during the summer solstice, and 8 hours 46 minutes during the winter solstice. The midday Sun stands 21.6° above the southern horizon at the December solstice, 68.4° at the June solstice, and exactly 45.0° at either equinox.

45th parallel south

The 45th parallel south is a circle of latitude that is 45° south of the Earth's equator.

It is the line that marks the theoretical halfway point between the equator and the South Pole. The true halfway point is 16.2 km (10.1 mi) south of this parallel because Earth is not a perfect sphere, but bulges at the equator and is flattened at the poles.Unlike its northern counterpart, almost all (97%) of it passes through open ocean. It crosses the South Atlantic Ocean, the Indian Ocean, Australasia (New Zealand and just south of Tasmania), the Southern Ocean, and Patagonia.

At this latitude, daytime lasts for 15 hours, 37 minutes during the December solstice and 8 hours, 46 minutes during the June solstice.

60th parallel north

The 60th parallel north is a circle of latitude that is 60 degrees north of Earth's equator. It crosses Europe, Asia, the Pacific Ocean, North America, and the Atlantic Ocean.

Although it lies approximately twice as far away from the Equator as from the North Pole, the 60th parallel is half as long as the Equator line. This is where the Earth bulges halfway as much as on the Equator.

At this latitude, the Sun is visible for 18 hours, 52 minutes during the June solstice and 5 hours, 52 minutes during the December solstice.

The maximum altitude of the Sun is 53.44° on 21 June and 6.56° on 21 December.The lowest latitude where white nights can be observed is approximately on this parallel.

Celestial equator

The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projection of the terrestrial equator into outer space. Due to Earth's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic (the plane of Earth's orbit).

An observer standing on Earth's equator visualizes the celestial equator as a semicircle passing through the zenith, the point directly overhead. As the observer moves north (or south), the celestial equator tilts towards the opposite horizon. The celestial equator is defined to be infinitely distant (since it is on the celestial sphere); thus, the ends of the semicircle always intersect the horizon due east and due west, regardless of the observer's position on Earth. (At the poles, the celestial equator coincides with the astronomical horizon.) At all latitudes, the celestial equator is a uniform arc or circle because the observer is only finitely far from the plane of the celestial equator, but infinitely far from the celestial equator itself.Astronomical objects near the celestial equator appear above the horizon from most places on earth, but they culminate (reach the meridian) highest near the equator. The celestial equator currently passes through these constellations:

These, by definition, are the most globally visible constellations.

Celestial bodies other than Earth also have similarly defined celestial equators.

Circle of latitude

A circle of latitude on Earth is an abstract east–west circle connecting all locations around Earth (ignoring elevation) at a given latitude.

Circles of latitude are often called parallels because they are parallel to each other; that is, any two circles are always the same distance apart. A location's position along a circle of latitude is given by its longitude. Circles of latitude are unlike circles of longitude, which are all great circles with the centre of Earth in the middle, as the circles of latitude get smaller as the distance from the Equator increases. Their length can be calculated by a common sine or cosine function. The 60th parallel north or south is half as long as the Equator (disregarding Earth's minor flattening by 0.3%). A circle of latitude is perpendicular to all meridians.

The latitude of the circle is approximately the angle between the Equator and the circle, with the angle's vertex at Earth's centre. The equator is at 0°, and the North Pole and South Pole are at 90° north and 90° south, respectively. The Equator is the longest circle of latitude and is the only circle of latitude which also is a great circle.

There are 89 integral (whole degree) circles of latitude between the equator and the Poles in each hemisphere, but these can be divided into more precise measurements of latitude, and are often represented as a decimal degree (e.g. 34.637°N) or with minutes and seconds (e.g. 22°14'26"S). There is no limit to how precisely latitude can be measured, and so there are an infinite number of circles of latitude on Earth.

On a map, the circles of latitude may or may not be parallel, and their spacing may vary, depending on which projection is used to map the surface of the Earth onto a plane. On an equirectangular projection, centered on the equator, the circles of latitude are horizontal, parallel, and equally spaced. On other cylindrical and pseudocylindrical projections, the circles of latitude are horizontal and parallel, but may be spaced unevenly to give the map useful characteristics. For instance, on a Mercator projection the circles of latitude are more widely spaced near the poles to preserve local scales and shapes, while on a Gall–Peters projection the circles of latitude are spaced more closely near the poles so that comparisons of area will be accurate. On most non-cylindrical and non-pseudocylindrical projections, the circles of latitude are neither straight nor parallel.

Arcs of circles of latitude are sometimes used as boundaries between countries or regions where distinctive natural borders are lacking (such as in deserts), or when an artificial border is drawn as a "line on a map", which was made in massive scale during the 1884 Berlin Conference, regarding huge parts of the African continent. North American nations and states have also mostly been created by straight lines, which are often parts of circles of latitudes. For instance, the northern border of Colorado is at 41°N while the southern border is at 37°N. Roughly half the length of border between the United States and Canada follows 49°N.

Decimal degrees

Decimal degrees (DD) express latitude and longitude geographic coordinates as decimal fractions and are used in many geographic information systems (GIS), web mapping applications such as OpenStreetMap, and GPS devices. Decimal degrees are an alternative to using degrees, minutes, and seconds (DMS). As with latitude and longitude, the values are bounded by ±90° and ±180° respectively.

Positive latitudes are north of the equator, negative latitudes are south of the equator. Positive longitudes are east of Prime Meridian, negative longitudes are west of the Prime Meridian. Latitude and longitude are usually expressed in that sequence, latitude before longitude.

Declination

In astronomy, declination (abbreviated dec; symbol δ) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the celestial equator, along the hour circle passing through the point in question.

The root of the word declination (Latin, declinatio) means "a bending away" or "a bending down". It comes from the same root as the words incline ("bend toward") and recline ("bend backward").In some 18th and 19th century astronomical texts, declination is given as North Pole Distance (N.P.D.), which is equivalent to 90 - (declination). For instance an object marked as declination -5 would have a NPD of 95, and a declination of -90 (the south celestial pole) would have a NPD of 180.

Ecliptic

The ecliptic is the mean plane of the apparent path in the sky that the Sun follows over the course of one year; it is the basis of the ecliptic coordinate system. This plane of reference is coplanar with Earth's orbit around the Sun (and hence the Sun's apparent path around Earth). The ecliptic is not normally noticeable from Earth's surface because the planet's rotation carries the observer through the daily cycles of sunrise and sunset, which obscure the Sun's apparent motion against the background of stars during the year.

Equinox

An equinox is commonly regarded as the instant of time when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun. This occurs twice each year: around 21 March and 22–23 September. In other words, it is the moment at which the center of the visible Sun is directly above the Equator. In the northern hemisphere, the equinox in March is called the Vernal or Spring Equinox; the September equinox is called the Autumnal or Fall Equinox.

However, because the Moon (and to a lesser extent the other planets) cause the motion of the Earth to vary from a perfect ellipse, the equinox is now officially defined by the Sun's more regular ecliptic longitude rather than by its declination. The instants of the equinoxes are currently defined to be when the longitude of the Sun is 0° and 180°. There are tiny (up to 1¼ arcsecond) variations in the Sun's latitude (discussed below), which means the Sun's center is rarely precisely over the equator under the official definition. The two understandings of the equinox can lead to discrepancies of up to 69 seconds.

On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the Sun, atmospheric refraction, and the rapidly changing duration of the length of day that occurs at most latitudes around the equinoxes. The word is derived from the Latin aequinoctium, from aequus (equal) and nox (genitive noctis) (night).

Galactic coordinate system

The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way galaxy, and the fundamental plane parallel to an approximation of the galactic plane but offset to its north. It uses the right-handed convention, meaning that coordinates are positive toward the north and toward the east in the fundamental plane.

Intertropical Convergence Zone

The Intertropical Convergence Zone (ITCZ), known by sailors as the doldrums or the calms, is the area encircling Earth near the Equator, where the northeast and southeast trade winds converge.

The ITCZ was originally identified from the 1920s to the 1940s as the "Intertropical Front" ("ITF"), but after the recognition in the 1940s and the 1950s of the significance of wind field convergence in tropical weather production, the term ITCZ was then applied. When it lies near the Equator, it is called the near-equatorial trough. Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon trough, a usage more common in Australia and parts of Asia. In the seamen's speech, the zone is referred to as the doldrums because of its erratic (monotonous) weather patterns with stagnant calms and violent thunderstorms.

The ITCZ appears as a band of clouds, usually thunderstorms, that encircle the globe near the Equator. In the Northern Hemisphere, the trade winds move in a southwestward direction from the northeast, while in the Southern Hemisphere, they move northwestward from the southeast. When the ITCZ is positioned north or south of the Equator, these directions change according to the Coriolis effect imparted by Earth's rotation. For instance, when the ITCZ is situated north of the Equator, the southeast trade wind changes to a southwest wind as it crosses the Equator. The ITCZ is formed by vertical motion largely appearing as convective activity of thunderstorms driven by solar heating, which effectively draw air in; these are the trade winds. The ITCZ is effectively a tracer of the ascending branch of the Hadley cell and is wet. The dry descending branch is the horse latitudes.

The location of the ITCZ gradually varies with the seasons, roughly corresponding with the location of the thermal equator. As the heat capacity of the oceans is greater than air over land, migration is more prominent over land. Over the oceans, where the convergence zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures. Sometimes, a double ITCZ forms, with one located north and another south of the Equator, one of which is usually stronger than the other. When this occurs, a narrow ridge of high pressure forms between the two convergence zones.

Latitude

In geography, latitude is a geographic coordinate that specifies the north–south position of a point on the Earth's surface. Latitude is an angle (defined below) which ranges from 0° at the Equator to 90° (North or South) at the poles. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Latitude is used together with longitude to specify the precise location of features on the surface of the Earth. On its own, the term latitude should be taken to be the geodetic latitude as defined below. Briefly, geodetic latitude at a point is the angle formed by the vector perpendicular (or normal) to the ellipsoidal surface from that point, and the equatorial plane. Also defined are six auxiliary latitudes which are used in special applications.

Line-crossing ceremony

The line-crossing ceremony is an initiation rite that commemorates a person's first crossing of the Equator. The tradition may have originated with ceremonies when passing headlands, and become a "folly" sanctioned as a boost to morale, or have been created as a test for seasoned sailors to ensure their new shipmates were capable of handling long rough times at sea. Equator-crossing ceremonies, typically featuring King Neptune, are common in the navy and are also sometimes carried out for passengers' entertainment on civilian ocean liners and cruise ships. They are also performed in the merchant navy and aboard sail training ships.

Throughout history, line-crossing ceremonies have sometimes become dangerous hazing rituals. Most modern navies have instituted regulations that prohibit physical attacks on sailors undergoing the line-crossing ceremony.

Longitude

Longitude (, AU and UK also ), is a geographic coordinate that specifies the east–west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ). Meridians (lines running from pole to pole) connect points with the same longitude. By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was allocated the position of 0° longitude. The longitude of other places is measured as the angle east or west from the Prime Meridian, ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. Specifically, it is the angle between a plane through the Prime Meridian and a plane through both poles and the location in question. (This forms a right-handed coordinate system with the z-axis (right hand thumb) pointing from the Earth's center toward the North Pole and the x-axis (right hand index finger) extending from the Earth's center through the Equator at the Prime Meridian.)

A location's north–south position along a meridian is given by its latitude, which is approximately the angle between the local vertical and the equatorial plane.

If the Earth were perfectly spherical and radially homogeneous, then the longitude at a point would be equal to the angle between a vertical north–south plane through that point and the plane of the Greenwich meridian. Everywhere on Earth the vertical north–south plane would contain the Earth's axis. But the Earth is not radially homogeneous and has rugged terrain, which affect gravity and so can shift the vertical plane away from the Earth's axis. The vertical north–south plane still intersects the plane of the Greenwich meridian at some angle; that angle is the astronomical longitude, calculated from star observations. The longitude shown on maps and GPS devices is the angle between the Greenwich plane and a not-quite-vertical plane through the point; the not-quite-vertical plane is perpendicular to the surface of the spheroid chosen to approximate the Earth's sea-level surface, rather than perpendicular to the sea-level surface itself.

Mercator projection

The Mercator projection is a cylindrical map projection presented by the Flemish geographer and cartographer Gerardus Mercator in 1569. It became the standard map projection for nautical navigation because of its ability to represent lines of constant course, known as rhumb lines or loxodromes, as straight segments that conserve the angles with the meridians. Although the linear scale is equal in all directions around any point, thus preserving the angles and the shapes of small objects (making it a conformal map projection), the Mercator projection distorts the size of objects as the latitude increases from the Equator to the poles, where the scale becomes infinite. So, for example, landmasses such as Greenland and Antarctica appear much larger than they actually are, relative to landmasses near the equator such as Central Africa.

Northern Hemisphere

The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's North Pole.Owing to the Earth's axial tilt, winter in the Northern Hemisphere lasts from the December solstice (typically December 21 UTC) to the March equinox (typically March 20 UTC), while summer lasts from the June solstice through to the September equinox (typically September 23 UTC). The dates vary each year due to the difference between the calendar year and the astronomical year.

Its surface is 60.7% water, compared with 80.9% water in the case of the Southern Hemisphere, and it contains 67.3% of Earth's land.

Orbital inclination

Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object.

For a satellite orbiting the Earth directly above the equator, the plane of the satellite's orbit is the same as the Earth's equatorial plane, and the satellite's orbital inclination is 0°. The general case for a circular orbit is that it is tilted, spending half an orbit over the northern hemisphere and half over the southern. If the orbit swung between 20° north latitude and 20° south latitude, then its orbital inclination would be 20°.

Right ascension

Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question.

When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system.

An old term, right ascension (Latin: ascensio recta) refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle. It contrasts with oblique ascension, the point on the celestial equator that rises with any celestial object as seen from most latitudes on Earth, where the celestial equator intersects the horizon at an oblique angle.

Tropic of Cancer

The Tropic of Cancer, which is also referred to as the Northern Tropic, is the most northerly circle of latitude on Earth at which the Sun can be directly overhead. This occurs on the June solstice, when the Northern Hemisphere is tilted toward the Sun to its maximum extent. It is currently 23°26′12.5″ (or 23.43681°) north of the Equator.

Its Southern Hemisphere counterpart, marking the most southerly position at which the Sun can be directly overhead, is the Tropic of Capricorn. These tropics are two of the five major circles of latitude that mark maps of Earth; the others being the Arctic and Antarctic Circles and the Equator. The positions of these two circles of latitude (relative to the Equator) are dictated by the tilt of Earth's axis of rotation relative to the plane of its orbit.

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