The latitude of the Equator is by definition 0° (zero degrees). The Equator is one of the five notable circles of latitude on Earth, with the others being the two Polar Circles (the Arctic Circle and the Antarctic Circle) and the two Tropical Circles (The Tropic of Cancer and the Tropic of Capricorn). The Equator is the only line of latitude that is also a great circle. The imaginary circle obtained when the Earth's equator is projected onto the sky is called the celestial equator.
In its seasonal apparent movement across the sky the sun passes over the Equator twice each year, at the March and September equinoxes. At the moment of the equinox, light rays from the center of the sun are perpendicular to the surface of the earth at the point on the equator experiencing solar noon.
The Equator, like the Tropics, is not fixed. the true equatorial plane is always perpendicular to the Earth's spin axis; this axis is fairly stable but its position drifts about 15 meters (49 feet) during a year and the equator shifts likewise.
Equatorial seasons and climate
Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution. During the year the northern and southern hemispheres are inclined toward or away from the sun according to Earth's position in its orbit. The hemisphere inclined 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 equinox the Earth's axis is not tilted toward the sun and the day is about 12 hours long over most of the Earth.
The Equator lies mostly on the three largest oceans: the Pacific Ocean, the Atlantic Ocean, and the Indian Ocean. The highest point on the Equator is at the elevation of 4,690 meters (15,387 feet), found on the slopes of Volcán Cayambe [summit 5,790 meters (18,996 feet)] in Ecuador. This is slightly above the snow line, and is the only place on the Equator where snow lies on the ground.
Lowlands around the Equator generally have a tropical rainforest climate, also known as an equatorial climate, through cold currents cause some regions to have tropical monsoon climates with a dry season in the middle of the year. Usually, average annual temperature is around 30 °C (86 °F) during the day and 23 °C (73 °F) at night. Rainfall is very high, usually from 2,500 to 3,500 mm per year. Average rainy days are around 200 per year and average annual sunshine hours around 2000.
Equatorial countries and territories
The Equator traverses the land and/or territorial waters of 14 countries. Starting at the Prime Meridian and heading eastwards, the Equator passes through:
Despite its name, no part of Equatorial Guinea's territory lies on the Equator. However, its island of Annobón is 155 kilometers (100mi) south of the Equator, and the rest of the country lies to the north.
Latitude
Latitude (φ) 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.
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| A graticule on a sphere or an ellipsoid. The lines from pole to pole are lines of constant longitude, or meridians. The circles parallel to the equator are lines of constant latitude, or parallels. The graticule determines the latitude and longitude of position on the surface. |
The graticule, the mesh formed by the lines of constant latitude and constant longitude, is constructed by reference to the rotation axis of the Earth. The primary reference points are the poles where the axis of rotation of the Earth intersects the reference surface. Planes which contain the rotation axis intersect the surface in the meridians and the angle between any one meridian plane and that through Greenwich (the Prime Meridian) defines the longitude: meridians are lines of constant longitude. The plane through the centre of the Earth and orthogonal to the rotation axis intersects the surface in a great circle called the equator. Planes parallel to the equatorial plane intersect the surface in circles of constant latitude; these are the parallels. The equator has a latitude of 0°, the North pole has a latitude of 90° north (written 90° N or +90°), and the South pole has a latitude of 90° south (written 90° S or −90°). The latitude of an arbitrary point is the angle between the equatorial plane and the radius to that point.
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| A perspective view of the Earth showing how latitude (φ) and longitude (λ) are defined on a spherical model. The graticule spacing is 10 degrees. |
Longitude
Longitude 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). Points with the same longitude lie in lines running from the North Pole to the South Pole. By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was intended to establish the position of zero degrees longitude. The longitude of other places was to be 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 containing the Prime Meridian and a plane containing the North Pole, South Pole and the location in question.
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| Drawing of Earth with Longitudes |
Circles of latitude
A circle of latitude on the Earth is an imaginary east-west circle connecting all locations (not taking into account elevation) having a given latitude. A location's position along a circle of latitude is given by its longitude.
Circles of latitude are often called parallels because they are parallel to each other – that is, any two parallels are everywhere the same distance apart. (Since the Earth is not spherical the distance from the equator to 10 degrees north is slightly less than the distance from 10 to 20 degrees north.)
Circles of latitude become smaller the farther they are from the equator and the closer they are to the poles. A circle of latitude is perpendicular to all meridians, and is hence a special case of a loxodrome.
The latitude of the circle is (roughly) the angle between the Equator and the circle, with the angle's vertex at the Earth's centre. The Equator is at 0°, and the North and South Pole are at 90° north and 90° south respectively. 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.
Great circle
A great circle, also known as an orthodrome or Riemannian circle, of a sphere is the intersection of the sphere and a plane which passes through the center point of the sphere. (A small circle is the intersection of the sphere and a plane which does not pass through the center.) Any diameter of any great circle coincides with a diameter of the sphere, and therefore all great circles have the same circumference as each other, and have the same center as the sphere. A great circle is the largest circle that can be drawn on any given sphere.
Solstice
A solstice is an astronomical event that occurs twice each year as the Sun reaches its highest or lowest excursion relative to the celestial equator on the celestial sphere. As a result, on the day of the solstice, the Sun appears to have reached its highest or lowest annual altitude in the sky above the horizon at local solar noon. The word solstice is derived from the Latin sol (sun) and sistere (to stand still), because at the solstices, the Sun stands still in declination; that is, the seasonal movement of the Sun's path (as seen from Earth) comes to a stop before reversing direction. The solstices, together with the equinoxes, are connected with the seasons. In many cultures the solstices mark either the beginning or the midpoint of winter and summer.
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Illumination of Earth by Sun at the northern solstice.
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| Illumination of Earth by Sun at the southern solstice. |
The term solstice can also be used in a broader sense, as the date (day) when this occurs. The day of the solstice is either the longest day of the year (in summer) or the shortest day of the year (in winter) for any place outside of the tropics.
Equinox
An equinox occurs twice a year (around 20 March and 22 September), when the plane of the Earth's equator passes the center of the Sun. At this time the tilt of the Earth's axis is inclined neither away from nor towards the Sun. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, night and day are about equal length. Times of sunset and sunrise vary with an observer's location (longitude and latitude), so these dates likewise depend on location and do not exist for locations close to the Equator. To avoid this ambiguity the term equilux is sometimes used in this sense.
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| During an equinox, the Earth is not tilted toward or away from the Sun and the length of the day is the same at all points on the Earth's surface. |
During an equinox, the Earth is not tilted toward or away from the Sun and the length of the day is the same at all points on the Earth's surface.
Sources:
http://en.wikipedia.org/wiki/Equator
http://en.wikipedia.org/wiki/Latitude
http://en.wikipedia.org/wiki/Longitude
http://en.wikipedia.org/wiki/Circle_of_latitude
http://en.wikipedia.org/wiki/Great_circle
http://en.wikipedia.org/wiki/Solstice
http://en.wikipedia.org/wiki/Equinox
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