Equator

equatorial planeThe Equatorequatorialequatorial zone0equatorial beltEquatorial lineLine0°
The equator of a rotating spheroid (such as a planet) is the parallel (circle of latitude) at which latitude is defined to be 0°.wikipedia
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Circle of latitude

parallelparallelscircles of latitude
The equator of a rotating spheroid (such as a planet) is the parallel (circle of latitude) at which latitude is defined to be 0°. 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).
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.

Southern Hemisphere

SouthernaustralS. Hemisphere
It is the imaginary line on the spheroid, equidistant from its poles, dividing it into northern and southern hemispheres.
The Southern Hemisphere is the half of Earth that is south of the Equator.

Northern Hemisphere

NorthernNorth HemisphereNorthern Hemispheric
It is the imaginary line on the spheroid, equidistant from its poles, dividing it into northern and southern hemispheres.
The Northern Hemisphere is the half of Earth that is north of the Equator.

Latitude

latitudesSouthlatitudinal
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).
Latitude is an angle (defined below) which ranges from 0° at the Equator to 90° (North or South) at the poles.

Tropics

tropicaltropictropical zone
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 tropics are the region of the Earth surrounding the Equator.

Spheroid

oblate spheroidoblateprolate spheroid
The equator of a rotating spheroid (such as a planet) is the parallel (circle of latitude) at which latitude is defined to be 0°.
The current World Geodetic System model uses a spheroid whose radius is at the Equator and at the poles.

Tropic of Capricorn

Capricorn23.44° southsouth
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).
Its latitude is currently south of the Equator, but it is very gradually moving northward, currently at the rate of 0.47 arcseconds, or 15 metres, per year.

Tropic of Cancer

Cancertropic23.44° north
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).
It is currently north of the Equator.

Arctic Circle

circumpolarArcticCircumpolar arctic
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 position of the Arctic Circle is not fixed and currently runs north of the Equator.

Rotation

rotatingrotatespin
In other words, it is the intersection of the spheroid with the plane perpendicular to its axis of rotation and midway between its geographical poles.
This rotation induces a centrifugal acceleration in the reference frame of the Earth which slightly counteracts the effect of gravity the closer one is to the equator.

Celestial equator

equatorialequatorial planeequatorial sky
The plane of Earth's equator, when projected outwards to the celestial sphere, defines the celestial equator.
The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth.

Sunset

sundownsetssetting
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.
As viewed from the Equator, the equinox Sun sets exactly due west in both Spring and Autumn.

Equatorial bulge

bulgesbulges slightlybulging
Earth bulges slightly at the equator; the "average" diameter of Earth is, but the diameter at the equator is about greater than at the poles.
An equatorial bulge is a difference between the equatorial and polar diameters of a planet, due to the centrifugal force exerted by the rotation about the body's axis.

Earth

Earth's surfaceterrestrialworld
On Earth, the equator is about long, of which 78.8% lies across water and 21.3% over land.
There is a small flattening at the poles and bulging around the equator due to Earth's rotation.

September equinox

autumnal equinoxSouthward equinox23 September
In the cycle of Earth's seasons, the equatorial plane runs through the Sun twice per year: on the equinoxes in March and September.
At the equinox, the Sun as viewed from the equator rises due east and sets due west.

Antarctic Circle

AntarcticcircumpolarAntarctic regions
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 position of the Antarctic Circle is not fixed and currently runs south of the Equator.

Season

seasonsseasonalfour seasons
In the cycle of Earth's seasons, the equatorial plane runs through the Sun twice per year: on the equinoxes in March and September.
The two instants when the Sun is directly overhead at the Equator are the equinoxes.

Geographical pole

polespolepolar
It is the imaginary line on the spheroid, equidistant from its poles, dividing it into northern and southern hemispheres. In other words, it is the intersection of the spheroid with the plane perpendicular to its axis of rotation and midway between its geographical poles.
As with Earth's North and South Poles, they are usually called that body's "north pole" and "south pole", one lying 90 degrees in one direction from the body's equator and the other lying 90 degrees in the opposite direction from the equator.

Axial tilt

obliquityobliquity of the eclipticaxis
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 m during a year.
In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane.

Spaceport

launch complexlaunch sitecosmodrome
Sites near the equator, such as the Guiana Space Centre in Kourou, French Guiana, are good locations for spaceports as they have a fastest rotational speed of any latitude, 460 m/s.
Rockets can most easily reach satellite orbits if launched near the equator in an easterly direction, as this maximizes use of the Earth's rotational speed (465 m/s at the equator).

Guiana Space Centre

KourouGuiana Space CenterCentre Spatial Guyanais
Sites near the equator, such as the Guiana Space Centre in Kourou, French Guiana, are good locations for spaceports as they have a fastest rotational speed of any latitude, 460 m/s.
The Guiana location has the significant benefit of greatly increased payload capability, owing to the near equatorial position.

Orbital inclination

inclinationinclinedtilted
The added velocity reduces the fuel needed to launch spacecraft eastward (in the direction of Earth's rotation) to orbit, while simultaneously avoiding costly maneuvers to flatten inclination during missions such as the Apollo moon landings.
For a satellite orbiting a planet, the plane of reference is usually the plane containing the planet's equator.

Geographical mile

mile (geographical)miles
The geographical mile is defined as one arc-minute of the equator, so it has different values depending on which radius is assumed.
The geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator.

Equatorial Guinea

EquatoguineanRepublic of Equatorial GuineaGuinea Ecuatorial
Despite its name, no part of Equatorial Guinea lies on the equator.
Formerly the colony of Spanish Guinea, its post-independence name evokes its location near both the Equator and the Gulf of Guinea.

French Guiana

GuyaneGuianaFrench Guianese
Sites near the equator, such as the Guiana Space Centre in Kourou, French Guiana, are good locations for spaceports as they have a fastest rotational speed of any latitude, 460 m/s.
The department was considered particularly suitable for the purpose because it is near the equator and has extensive access to the ocean as a buffer zone.