Equator

Countries and territories that touch the Equator (red) or the prime meridian (blue), which intersect at "Null Island".
Road sign marking the equator near Nanyuki, Kenya
The equator marked as it crosses Ilhéu das Rolas, in São Tomé and Príncipe
The Marco Zero monument marking the equator in Macapá, Brazil
GPS reading taken on the Equator close to the Quitsato Sundial, in Cayambe, Ecuador
Diagram of the seasons, depicting the situation at the December solstice. Regardless of the time of day (i.e. Earth's rotation on its axis), the North Pole will be dark, and the South Pole will be illuminated; see also polar night. In addition to the density of incident light, the dissipation of light in atmosphere is greater when it falls at a shallow angle.

Circle of latitude, about 40,075 km in circumference, that divides Earth into the Northern and Southern hemispheres.

- Equator

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Equinox

Gregorian calendar leap solstice
Contour plot of the hours of daylight as a function of latitude and day of the year, showing approximately 12 hours of daylight at all latitudes during the equinoxes
Earth at the March 2019 equinox
Celestial sphere
Diagram of the difference between the Sun's celestial longitude being zero and its declination being zero. Its celestial latitude never exceeds 1.2 arcseconds, but is exaggerated in this diagram.
When Saturn is at equinox its rings reflect little sunlight, as seen in this image by Cassini in 2009.
Illumination of Earth by the Sun at the equinox
The relation between the Earth, Sun, and stars at the March equinox. From Earth's perspective, the Sun appears to move along the ecliptic (red), which is tilted compared to the celestial equator (white).
Diagram of the Earth's seasons as seen from the north. Far right: December solstice.
Diagram of the Earth's seasons as seen from the south. Far left: June solstice.
Day arc at 0° latitude (equator)
Day arc at 20° latitude
Day arc at 50° latitude
Day arc at 70° latitude
Day arc at 90° latitude (pole)

A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator (rather than north or south of the equator).

Spheroid

Ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters.

The planet Jupiter is an oblate spheroid with a flattening of 0.06487
A rugby ball.

The current World Geodetic System model uses a spheroid whose radius is 6,378.137 km at the Equator and 6,356.752 km at the poles.

Tropics

World map with the intertropical zone highlighted in crimson
Areas of the world with tropical climates
A graph showing the zonally averaged monthly precipitation. The tropics receive more precipitation than higher latitudes. The precipitation maximum, which follows the solar equator through the year, is under the rising branch of the Hadley circulation; the sub-tropical minima are under the descending branch and cause the desert areas.
Aerial view of Bora Bora, French Polynesia
Tropical sunset over the sea in Kota Kinabalu, Malaysia
Coconut palms in the warm, tropical climate of northern Brazil
Distribution of tropical wet forests
Juruá River in Brazil surrounded by dense tropical rainforests. The Brazilian rainforests are home to uncontacted tribes to this day.

The tropics are the regions of Earth surrounding the Equator.

Tropic of Capricorn

Circle of latitude that contains the subsolar point at the December (or southern) solstice.

World map showing the Tropic of Capricorn
Relationship of Earth's axial tilt (ε) to the tropical and polar circles

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

Also referred to as the Northern Tropic, is the most northerly circle of latitude on Earth at which the Sun can be directly overhead.

World map showing the Tropic of Cancer
Relationship of Earth's axial tilt (ε) to the tropical and polar circles
Carretera 83 (Vía Corta) Zaragoza-Victoria, km 27+800. Of the Tropic of Cancer's intersections with Mexican federal highways, this is the only one where it is precisely marked and the drift from 2005 to 2010 can be seen.
Road sign south of Dakhla, Western Sahara marking the Tropic of Cancer. The sign was placed by Budapest-Bamako rally participants; thus, the inscription is in English and Hungarian.
Road Sign near Mehsana City in Gujarat State, India Mehsana, Gujarat
Sign marking the Tropic of Cancer a few kilometres from Rann of Kutch, Gujarat, India
Sign marking the Tropic of Cancer in Madhya Pradesh, India
Sign marking the Tropic of Cancer on National Highway 34 in Nadia District, West Bengal, India
Ruisui Tropic of Cancer Marker in Ruisui Township, Hualien County, Taiwan

Using a continuously updated formula, the circle is currently north of the Equator.

Northern Hemisphere

Northern Hemisphere shaded blue. The hemispheres appear unequal here because Antarctica is not shown.
Northern Hemisphere from above the North Pole
Northern hemisphere glaciation during the last ice ages. The setup of 3 to 4 kilometer thick ice sheets caused a sea level lowering of about 120 m.
Canadian Rockies in North America

The Northern Hemisphere is the half of Earth that is north of the Equator.

Celestial equator

The celestial equator is currently inclined by about 23.44° to the ecliptic plane. The image shows the relations between Earth's axial tilt (or obliquity), rotation axis, and orbital plane.

The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth.

Arctic Circle

One of the two polar circles and the most northerly of the five major circles of latitude as shown on maps of Earth.

Map of the Arctic, with the Arctic Circle in blue and the 10°C July mean isotherm in red
Relationship of Earth's axial tilt (ε) to the tropical and polar circles
At night, bright aurora borealis are a fairly common sight in the Arctic Circle. The picture of the northern lights in Rovaniemi.
Cylindrical projection showing the Arctic Circle in red
Northern Polar Circle Globe on Vikingen island marking the Arctic Circle in Norway
Arctic Circle sign by the Inland Line railway, Sweden
The white borderline of the Arctic Circle at the Santa Claus Village in Rovaniemi, Finland
Arctic Circle sign in the Republic of Karelia, Russia
Arctic Circle sign by the Yamalo-Nenets Autonomous Okrug, Russia
A sign in the Sakha Republic (Yakutia), Russia
Arctic Circle marker on island of Grímsey in Iceland

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

Rotation

Circular movement of an object around an axis of rotation.

A sphere rotating (spinning) about an axis
Rotation (angular displacement) of a planar figure around a point
Rotational Orbit v Spin
Relations between rotation axis, plane of orbit and axial tilt (for Earth).
Star trails caused by the Earth's rotation during the camera's long exposure time.
Euler rotations of the Earth. Intrinsic (green), Precession (blue) and Nutation (red)
The principal axes of rotation in space

This rotation induces a centrifugal acceleration in the reference frame of the Earth which slightly counteracts the effect of gravitation the closer one is to the equator.

Antarctic Circle

Most southerly of the five major circles of latitude that mark maps of Earth.

Map of the Antarctic with the Antarctic Circle in blue.
Relationship of Earth's axial tilt (ε) to the tropical and polar circles
An iceberg near the Antarctic Circle north of Detaille Island

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