Editing Latitude (section)

==Latitude on the sphere{{anchor|Spherical}}==
[[File:latitude and longitude graticule on a sphere.svg|thumb|upright=0.9|right|A perspective view of the Earth showing how latitude (<math>\phi</math>) and longitude (<math>\lambda</math>) are defined on a spherical model. The graticule spacing is 10 degrees.]]

===The graticule on the sphere===
The graticule is formed by the lines of constant latitude and constant longitude, which are constructed with reference to the rotation axis of the Earth. The primary reference points are the [[Geographical pole|poles]] where the axis of rotation of the Earth intersects the reference surface. Planes which contain the rotation axis intersect the surface at the [[Meridian (geography)|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 perpendicular to the rotation axis intersects the surface at 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°&nbsp;N or +90°), and the [[South Pole]] has a latitude of 90° South (written 90°&nbsp;S or −90°). The latitude of an arbitrary point is the angle between the equatorial plane and the normal to the surface at that point: the normal to the surface of the sphere is along the radial vector.

The latitude, as defined in this way for the sphere, is often termed the spherical latitude, to avoid ambiguity with the geodetic latitude and the auxiliary latitudes defined in subsequent sections of this article.

===Named latitudes on the Earth===
[[File:December solstice geometry.svg|thumb|upright=1.35|right|The orientation of the Earth at the December solstice]]
Besides the equator, four other parallels are of significance:
:{| class="wikitable"
| [[Arctic Circle]] || 66° 34′ (66.57°) N
|-
| [[Tropic of Cancer]] ||23° 26′ (23.43°) N
|-
| [[Tropic of Capricorn]] || 23° 26′ (23.43°) S
|-
| [[Antarctic Circle]] || 66° 34′ (66.57°) S
|}
The plane of the Earth's orbit about the Sun is called the [[ecliptic]], and the plane perpendicular to the rotation axis of the Earth is the equatorial plane. The angle between the ecliptic and the equatorial plane is called variously the axial tilt, the obliquity, or the inclination of the ecliptic, and it is conventionally denoted by {{mvar|i}}. The latitude of the tropical circles is equal to {{mvar|i}} and the latitude of the polar circles is its complement (90° - ''i''). The axis of rotation varies slowly over time and the values given here are those for the current [[Epoch (astronomy)|epoch]]. The time variation is discussed more fully in the article on [[axial tilt]].{{efn|The value of this angle today is {{circle of latitude|tropical|convert}}. This figure is provided by [[Template:Circle of latitude]].}}

The figure shows the geometry of a [[cross section (geometry)|cross-section]] of the plane perpendicular to the ecliptic and through the centres of the Earth and the Sun at the December [[solstice]] when the Sun is overhead at some point of the [[Tropic of Capricorn]]. The south polar latitudes below the [[Antarctic Circle]] are in daylight, whilst the north polar latitudes above the Arctic Circle are in night. The situation is reversed at the June solstice, when the Sun is overhead at the Tropic of Cancer. Only at latitudes in between the two [[tropics]] is it possible for the Sun to be directly overhead (at the [[zenith]]).

On [[map projections]] there is no universal rule as to how meridians and parallels should appear. The examples below show the named parallels (as red lines) on the commonly used [[Mercator projection]] and the [[Transverse Mercator projection]]. On the former the parallels are horizontal and the meridians are vertical, whereas on the latter there is no exact relationship of parallels and meridians with horizontal and vertical: both are complicated curves.
{| style="text-align:left" style="margin: 1em auto 1em auto"
|-valign=top
! width="1%" |
! width="36%"|Normal Mercator
! width="3%"|
! width="36%" |Transverse Mercator
! width="1%" |
|-valign=top
|
| align="center" width="200px" | [[File:MercNormSph enhanced.png|center|200px]]
|
\
| align="center" width="200px" | [[File:MercTranSph enhanced.png|center|200px]]
|}