# SelenographicCoordinates

``public struct SelenographicCoordinates``

AA, Chapter 53 (p.371): The mean period of rotation of the Moon is equal to the mean sidereal period of revolution around the Earth, and the mean plane of the lunar equator intersects the ecliptic at a constant incliation (see @MeanLunarEquatorInclination) […]. On the average, therefore, the same hemisphere of the Moon is always turned towards the Earth. However, apparent oscillations known as optical librations, which are due to variations in the geometric position of the Earth relative to the lunar surface during the course of the orbital motion of the Moon, allow about 59% of the surface to be observed from tbe Earth. The mean center of the Moon’s apparent disk is the origin of the system of selenographic coordinates on the surface of the Moon [see below]. The displacement, at any time, of the mean center of the disk from the apparent center, represents the amount of libration, and is measured by the selenographic coordinates of the apparent center of the disk at that time. AA (p.372): The selenographic longitude and latitude of the Earth, as given in the almanacs, are the geocentric selenographic coordinates of the apparent central point of the disk. At this point on the surface of the Moon, the Earth is in the zenith.

• ``` longitude ```

Selenographic longitude are measured from the lunar meridian that passes through the mean center of the apparent disk, positive in the direction of towards Mare Crisium, that is towards the west of geocentric celestial sphere.

#### Declaration

Swift

``public let longitude: Degree``
• ``` latitude ```

Selenographic latitudes are measured from the lunar equator, positive towards the north, that is, they are positive in the hemisphere containing Mare Serenitatis.

#### Declaration

Swift

``public let latitude: Degree``
• ``` colongitude ```

The associated colongitude.

#### Declaration

Swift

``public var colongitude: Degree { get }``
• ``` zero ```

A Selenographic coordinates centered at the equator and prime meridian.

#### Declaration

Swift

``public static var zero: SelenographicCoordinates { get }``
• ``` init(longitude:latitude:) ```

Returns a SelenographicCoordinates object.

#### Declaration

Swift

``public init(longitude: Degree, latitude: Degree)``

#### Parameters

 ``` longitude ``` The longitude. ``` latitude ``` The latitude