Notes on Navigation: Longitude of a Place ...

Is the arc of the Equator or the angle at the poles contained between the Prime meridians through that place. Longitudes are measured from 0° to 180°,and named  East or West according to the place being East or West of the Prime meridian.

This cutaway drawing shows that the latitude and longitude of any place are based on the sizes of two angles that originate at the center of the Earth. For New Orleans these angles are 30 degrees (north latitude) and 90 degrees (west longitude).

Notes on Navigation: Geocentric and Geographic Latitude of a place:

Geocentric  Latitude of a place

The angular distance between a point on the Earth's surface and the equator, using the center of the Earth as the vertex. It differs from geodetic latitude in that it accounts for the ellipsoidal shape of the Earth rather than considering it as a perfect sphere


Geographic Latitude of a place

The angle between the plane of the Equator and the vertical at that place. In navigation, the term Latitude implies as observed, that is the geographic latitude. 

Notes On Navigation: Meridians

Meridians are semi-great circles on the Earth, joining the two poles. The other half of the same great circle forms yet another meridian.

Prime Meridian:

Is the meridian which passes through Greenwich. The other meridians are named East or West from the prime meridian.

Notes on Navigation: Parallels of Latitude

Parallels of Latitude are small circles on Earth’s surface, the planes of which are parallel to the plane of the Equator. All parallels therefore run East-West. 

Notes on Navigation: Latitude and Longitude: The Geographic Grid

In order to measure accurately the position of any place on the surface of the earth, a grid system has been set up. It pinpoints location by using two coordinates: latitude and longitude. B. It is purely a human invention, but it is tied to two fixed points established by earth motions: the poles, or ends of the earth's rotational axis. 

1. Longitude represents east-west location, and it is shown on a map or globe by a series of north-south running lines that all come together at the North Pole and at the South Pole and are the widest apart at the equator -- these lines of longitude are called "meridians." Figure 1 -- meridians of longitude

2. Latitude represents north-south location, and it is shown on a map or globe by a series of east-west running lines that parallel the equator, which marks the midpoint between the two poles all around the earth's circumference -- these lines of latitude are called "parallels." Figure 2 -- parallels of latitude