The angle between the equatorial plane and a line joining a point on the ellipsoid to the center of the ellipsoid is called ____ latitude.

• A. Astronomic
• B. Authalic
• C. Geodetic
• D. Geocentric

Answer: (D)

The line from the Earth's center to a point on the surface defines a radius vector, and the angle that this radius vector makes with the equatorial plane is called the geocentric latitude. This name comes from using the center of the Earth as the reference point for measuring the angle. It differs from the geodetic latitude, which is defined by the angle between the equatorial plane and the normal to the reference ellipsoid at that surface point (not the line to the center). On a perfect sphere these two latitudes would coincide, but for the oblate Earth they are generally different. Astronomical latitude relates to observations of stars and the local vertical, while authalic latitude is tied to a sphere of equal area to the ellipsoid, so those definitions involve different reference constructions.