Perhaps you can't believe that longitude is really all that simple. So as not to disappoint anyone, we'll conclude by mentioning that there are several different types of longitude. Fortunately, the differences implied by these qualifying terms is very small, and in most cases non-existent. However, to understand the terminology, we should understand the differences between astronomical, geodetic, geographic, and geocentric longitude. Geodetic longitude is that which was defined above.
Geographic longitude is another name for geodetic longitude.
Whenever the term longitude is used in an unqualified manner, it should be taken to mean geodetic longitude as defined above. While the definition of geocentric longitude is somewhat different than that given above, the end result turns out to be the same as geodetic longitude. P, the angle between the line from point P to the center of the earth and the prime meridian determines the longitude. Due to the nature of the ellipsoid model which we use, i.e. an ellipsoid of revolution, this turns out to be the same as the angle between the planes we used above. A different model, a triaxial ellipsoid for example, would cause a difference in these two longitudes.