We have made a semi-empirical analysis of our extensive body of observed cosmic ray intensity data from Pioneers 10 and 11 and related spectral information from other authors in order to infer the radius R of the modulation region surrounding the sun. During the period 1972--1985 the inferred values of R vary with time systematically and in a manner generally similar to that of sunspot numbers. The range of values of R is from 42 AU at the time of minimum solar activity (~1976) to 88 AU about 1.5 yr following the time of maximum solar activity (~1980). A specific, testable prediction is that Pioneer 10 will reach the modulation boundary in 1988 and will remain in its vicinity for several years thereafter.