We have assembled a data set of 1821 magnetopause crossings. Separate fits to subsets of this data set determine the magnetopause locations as a function of solar wind dynamic pressure and interplanetary magnetic field orientation. Solar wind dynamic pressure variations produce self-similar magnetopause motion on time scales of one hour or longer. We verify the pressure balance relationship between the solar wind dynamic pressure and the location of the subsolar magnetopause. We quantify the relationship between the IMF Bz, region 1 Birkeland current strength, the position of the subsolar magnetopause, and the shape of the dayside magnetosphere. Cross sections of the dayside magnetopause in planes perpendicular to the Earth-Sun line are oblate. ¿ American Geophysical Union 1991