Configuration of a microlensing event as seen by the observer, who does not resolve any of the components. The filled red circles represent the source S at equal time intervals as it moves relative to the lens L (black circle). The relative motion of the source consists of uniform proper motion in a straight line combined with annual parallax, which is why the source appears to move in a non-linear fashion. The major and minor images, I₁ and I₂, respectively, of the source are plotted as orange ellipses. Although S, L, I₁ and I₂ are all plotted with a finite size for illustrative purposes, the analysis in this paper ignores finite-size effects. At any single instant, I₁, S, L and I₂ form a straight line on the sky along with the centroids C_mic (asterisks), C_LS (plus signs) and C_LI2 (crosses). The centroids C_mic, C_LS and C_LI2 have been plotted for f_L/ f_S = 1. Angular distances defined in the text are marked in the diagram.