In practice, real-world objects are not point sources, and finite source size effects will set a limit to how large an amplification can occur for very close alignment, but some microlensing events can cause a brightening by a factor of hundreds. Unlike gravitational macrolensing where the lens is a galaxy or cluster of galaxies, in microlensing u changes significantly in a short period of time. The relevant time scale is called the Einstein time , and it's given by the time it takes the lens to traverse an angular distance relative to the source in the sky. For typical microlensing events, is on the order of a few days to a few months. The function u(t) is simply determined by the Pythagorean theorem: The minimum value of u, called umin, determines the peak brightness of the event. In a typical microlensing event, the light curve is well fit by assuming that the source is a point, the lens is a single point mass, and the lens is moving in a straight line: the point source-point lens approximation. In these events, the only physically significant parameter that can be measured is the Einstein timescale . Since this observable is a degenerate function of the lens mass, distance, and velocity, we cannot determine these physical parameters from a single event. However, in some extreme events, may be measurable while other extreme events can probe an additional parameter: the size of the Einstein ring in the plane of the observer, known as the Projected Einstein radius: . This parameter describes how the event will appear to be different from two observers at different locations, such as a satellite observer. The projected Einstein radius is related to the physical parameters of the lens and source by It is mathematically convenient to use the inverses of some of these quantities. These are the Einstein proper motion and the Einstein parallax These vector quantities point in the direction of the relative motion of the lens with respect to the source. Some extreme microlensing events can only constrain one component of these vector quantities. Should these additional parameters be fully measured, the physical parameters of the lens can be solved yielding the lens mass, parallax, and proper motion as Extreme microlensing events In a typical microlensing event, the light curve is well fit by assuming that the source is a point, the lens is a single point