# Geometry as Motion

Nothing seems as static and as solid as geometry—there is even a subfield of geometry known as “solid geometry”. Geometric objects seem fixed in time and in space. Yet the very first algebraic description of geometry was born out of kinematic constructions of curves as René Descartes undertook the solution of an ancient Greek problem posed by Pappus of Alexandria (c. 290 – c. 350) that had remained unsolved for over a millennium. In the process, Descartes’ invented coordinate geometry.

Descartes used kinematic language in the process of drawing  curves, and he even talked about the speed of the moving point. In this sense, Descartes’ curves are trajectories.

The problem of Pappus relates to the construction of what were known as loci, or what today we call curves or functions. Loci are a smooth collection of points. For instance, the intersection of two fixed lines in a plane is a point. But if you allow one of the lines to move continuously in the plane, the intersection between the moving line and the fixed line sweeps out a continuous succession of points that describe a curve—in this case a new line. The problem posed by Pappus was to find the appropriate curve, or loci, when multiple lines are allowed to move continuously in the plane in such a way that their movements are related by given ratios. It can be shown easily in the case of two lines that the curves that are generated are other lines. As the number of lines increases to three or four lines, the loci become the conic sections: circle, ellipse, parabola and hyperbola. Pappus then asked what one would get if there were five such lines—what type of curves were these? This was the problem that attracted Descartes.

What Descartes did—the step that was so radical that it reinvented geometry—was to fix lines in position rather than merely in length. To us, in the 21st century, such an act appears so obvious as to remove any sense of awe. But by fixing a line in position, and by choosing a fixed origin on that line to which other points on the line were referenced by their distance from that origin, and other lines were referenced by their positions relative to the first line, then these distances could be viewed as unknown quantities whose solution could be sought through algebraic means. This was Descartes’ breakthrough that today is called “analytic geometry”— algebra could be used to find geometric properties.

Newton too viewed mathematical curves as living things that changed in time, which was one of the central ideas behind his fluxions—literally curves in flux.

Today, we would call the “locations” of the points their “coordinates”, and Descartes is almost universally credited with the discovery of the Cartesian coordinate system. Cartesian coordinates are the well-known grids of points, defined by the x-axis and the y-axis placed at right angles to each other, at whose intersection is the origin. Each point on the plane is defined by a pair of numbers, usually represented as (x, y). However, there are no grids or orthogonal axes in Descartes’ Géométrie, and there are no pairs of numbers defining locations of points. About the most Cartesian-like element that can be recognized in Descartes’ La Géométrie is the line of reference AB, as in Fig. 1. Fig. 1 The first figure in Descartes’ Géométrie that defines 3 lines that are placed in position relative to the point marked A, which is the origin. The point C is one point on the loci that is to be found such that it satisfies given relationships to the 3 lines.

In his radical new approach to loci, Descartes used kinematic language in the process of drawing the curves, and he even talked about the speed of the moving point. In this sense, Descartes’ curves are trajectories, time-dependent things. Important editions of Descartes’ Discourse were published in two volumes in 1659 and 1661 which were read by Newton as a student at Cambridge. Newton also viewed mathematical curves as living things that changed in time, which was one of the central ideas behind his fluxions—literally curves in flux.