"The figure A D B E [in Figure 9] represents the form of a planetary orbit, which is that of an oval or ellipse. The longest diameter is A B; the shorter diameter D E. The two points F and G are called the foci of the ellipse, around which, as two central points, the ellipse is formed. The sun is not placed in C, the center of the orbit, but at F, one of the foci of the ellipse. When the planet, therefore, is at A it is nearest the sun, and is said to be in its perihelion; its distance from the sun gradually increases until it reaches the point, B, when it is at its greatest distance the sun, and is said to be in its aphelion; when it arrives at the points D and E of its orbit, it said to be at the mean distance. ...F D , the mean distance of the planet from the sun... The orbits of some of the planets are more elliptical than others. ...Most of the orbits, except those of some of the new planets, approach very nearly to the circular form."