在天文动力学，架构在标准假说下的任何轨道都必须是圆锥切面的形状。圆锥切面的离心率，轨道离心率是定义轨道形状的重要参数，而且定义了绝对的形状。离心率可以解释为形状从圆形偏离了多少的程度。

架构在标准假说下，离心率（偏心率，）是严格的定义了圆、椭圆、抛物线和双曲线，并且有如下的数值：

椭圆的离心率是圆的角度投映。例如说，水星的离心率是（0.2056），简单的计算正弦的反函数可以得到投映的角度是11.86度，然后以那个数值的倾斜角度观察任何的圆形物体（例如从上面观看的咖啡杯），投射到你的眼中所看见的椭圆就会有相同的离心率。

The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptical orbit, 1 is a parabolic escape orbit, and greater than 1 is a hyperbola. The term derives its name from the parameters of conic sections, as every Kepler orbit is a conic section. It is normally used for the isolated two-body problem, but extensions exist for objects following a rosette orbit through the galaxy.