Glossary term: 椭圆
Description: 椭圆是类似于被压扁或拉长的圆的二维形状。椭圆上的最宽距离被称为长轴，最短距离被称为短轴。一个椭圆有两个焦点，它们位于长轴上，且与最宽点的距离相同。椭圆上任意一点到两个焦点的距离之和都是恒定的。椭圆的离心率e描述了它的扁平程度，取值范围为0
封闭轨道，如地球绕太阳运行的轨道，其形状就是椭圆。轨道的主要特征可用半长轴（长轴的一半）和离心率来描述，但想要全面描述一个轨道，还需要知道椭圆的方向。
非封闭轨道，如被抛向星际空间之前只访问太阳系内部一次的彗星的轨道，其轨道为抛物线（e=1）或双曲线（e>1）。
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Term and definition status: The original definition of this term in English have been approved by a research astronomer and a teacher The translation of this term and its definition is still awaiting approval
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