Glossary term: 橢圓
Description: 橢圓是類似於被壓扁或拉長的圓的二維形狀。橢圓上的最寬距離被稱為長軸，最短距離被稱為短軸。一個橢圓有兩個焦點，它們位於長軸上，且與最寬點的距離相同。橢圓上任意一點到兩個焦點的距離之和都是恆定的。橢圓的離心率e描述了它的扁平程度，取值範圍為0
封閉軌道，如地球繞太陽運行的軌道，其形狀就是橢圓。軌道的主要特徵可用半長軸（長軸的一半）和離心率來描述，但想要全面描述一個軌道，還需要知道橢圓的方向。
非封閉軌道，如被拋向星際空間之前只訪問太陽系內部一次的彗星的軌道，其軌道為拋物線（e=1）或雙曲線（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
This is an automated transliteration of the simplified Chinese translation of this term
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