Glossary term: 中天
Description: 在天文学中，中天是指天体通过观测者当地子午线的时刻。当一个天体穿过子午线时，它要么处于最高点，要么处于最低点。
从观测者的角度看，天球似乎是围绕地球旋转的。这意味着，天体在一天中的运行轨迹是圆形的。大多数天体从东方升起，在天空中移动到更高的位置，直到穿过子午线，然后在天空中移动到更低的位置，在西方落下。拱极天体是指距离天极之一足够近的天体，观测者可以在一个恒星日（略少于 24 小时）内看到其完整的圆形路径。在任何情况下，天体到达最高点都是在穿过子午线的瞬间。到达最高点的瞬间被称为上中天。12 个小时（恒星时）后，当天体位于天空最低点时（通常低于地平线），这一瞬间被称为下中天。
由于中天是天体穿过观测者当地子午线的时刻，因此也通常被称为子午线经过或子午线穿越。天体的时角是相对于观测者的当地子午线定义的，因此根据定义，上中天发生在时角为 0 时的瞬间，下中天发生在时角为 12 时的瞬间。
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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
The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".
