Glossary term: 日
Description: 在英语和许多其他语言中，“日”（day）有多重含义。“日”的“白天”这一重含义是指太阳为我们提供自然光的时间，与之相对的是太阳落山、一片漆黑的“夜晚”。根据这一定义，北半球的白天在冬季较短、在夏季较长，而南半球则相反。在遥远的北方或遥远的南方，每年总有一段时间，太阳根本不落山——在两极地区，太阳有六个月不落山！
“日”也是我们在日历中使用的时间单位的名称，长度为24小时。在天文学的角度上，从一个地方正午到下一个地方正午的时间，也就是太阳连续两次到达地平线以上最高位置的时间间隔，叫做视太阳日。地方日的长度在一年中因时而异，这是因为：（1）地球的轨道是椭圆形的，且地球离太阳越近，移动速度越快；（2）太阳在天空中的视轨迹与地球赤道的夹角不同。在实际应用中，计时使用的是视太阳日的平均长度，即平太阳日。根据平太阳日所划分的每天24小时的时间称为平太阳时。
恒星日以天球上的“固定”背景恒星为基础。恒星日是指一颗“固定”恒星连续两次到达天空最高点的时间间隔。一个恒星日大约持续23小时56分4秒，它与视太阳日（24小时）之间的差异是由太阳相对于“固定”背景恒星的视运动造成的。
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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".
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Related Activities
Why Do We Have Day and Night?
astroEDU educational activity (links to astroEDU website) Description: Explore day and night of Earth.License: CC-BY-4.0 Creative Commons 署名 4.0 国际 (CC BY 4.0) icons
Tags: Tilt Age Ranges: 6-8 , 8-10 , 10-12 Education Level: Primary , Secondary Areas of Learning: Modelling , Structured-inquiry learning , Social Research Costs: Low Cost Duration: 1 hour 30 mins Group Size: Group Skills: Asking questions , Communicating information
