词条 地平
描述: 地平线是将天空与地球表面分开的边界线。在地球上的任何位置，我们都只能看到地表有限的一部分。我们所能看到的部分与我们所不能看到的部分之间的分界线通常称为地平线。在天文学中，这一定义细化如下：我们在地球上的某一位置处，先用铅垂线找出向下的方向，接着将垂直于下方的平面定义为地平面。地平面与天球的交点即为我们的天文本地地平。地平坐标系利用地平面来定义天体在天空中的位置。我们看到天体的视线与地平面之间的夹角称为天体的高度角；视线在地平面上的投影与真北之间的夹角称为天体的方位角。
相关词条
用其他语言查看该词条
词条和定义状态： 该术语的英文原始定义已通过天文研究人员和教师的审核 该术语及其定义的译文仍待审核
OAE多语言词汇表是由国际天文学联合会（IAU） 天文教育办公室（OAE） 和 国际天文学联合会（IAU） 天文推广办公室（OAO）合作的项目。. 术语和定义由 OAE中心, OAE 国家天文教育协调员（NAECs） 以及其他志愿者共同选择、撰写和审阅。您可以在 找到完整的致谢列表。所有词汇表术语及其定义均根据 Creative Commons CC BY-4.0许可证发布 且版权归属于 “IAU OAE”。
如果您发现此词汇术语或定义中的事实或翻译错误， 请向 发邮件 联系.
相关图表
Horizontal Coordinate System
图注： This image shows the horizontal coordinate system of an observer on Earth. On the right we see the coordinate system in the local context of the observer. The observer appears here as a point surrounded by the celestial sphere. The ground appears as a plane, while the Earth is round, we can approximate the ground in the area around the observer as a plane. The line where this plane intersects with the celestial sphere is the horizon. The point directly above the observer is the zenith, the point directly below the observer is the nadir which is hidden by the ground.
Two coordinates define this coordinate system, altitude and azimuth, hence the reason this sometimes called an alt-az coordinate system. The altitude is zero at the horizon, maximum (90°) at the zenith and minimum (-90°) at the nadir. Azimuth is the angle around the horizon. Most commonly this is defined to be zero in the direction of north. Any point on the celestial sphere can be defined by these two coordinates. However what objects appear on these position will depend on the time and the location of the observer.
On the left-hand part of the diagram, we can see that when we put the observer and their local horizontal coordinate system in a global context, that the parts of the sky the observer can see depend on their position on the Earth and on the rotation of the Earth. The zenith points perpendicular to the Earth's surface so the position on the sky the zenith and nadir correspond to is dependent on the observer's latitude. The horizon also depends on the observer's position on the Earth.
来源： Maria Cristina Fortuna/IAU OAE
