Horizontal Coordinate System

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为 OAE 制作

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

词汇表: 高度角 , 方位角 , 地平 , 天顶 , 天底
分类： 肉眼天文学 , 观测天文学

授权许可： 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 图标

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Celestial Coordinates

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为 OAE 制作

图注： Here we see how celestial coordinates map positions on the celestial sphere. The celestial sphere is an imaginary, hollow sphere that surrounds the Earth. Celestial objects appear to be attached to the inside of the celestial sphere, and the planets, Sun, and Moon appear to move slowly across it. The celestial sphere has a spherical coordinate system similar to the latitude and longitude we see on the Earth. The celestial equator is the projection of the Earth's equator onto the celestial sphere while the north and south celestial poles are the projections of the Earth's north and south poles onto the celestial sphere. Declination is similar to latitude on the Earth in that it marks the angle above and below the celestial equator. Declination is zero on the celestial equator, is positive to the north of the celestial equator and negative to the south of it. It is commonly measured in degrees and sometimes in radians. Right Ascension is similar to longitude as it marks the angle around the celestial equator. As with longitude the zero point of right ascension (shown here as a dashed line) is a matter of choice. Traditionally the zero point of right ascension was the position of the Sun at the northern hemisphere vernal equinox (spring equinox) in March. It is positive and increases to the east, unlike longitude on Earth which is defined as east or west of the prime meridian. It is most commonly measured in hours, minutes and seconds or occasionally in degrees or radians. The ecliptic marks the Sun's path across the celestial sphere when viewed from the Earth. The Earth rotates within the celestial sphere. This leads to objects like stars or galaxies, which appear static on the celestial sphere, rising and setting when viewed from the Earth but remaining in the same position on the celestial sphere. The Earth's axis precesses within the celestial sphere. This slowly moves the celestial equator and the celestial poles meaning that the traditional celestial coordinate system changes slowly over time meaning that stars and galaxies that do not move will have positions that change. Astronomers have used coordinate systems defined by the position of the celestial poles and equator at a fixed point in time. Now celestial positions are most commonly measured on a fixed coordinate system defined by the position of the celestial poles and equator on the 1st of January 2000.
来源： Maria Cristina Fortuna/IAU OAE

分类： 肉眼天文学 , 观测天文学

授权许可： 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 图标

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Evening Sky Watching for Students

astroEDU 教育活动（链接至 astroEDU 网站）
Origin: astroEDU
描述: Let's observe the evening sky with the naked eye

词汇表: 光污染 , 观测
分类： 观测天文学 , 地球
标签： Art , Motion of star , Observation of sky , Sky watching
年龄段： 4-6 , 6-8
受教育等级: 学前 , Primary
学习领域： 讨论小组 , 以观察为主 , 结构化探究式学习
费用: 零成本
时间跨度: 30 分钟
组别大小： 小组
技能： 提出问题 , 传达信息
作者： Akihiko Tomita

授权许可： 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 图标