Latitude and Longitude

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图注： Two views of the Earth, one viewing the Northern Hemisphere, one viewing the Southern Hemisphere. The Earth rotates around its axis, an imaginary line that runs through the Earth from the North Pole to the South Pole. The Equator is an imaginary line that is the same distant from both the North Pole and the South Pole. The positions of two cities, Rome in the Northern Hemisphere and Sydney in the Southern Hemisphere are marked here with red dots. Arrows indicate the two spherical coordinates latitude and longitude. Latitude marks the position north or south of the equator. Here we can see Rome has the letter N in its latitude as it is in the Northern Hemisphere and Sydney has the letter S in its latitude as it is in the Southern Hemisphere. Latitude can vary from 90° N at the North Pole to 0° at the Equator to 90° S at the South Pole. Longitude measures the position around the equator. While the choice of the zero point for latitude as the equator seems obvious, the choice of the zero point for longitude is more subjective. By convention the zero point in longitude is the prime meridian which passes through the Royal Greenwich Observatory in the UK. This is marked here as a solid line originating at the North Pole. Longitude is measured in degrees east or west of the prime meridian. As both Rome and Sydney lie to the east of Greenwich, they have the letter E as part of their longitude. Moving west to east, longitude varies from 180° W on the other side of the world from the prime meridian to 0° on the prime meridian before reaching 180° E again on the other side of world from the prime meridian. This diagram shows the Earth at the December solstice. Two views are presented, one viewing the Northern Hemisphere and one viewing the Southern Hemisphere about nine hours later. The shaded region shows the night side of the Earth, with the day side being lighter. As it is the December solstice, the Sun appears overhead at the Tropic of Capricorn. This is a line of constant latitude at 23°26′09.3″ S. Six months later, at the June solstice, the Sun will appear to be overhead at the Tropic of Cancer at 23°26′09.3″ N. As the Sun appears over the Tropic of Capricorn more of the Southern Hemisphere is illuminated than the Northern Hemisphere. Indeed below the Antarctic Circle (the Polar Circle around the South Pole) the Sun does not set at this time of year leading to a Polar Day. Conversely, north of the Arctic Circle (the Polar Circle around the North Pole) the Sun does not rise at this time of year, leading to a Polar Night.
来源： Maria Cristina Fortuna/IAU OAE

词汇表: 地球自转 , 赤道 , 纬度 , 经度 , 极圈 , 地轴 , 回归线 , 极昼 , 极夜 , 北回归线 , 南回归线 , 热带地区 , 北极 , 南极
分类： 肉眼天文学

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蛇夫座星图

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图注： 蛇夫座及其明亮恒星与周边星座示意图。从顶部开始沿顺时针方向，蛇夫座周边的星座依次为：武仙座、巨蛇头、天蝎座、人马座和巨蛇尾。蛇夫座并不属于传统的黄道十二星座，但它也位于黄道上。从地球看来，太阳以一年为周期在天球上移动，所经过的路径称为黄道，在图中以蓝色实线标注。太阳在每年十一月下旬到十二月中旬位于蛇夫座。太阳系中的其他行星也经常出现在蛇夫座。 蛇夫座横跨天赤道，因此在一年中的某些时候，整个地球都能看到蛇夫座的一部分。蛇夫座在北半球的夏季和南半球的冬季夜晚最适宜观测——不过此时，北极的大部分地区正处于极昼，因此无法看到蛇夫座的恒星。在赤道地区和南北两个半球的温带地区都能看到整个蛇夫座。 由于蛇夫座靠近银河系中心，因此包含了许多球状星团，如M9、M10、M12、M14、M19、M62和M107。这些球状星团在图中以叠加了十字符号的黄色圆圈标注。 该图的纵轴为赤纬，横轴为赤经，方向为上北下南左东右西。图中标注的恒星大小对应其视星等——衡量天体视亮度的标准，较大的圆点代表着较亮的恒星。图中的希腊字母标注着星座中最亮的恒星。这些恒星按亮度排序，最亮的一般被标记为α星，第二亮的一般为β星，等等，不过这种字母排序并不总是和实际亮度排序完全一致。图中虚线标注的是国际天文学联合会划定的星座边界，绿色实线则是一种常见的星座形象连线。需要注意的是，在实际观测时，这些边界与连线都不会出现在天空中。
来源： 国际天文学联合会天文教育办公室（IAU OAE）根据国际天文学联合会和《天空与望远镜》的原文改编
来源链接

词汇表: 视星等 , 天球坐标 , 天赤道 , 星座 , 赤纬 , 黄道 , 银河系中心 , 球状星团 , 赤经（RA） , 天蝎座 , 黄道十二宫 , 人马座
分类： 肉眼天文学

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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International Date Line

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图注： The International Date Line (shown here as a solid line) is a specific meridian which roughly follows the meridian of longitude 180°. The International Date Line passes north–south between Russia and Alaska, through the Pacific Ocean including parts of Micronesia and Polynesia, and to the east of Australia/New Zealand before reaching the South Pole on Antarctica. The International Date Line marks the boundary where calendar dates change by one. Therefore, regions to the west of the International Date line are one calendar day ahead of regions to the east. The International Date Line does not always exactly follow the meridian of longitude 180°. This is partly to stop some territory of a country falling on the other side of the International Date Line than the rest of that country (the Aleutian Islands in the United States being a good example). Some countries also choose which side of the International Date Line they lie on. Samoa changed which side of the date line it lay on in 2011. At this point the International Date Line was moved to lie to the east of Samoa having previously passed to the west of Samoa. On the opposite side of the world the dashed line marks the prime meridian (the meridian of longitude 0°). This is a line that passes through the Royal Greenwich Observatory in the UK. All time zones are measure relative to Universal Time which is the zero point for all timezones. The time zones marked at the top of the diagram are the timezones in the shaded regions excluding the effects of daylight savings time.
来源： Maria Cristina Fortuna/IAU OAE

词汇表: 时区 , 世界时（UT） , 本初子午线 , 国际日期变更线
分类： 天文学与社会 , 地球

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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Horizontal Coordinate System

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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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Seasons

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图注： This diagram shows the Earth at the June solstice which is known as the summer solstice in the Northern Hemisphere and the winter solstice in the Southern Hemisphere. The Earth’s axis of rotation is tilted from the axis of the Earth’s orbit around the Sun by 23.4°. Here we see light from the sun represented as parallel lines, in the diagram the Sun would lie far off the right-hand edge. Five lines are marked with angles. These lines hit the Earth at special latitudes. From top to bottom these lines hit the Earth at the North Pole, the Tropic of Cancer, the Equator, the Tropic of Capricorn and the Antarctic Circle. The lines are shown hitting the Earth at the longitude where it is currently noon. For the last four lines, the angles marked show the angle above the horizon (the altitude) which the Sun appears in the sky at at local noon. At the June solstice, the Sun will appear directly overhead at local noon on every point on the Tropic of Capricorn. By contrast the Sun reaches a maximum altitude of 0° at local noon on every point in the Antarctic Circle. This means the Sun never rises and every point south of the Antarctic Circle experiences a long Polar Night. By contrast the Sun never sets on this day north of the Arctic Circle and the regions here experience a Polar Day. The at the North Pole the Sun will remain at a constant altitude all day. This altitude will be the angle 180°-156.6° (23.4°). The altitude the Sun appears at is important as it defines how spread out the Sun’s rays are on the Earth’s surface. Look at the thick, faint parallel lines in the background. Notice how at the Tropic of Cancer the area the lines intersect with the Earth’s surface perpendicularly and the line only covers a small part of the Earth’s surface. Now look at a line near the Tropic of Capricorn and see that this intersects with the Earth at a grazing angle and the line is spread out, covering a much wider area of the Earth’s surface. This means that a given amount of light from the Sun that hits the Tropic of Cancer is more concentrated and the same amount of light hitting the Tropic of Capricorn would be more spread out. This leads to more energy per unit area from the Sun hitting the Tropic of Cancer and less energy per unit area hitting the Tropic of Capricorn. This means it will be warmer at the Tropic of Cancer than at the Tropic of Capricorn. It is thus summer at the Tropic of Cancer and winter at the Tropic of Capricorn. Six months later this situation will be reversed and it will be summer at the Tropic of Capricorn and winter at the Tropic of Cancer.
来源： Maria Cristina Fortuna/IAU OAE

词汇表: 赤道 , 二至点 , 夏至 , 冬季 , 冬至 , 北极圈 , 南极圈 , 北回归线 , 南回归线 , 北极

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