词条 二至点
描述: 地轴并非垂直于地球围绕太阳的轨道平面，而是与垂直方向呈23.4度的倾斜角。因此，在地球轨道的不同位置上，地轴与太阳视线之间的角度在一年内是不同的。这种现象的实际效果是，对于地球上的观测者来说，太阳在一天中达到地平线上最高点的高度会随时间而变化。北半球夏至，同时也是南半球的冬至，大约发生在6月21日，此时太阳在北半球的高度到达一年中的最高点，同时在南半球的高度则到达最低点。而南半球夏至，同时也是北半球的冬至，大约发生在12月21日，此时太阳在南半球的高度到达最高点，同时在北半球的高度则到达最低点。
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每月日落的地方
图注： 这组图片是在菲律宾布拉干省的圣拉斐尔用智能手机拍摄的，它捕捉到了地球轴向倾斜和环绕太阳运行所导致的日落位置在一年中的变化。从 2022 年 1 月到 12 月，每张图片都反映了黄昏时分背景和太阳位置的变化。在十月，可以观察到树叶的变化和一根电线杆的出现。这组作品是地球与太阳之间的天体舞蹈以及地面自然景观变化的视觉见证。
来源： John Paul Pile/IAU OAE (CC BY 4.0)
太阳的轨迹
图注： 这幅令人陶醉的图片于 2018 年 6 月 21 日至 2018 年 12 月 21 日期间在德国拍摄，是艺术与科学的诗意融合，展现了大自然的韵律和时间的流逝。它合并了六个月内每天不同时间段拍摄的图片。每条曲线都追踪了太阳在特定一天内穿越天空的路径（或弧线）。在这里，太阳划过的弧线被投影扭曲，使其看起来像一条曲线。每天划过天空的弧线位置随着太阳在天空中的季节性移动而变化。这幅图片是用一个咖啡罐制成的简易针孔摄像机拍摄的，它展示了太阳在六个月时间里的旅程，以及日弧从夏季较高位置（太阳在天空中的位置较高）逐渐移动到冬至最低位置的过程。太阳路径上的间隙代表太阳被云层遮挡的日子或时段。这幅图片生动地表现了四季的变化，展示了太阳在天空中随着白天变短或变长而舞动的轨迹。
来源： Frank Niessen/IAU OAE (CC BY 4.0)
日至和日分
图注： 这张引人入胜的照片记录了在2007年12月21日至2008年6月20日期间，太阳在土耳其布尔萨的冬至（下）、春分（中）以及夏至（上）在天空中运动的轨迹。摄影师使用鱼眼镜头，在这三天中以固定的时间间隔拍摄太阳从日出到日落的位置，生动展示了一年中昼夜长短的变化。太阳每日在天空中的运行轨迹、正午高度、日出和日落的位置，以及白昼的长度，均由地球自转轴与其绕太阳轨道的倾斜角度决定。这一系列照片拍摄于布尔萨的一座屋顶，当地的地形偶尔会遮挡地平线，为画面增添了独特的地域特征。照片中的方向以北朝上、以南朝下，这组视觉日记成为中北纬地区居民欣赏太阳运行轨迹变化的精彩呈现。
来源： 通奇·特泽尔/国际天文学联合会教育办公室 (CC BY 4.0)
西落幅度
图注： 这张合成照片拍摄于意大利西西里岛的 Gatto Corvino 村，展示了从2016年到2017年冬至到夏至期间，太阳在海面上不同位置的落日景象。中心框为正西方。随着地球绕太阳公转，太阳的落下（和升起）位置从夏季的正西（东）方向偏北，变化到冬季的正西（东）方向偏南。在春分和秋分时，白昼和黑夜的长度相等，太阳从正东方升起，正西方落下。太阳在地平线上位置的变化是由于地球绕太阳运行时的自转轴倾斜。在地球的温带地区，夏至和冬至时，正午太阳在地平线以上的高度角分别达到最高和最低值。
来源： Marcella Giulia Pace/IAU OAE (CC BY 4.0)
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Solstices and Equinoxes
图注： This diagram demonstrates the relative positions of the Earth and Sun at the two solstices and two equinoxes. The Earth’s rotation axis is tilted by 23.4° from the axis of its orbit. This means that for half the Earth’s annual orbit around the Sun, the Southern Hemisphere is tilted towards the Sun and the Northern Hemisphere is tilted away from the Sun. During this time the Sun appears to lie below the celestial equator. Due to this at any particular point in the Southern Hemisphere the Sun will appear to be higher above the horizon at a particular time of day and the days will be longer. Conversely during this time, the Sun appears lower in the sky at any particular point in the day in the Northern Hemisphere and the days are shorter.
This effect is most pronounced in late December when the Sun appears at its most southerly point in the sky, corresponding to the longest day of the year in the Southern Hemisphere. This normally occurs on the 21st or 22nd of December (UTC), depending on when the most recent leap year was, but can very occasionally occur on the 20th or 23rd of December. This event is known as the December solstice. As the Sun’s rays hit the Southern Hemisphere at a more perpendicular angle during this time, the Southern Hemisphere is typically warmer in this time of year than at other times of year. This is often referred to as summer in temperate and antarctic regions of the Southern Hemisphere and thus the December solstice is known as the summer solstice. South of the Antarctic Circle at this time the Sun never sets and a Polar Day can last several months. Conversely in the Northern Hemisphere, the Sun’s rays hit the Earth at a more grazing angle than at other times of year so it is colder and this solstice is known as the winter solstice. North of the Arctic Circle the Sun never rises during this time and the Polar Night can last several months.
As the year progresses the Earth moves in its orbit and the tilt of the Earth’s axis moves so it appears more side-on to the Sun. From the perspective of an observer on Earth this means that the Sun moves north in the sky, eventually reaching the celestial equator in late March, This usually occurs on the 20th of March (UTC) but can sometimes occur on the 19th or 21st of March. During this time all places on Earth will experience night and day that is approximately 12 hours long. The Sun’s rays now hit the Southern hemisphere at a more shallow angle than they did in December and hit the Northern Hemisphere at a more perpendicular angle. This means that the Northen Hemisphere will have warmed since December and the Southern Hemisphere cooled. This leads to the March equinox being referred to as the autumn equinox in the Southern Hemisphere and the spring or vernal equinox in the Northern Hemisphere.
As the year progresses further the Sun moves further north on the sky, the Northern Hemisphere days lengthen and the Southern Hemisphere days shorten. By June the Earth’s orbit has progressed to the point where the Northern Hemisphere points towards the Sun and the Southern Hemisphere away from the Sun. At one point between the 20th and 22nd of June (UTC) the Sun reaches its most northerly point in the sky, this is known as June solstice. It is now summer in northern temperate and artic regions so this is known as the summer solstice in the Northern Hemisphere and the winter solstice in the Southern Hemisphere. This is the longest day of the year in the Northern Hemisphere and the shortest day of the year in the Southern Hemisphere. During this time regions north of the Arctic Circle experience a long Polar Day and regions south of the Antarctic Circle experience polar night.
The year progresses further and the Earth moves in its orbit so that the tilt of the Earth’s axis again appears side-on for the Sun. The Sun moves south in the sky and again crosses the celestial equator between the 21st and 24th of September (UTC). At this time all places on the Earth experience equal lengths of day and night. This is known as the September equinox, also called the spring or vernal equinox in the Southern Hemisphere and the autumn equinox in the Northern Hemisphere.
Note the sizes of the Earth, Sun and the Earth’s orbit around the Sun are not to scale in this diagram.
来源： Maria Cristina Fortuna/IAU OAE
Seasons
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
