土星的一季

土星的一季

ByNASA中文

2023年8月25日 A Season of Saturn Image Credit & Copyright: Andy Casely Explanation: Ringed planet Saturn will be at its 2023 opposition, opposite the Sun in Earth’s skies, on August 27. While that puts the sixth planet from the Sun at its brightest and well-placed for viewing, its beautiful ring system isn’t visible to the unaided eye. Still, this sequence of telescopic images taken a year apart over the last six years follows both Saturn and rings as seen from inner planet Earth. The gas giant’s ring plane tilts from most open in 2018 to approaching edge-on in 2023 (top to bottom). That’s summer to nearly the autumn equinox for Saturn’s northern hemisphere. In the sharp planetary portraits, Saturn’s northern hexagon and a large storm system…

秋分: 卡拉尼什巨石阵上空的日行迹

秋分: 卡拉尼什巨石阵上空的日行迹

ByNASA中文

2022年9月18日 Analemma over the Callanish Stones Image Credit & Copyright: Giuseppe Petricca Explanation: If you went outside at the same time every day and took a picture that included the Sun, how would the Sun’s position change? A more visual answer to that question is an analemma, a composite image taken from the same spot at the same time over the course of a year. The featured analemma was composed from images taken every few days at noon near the village of Callanish in the Outer Hebrides in Scotland, UK. In the foreground are the Callanish Stones, a stone circle built around 2700 BC during humanity’s Bronze Age. It is not known if the placement of the Callanish Stones has or had astronomical significance. The…

土星环与土星的季节

土星环与土星的季节

ByNASA中文

2021年9月19日 Rings and Seasons of Saturn Image Credit & Copyright: Damian Peach/SEN Explanation: On Saturn, the rings tell you the season. On Earth, Wednesday marks an equinox, the time when the Earth’s equator tilts directly toward the Sun. Since Saturn’s grand rings orbit along the planet’s equator, these rings appear most prominent — from the direction of the Sun — when the spin axis of Saturn points toward the Sun. Conversely, when Saturn’s spin axis points to the side, an equinox occurs and the edge-on rings are hard to see from not only the Sun — but Earth. In the featured montage, images of Saturn between the years of 2004 and 2015 have been superposed to show the giant planet passing from southern summer toward…

近日点与远日点

近日点与远日点

ByNASA中文

2021年7月8日 Perihelion to Aphelion Image Credit & Copyright: Richard Jaworski Explanation: Aphelion for 2021 occurred on July 5th. That’s the point in Earth’s elliptical orbit when it is farthest from the Sun. Of course, the distance from the Sun doesn’t determine the seasons. Those are governed by the tilt of Earth’s axis of rotation, so July is still summer in the north and winter in the southern hemisphere. But it does mean that on July 5 the Sun was at its smallest apparent size when viewed from planet Earth. This composite neatly compares two pictures of the Sun, both taken with the same telescope and camera. The left half was captured close to the date of the 2021 perihelion (January 2), the closest point in…

行星上的季节变化

行星上的季节变化

ByNASA中文

Credit：NASA 9月22日是北半球的秋分，同时也是南半球的春分，也就是说昨天是北半球入秋第一天，南半球入春第一天。 地球上有着明显的春夏秋冬，四季分明，但不是太阳系中每颗行星都有明显的四季变化。 Credit：NASA 行星上的季节变化由两个因素引起，轴倾角（axial tilt）和轨道偏心率（orbital eccentricity）。轴倾角指的是行星的自转轴相对于轨道面的倾斜程度。水星，金星和木星的轴倾角很小，地球，火星，土星，海王星的轴倾角都是二十多度，而天王星最夸张，轴倾角82度，和其他行星兄弟相比，几乎是躺着。 轨道偏心率简单说指的是远日点和近日点的变化比，太阳系所有的行星的轨道都是椭圆，轨道偏心率决定了椭圆轨道的扁平程度。 拿我们地球来说，轨道偏心率为0.02，环绕太阳的轨道几乎是圆形，所以这点对地球上的气候影响很小。地球的轴倾角为23.5°，我们星球上的季节变化主要由它决定，当北极偏向太阳时，北半球迎来了夏天，当北极偏离太阳时，北半球迎来了冬天。 水星，金星和木星上的轴倾角很小，这三颗行星上的季节变化很小。 水星，几乎没有季节变化。水星上的一天（一昼夜）相当于177个地球日，一年相当于88个地球日。另外由于没有大气层保护，水星昼夜温差极大，面向太阳的一面，白天温度可答430摄氏度，背向太阳的一面，夜晚的温度低至零下180摄氏度。 金星有着浓厚的大气层，导致其表面温度高达470摄氏度。多提一句，大气层中的温度随着距离地面的高度而不同，在其距离地面约30英里的大气层中，温度和地球相当。 火星的轴倾角是 24°，略高于地球，但火星轨道偏心率是0.09，是地球的四倍多，火星环绕太阳的轨道在1.64 AU和1.36 AU之间，这两种情况的加持下，导致火星上的季节变化比我们地球大很多。 我们都知道，火星上的大气很稀薄，约为地球的1%。另外引起我们注意的是，火星上的大气压力随着季节发生变化，火星上冬季的全球大气压力比夏季低25%，之所以有这种情况发生，是因为火星的轨道偏心率，以及火星两极干冰冰冠和二氧化碳的交换。火星北半球在冬至比在夏至，距离太阳近约10%，火星北半球冬至时，北极冰冠吸收的二氧化碳，比南极在同期吸收的多。 天王星上一年相当于84个地球年，轨道偏心率为0.06，它环绕太阳的轨道几乎也是圆形，但天王星的轴倾角为82度，这就导致天王星上长达四分之一的时间里有着极端的季节变化，当太阳光直射天王星极区时，其他地区就陷入了漫长、黑暗、寒冷的冬天。 参考： https://science.nasa.gov/science-news/science-at-nasa/2000/interplanetaryseasons

近日点与远日点的太阳

近日点与远日点的太阳

ByNASA中文

2020 January 9 Perihelion to Aphelion Image Credit & Copyright: Ian Griffin (Otago Museum) Explanation: Perihelion for 2020, the point in Earth’s elliptical orbit when it is closest to the Sun, occurred on January 5th. The distance from the Sun doesn’t determine the seasons, though. Those are governed by the tilt of Earth’s axis of rotation, so January is still winter in the north and summer in southern hemisphere. But it does mean that on January 5 the Sun was at its largest apparent size. This composite neatly compares two pictures of the Sun, both taken from planet Earth with the same telescope and camera. The left half was captured on the date of the 2020 perihelion. The right was recorded only a week before…