《授时历》外行星计算精度

目的 研究元代<授时历>推算木星、火星和土星赤经精度.方法 文献考证和比较研究.结果 在1282-1302年间,<授时历>推算木星、火星和土星赤经的绝对值平均误差分别为0°.61,0°.98和0°.38,木星、火星和土星赤经的绝对值最大误差分别为1°.69,4°.20和1°.13.结论 <授时历>外行星计算"精度"达不到南宋历法家制定的"差天一度"这个标准.     

北宋的行星计算精度——以《纪元历》外行星计算为例

行星理论是中国传统历法中的重要内容,历法家从汉魏时期开始就已经将五星预报的精度视为检验历法优劣的标准之一.北宋时期.历法家对五星测算误差的要求是差天二度以内,而南宋时期的标准已提高到差天一度.通过全面考察<纪元历>行用期间外行星计算精度,指出<纪元历>木星和土星计算误差达到了历法家制定的精度要求,而火星的计算误差则没有完全达到历法家制定的精度要求.     

《皇极历》外行星算法及精度分析北大核心CSCDCSSCI

行星运动理论是中国古代历法的重要组成部分。北齐天算家张子信发现太阳视运动与行星公转不均匀现象之后,刘焯首次将该重大发现引入到《皇极历》五星推步算法中。本文在解读《皇极历》步五星术文的基础上,利用Python语言模拟《皇极历》算法推算木星、火星和土星视位置,讨论了《皇极历》推算约600—630年间行星真黄经的误差,其结果为木星、火星和土星的黄经最大绝对误差分别为5.27°、17.67°、5.10°。分析表明,《皇极历》外行星视位置的误差结果可能是由行星的定见时刻推算、行星动态描述的分段处理方案,以及留段目的时长等因素造成的。     

宋元时期内行星计算精度——以《纪元历》和《授时历》为例

目的研究宋元时期内行星视位置计算精度。方法利用计算机编程计算《纪元历》和《授时历》内行星视位置,并与理论结果进行比较。结果《纪元历》金星和水星计算误差符合"差天二度"这个标准的天数分别占到总数据量的96.24%和33.27%,《授时历》金星和水星计算误差符合"差天二度"这个标准的天数分别占总数据量的97.05%和39.09%。结论宋元时期金星计算误差基本能达到北宋历法家制定的"差天二度"这个标准,但水星计算误差达不到这个标准。     

天王星的发现

中国古代天文学家通过肉眼和天文仪器观测到了水星、金星、火星、木星和土星,统称为“五星”,并且把它们与太阳、月球一起称为“七曜”。意思是说,它们是天上7颗光耀人间的天体。l543年,波兰天文学家哥白尼发表《天体运行论》,提出了日心说,认识到水星、金星、火星、木星、土星和我们人类的故乡地球,都是围绕太阳运行的行星,而月球则是地球的～颗卫星。     

《纪元历》等8部宋代历法的定朔推步及精度分析

中国古代历法中,定朔是一个极其重要的概念,定朔的推算精度直接影响推算日月食的精度.从中国古代定朔算法模型入手,清晰梳理了<纪元历>定朔推步过程,并严格按照其推朔过程编写计算机程序,计算出<纪元历>等8部历法颁行期间的每次定朔时刻.将这些结果与理论值相比较,得出各部历法推算定朔的精度,就此对各历的优劣给出客观的评价,并指出精度的降低是宋代频繁改历的一个重要原因.     

寻找第九行星

正经过天文学界多年的争论和12天的激烈辩论,国际天文学联合会在2006年通过关于太阳系行星的新定义,而冥王星将不再被定义为"行星"。太阳系行星家族只剩下水星、金星、地球、火星、木星、土星、天王星和海王星8名成员。由此,冥王星惨遭"降级",被驱逐出了行星家族,归入"矮行星"行列。     

观测、理论与推算——从《三统历》到《皇极历》的火星运动研究

行星运动是中国古代历法推算的重要内容。以火星运动为例,考察自汉代《三统历》(《太初历》)至隋代《皇极历》5部历法中的火星运动计算,由此探讨中国古代天文观测、理论和推算之间的关系。从《三统历》开始,火星动态表逐步精致化:增加会合周期中的段数,调整每一段的起始点时间及其中的视速度大小,引入"合"的概念等。分析这些运动表的构造,探讨其所依据的实际观测数据和观测技术。特别是动态表中"留"的数据,反映了中国古代天文观测的精度。中国在公元6世纪观测发现了太阳运动和五星运动的不均匀性。通过《皇极历》分析这种不均匀性是如何体现在关于火星的历法计算之中。动态表不再固定,其中每段中的速度随着"晨始见"的时刻而变化,说明计算模型与实际观测的关系更加密切。     

中国古代五星定见算法的沿革与分期

张子信之前的历法家以五星和太阳的平均运动推算五星始见时刻,从隋代《皇极历》开始,历法家在设计五星定见算法时,逐渐考虑了五星和太阳的不均匀运动,在宋代的《崇天历》中,五星定见算法基本定型.梳理了中国古代五星定见算法的历史沿革,并根据五星定见算法的特征,以《皇极历》、《大衍历》和《崇天历》为关节点,将五星定见算法的历史分为四个阶段.     

《纪元历》定朔算法模型及分析

目的 用数学公式将<纪元历>定朔推步过程完整地表达出来.方法 数学建模与算法分析.结果 <纪元历>是宋代具有代表性的历法,其推朔模型及精度可以代表宋代历法的整体水平;从中国古代定朔算法模型入手,将<纪元历>的定朔算法公式化,并给出在计算机上实现定朔算法的基本思路.结论 根据<纪元历>的定朔算法编写的计算机程序,为进一步的考证工作奠定了基础.     

ENSO循环与行星位置的相互关系Ⅱ:行星视赤经与赤道东太平洋海温的联系

 利用通用天文软件SKYMap Pro Version 8提供的大行星冲日资料和计算得到的各大行星视赤经资料,分析了行星相对位置与ENSO之间的相互关系,结果发现两者之间存在明显联系,主要有:三大行星(火、木、土)在3月冲日,有利于在当年发生EL-Nino事件,且火星和木星对应的事件持续时间都较长,特别是木星3月冲日与1990s以后的2次EL-Nino群发期有很好的对应,与土星对应的事件持续较短.火星和木星在8月冲日有利于次年发生EL-Nino现象,土星则不明显.火星和木星在9月冲日容易在当年发生LA-Nina现象,而土星在9月冲日的头一年易发生EL-Nino,次年易发生LA-Nina.对行星的视赤经与赤道太平洋海温关键区之间相互关联的非偶然性检验表明,只有木星和土星存在相关区.木星视赤经相对于Nino 4海温距平大于0的概率呈现偏心结构,并存在2个奇对称区域;土星的对应区相对比较均匀,存在3个偶对称区域,且彼此相差60°,在空间上构成六角型结构.     

Chaos-assisted capture of irregular moons

It has been thought that the capture of irregular moons--with non-circular orbits--by giant planets occurs by a process in which they are first temporarily trapped by gravity inside the planet's Hill sphere (the region where planetary gravity dominates over solar tides). The capture of the moons is then made permanent by dissipative energy loss (for example, gas drag) or planetary growth. But the observed distributions of orbital inclinations, which now include numerous newly discovered moons, cannot be explained using current models. Here we show that irregular satellites are captured in a thin spatial region where orbits are chaotic, and that the resulting orbit is either prograde or retrograde depending on the initial energy. Dissipation then switches these long-lived chaotic orbits into nearby regular (non-chaotic) zones from which escape is impossible. The chaotic layer therefore dictates the final inclinations of the captured moons. We confirm this with three-dimensional Monte Carlo simulations that include nebular drag, and find good agreement with the observed inclination distributions of irregular moons at Jupiter and Saturn. In particular, Saturn has more prograde irregular moons than Jupiter, which we can explain as a result of the chaotic prograde progenitors being more efficiently swept away from Jupiter by its galilean moons.     

Origin of the orbital architecture of the giant planets of the Solar System

Planetary formation theories suggest that the giant planets formed on circular and coplanar orbits. The eccentricities of Jupiter, Saturn and Uranus, however, reach values of 6 per cent, 9 per cent and 8 per cent, respectively. In addition, the inclinations of the orbital planes of Saturn, Uranus and Neptune take maximum values of approximately 2 degrees with respect to the mean orbital plane of Jupiter. Existing models for the excitation of the eccentricity of extrasolar giant planets have not been successfully applied to the Solar System. Here we show that a planetary system with initial quasi-circular, coplanar orbits would have evolved to the current orbital configuration, provided that Jupiter and Saturn crossed their 1:2 orbital resonance. We show that this resonance crossing could have occurred as the giant planets migrated owing to their interaction with a disk of planetesimals. Our model reproduces all the important characteristics of the giant planets' orbits, namely their final semimajor axes, eccentricities and mutual inclinations.     

The formation of Uranus and Neptune in the Jupiter-Saturn region of the Solar System

Planets are believed to have formed through the accumulation of a large number of small bodies. In the case of the gas-giant planets Jupiter and Saturn, they accreted a significant amount of gas directly from the protosolar nebula after accumulating solid cores of about 5-15 Earth masses. Such models, however, have been unable to produce the smaller ice giants Uranus and Neptune at their present locations, because in that region of the Solar System the small planetary bodies will have been more widely spaced, and less tightly bound gravitationally to the Sun. When applied to the current Jupiter-Saturn zone, a recent theory predicts that, in addition to the solid cores of Jupiter and Saturn, two or three other solid bodies of comparable mass are likely to have formed. Here we report the results of model calculations that demonstrate that such cores will have been gravitationally scattered outwards as Jupiter, and perhaps Saturn, accreted nebular gas. The orbits of these cores then evolve into orbits that resemble those of Uranus and Neptune, as a result of gravitational interactions with the small bodies in the outer disk of the protosolar nebula.     

木星平运动共振和Ⅰ型迁移对土星核形成的作用

采用三维N体模拟研究了在太阳星云盘中木星完全形成后土星核的快速形成.除了考虑太阳,木星及行星胚胎间的引力相互作用,还考虑了使行星胚胎发生Ⅰ型迁移和轨道圆化效应的气体盘潮汐作用.模拟表明:木星的平运动共振构型和行星Ⅰ型迁移大大地提高了行星胚胎的碰撞吸积率,同时木星的引力摄动有效地阻止大行星胚胎过快向内迁移而落入太阳中,最终在两百万年的时间内有可能在雪线之外靠近木星3:2平运动共振处吸积形成一颗土星核.     

An interplanetary shock traced by planetary auroral storms from the Sun to Saturn

A relationship between solar activity and aurorae on Earth was postulated long before space probes directly detected plasma propagating outwards from the Sun. Violent solar eruption events trigger interplanetary shocks that compress Earth's magnetosphere, leading to increased energetic particle precipitation into the ionosphere and subsequent auroral storms. Monitoring shocks is now part of the 'Space Weather' forecast programme aimed at predicting solar-activity-related environmental hazards. The outer planets also experience aurorae, and here we report the discovery of a strong transient polar emission on Saturn, tentatively attributed to the passage of an interplanetary shock--and ultimately to a series of solar coronal mass ejection (CME) events. We could trace the shock-triggered events from Earth, where auroral storms were recorded, to Jupiter, where the auroral activity was strongly enhanced, and to Saturn, where it activated the unusual polar source. This establishes that shocks retain their properties and their ability to trigger planetary auroral activity throughout the Solar System. Our results also reveal differences in the planetary auroral responses on the passing shock, especially in their latitudinal and local time dependences.     

Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter

It has often been stated that Saturn's magnetosphere and aurorae are intermediate between those of Earth, where the dominant processes are solar wind driven, and those of Jupiter, where processes are driven by a large source of internal plasma. But this view is based on information about Saturn that is far inferior to what is now available. Here we report ultraviolet images of Saturn, which, when combined with simultaneous Cassini measurements of the solar wind and Saturn kilometric radio emission, demonstrate that its aurorae differ morphologically from those of both Earth and Jupiter. Saturn's auroral emissions vary slowly; some features appear in partial corotation whereas others are fixed to the solar wind direction; the auroral oval shifts quickly in latitude; and the aurora is often not centred on the magnetic pole nor closed on itself. In response to a large increase in solar wind dynamic pressure Saturn's aurora brightened dramatically, the brightest auroral emissions moved to higher latitudes, and the dawn side polar regions were filled with intense emissions. The brightening is reminiscent of terrestrial aurorae, but the other two variations are not. Rather than being intermediate between the Earth and Jupiter, Saturn's auroral emissions behave fundamentally differently from those at the other planets.     

A low mass for Mars from Jupiter's early gas-driven migration

Jupiter and Saturn formed in a few million years (ref. 1) from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only ～100,000 years (ref. 2). Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration. The terrestrial planets finished accreting much later, and their characteristics, including Mars' small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun (1 au is the Earth-Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5 au, and its subsequent outward migration, lead to a planetesimal disk truncated at 1 au; the terrestrial planets then form from this disk over the next 30-50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3 au and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.     

Jovian-like aurorae on Saturn

Planetary aurorae are formed by energetic charged particles streaming along the planet's magnetic field lines into the upper atmosphere from the surrounding space environment. Earth's main auroral oval is formed through interactions with the solar wind, whereas that at Jupiter is formed through interactions with plasma from the moon Io inside its magnetic field (although other processes form aurorae at both planets). At Saturn, only the main auroral oval has previously been observed and there remains much debate over its origin. Here we report the discovery of a secondary oval at Saturn that is approximately 25 per cent as bright as the main oval, and we show this to be caused by interaction with the middle magnetosphere around the planet. This is a weak equivalent of Jupiter's main oval, its relative dimness being due to the lack of as large a source of ions as Jupiter's volcanic moon Io. This result suggests that differences seen in the auroral emissions from Saturn and Jupiter are due to scaling differences in the conditions at each of these two planets, whereas the underlying formation processes are the same.     

ENSO循环与行星位置的相互关系(Ⅰ):木星、土星视赤纬与赤道东太平洋海温的联系

 利用通用天文软件SKYMap Pro Version 8计算了木星和土星的月视赤纬,将其与ENSO循环及赤道太平洋关键区海温距平进行对比分析,发现两者存在明显联系,主要包括:木星视赤纬由正经过赤道变为负时,容易发生EL-Nino事件,而且事件持续时间较长,1980年代及以前发生过3次这样的情况都对应持续较长的EL-Nino事件,1980年代以后的2次EL-Nino事件群发期,其中心都发生在木星视赤纬从正向负转化期间.EL-Nino易发生在木星视赤纬的正极值和赤道附近,而负极值附近很少发生.LA-Nnia事件在1980年代及以前发生在木星视赤纬极值和赤道之间的中间视赤纬区域内,但1980年代以后,则容易发生在木星视赤纬负极值附近.一些EL-Nino事件发生年的木星视赤纬位置近乎在1条直线上,过去60年中有4条这样的直线,将EL-Nino事件分成4个组,每个组内都准确地由4个事件组成.当土星视赤纬靠近或离开赤道时(大概在±5°~10°),都会有EL-Nino发生,但在赤道区域却几乎不发生EL-Nino事件.