雅克·卡西尼Jacques Cassini

雅克·卡西尼是乔凡尼·多美尼科·卡西尼和Geneviève de Laistre的儿子。为避免与卡西尼家族的其他成员（他的乔凡尼·多美尼科·卡西尼、塞萨尔-弗朗索瓦·卡西尼·图里和让-多米尼克·德·卡西尼）混淆，他有时被称为卡西尼 II。乔凡尼·多美尼科·卡西尼在1674年与Geneviève结婚时是巴黎天文台的负责人，并且在此两年前已成为法国公民，将名字从乔凡尼·多美尼科·卡西尼更改。Geneviève是Clermont伯爵领地的中将之女，他们拥有瓦兹省的塞萨尔-弗朗索瓦·卡西尼·图里城堡，该城堡成为他们的夏季住所。卡西尼，他父母两个儿子中的第二个，出生在巴黎天文台的家中。这是一个富裕的家庭，卡西尼接受了尽可能最好的教育。

他家人居住的巴黎天文台为教育一个对科学充满热情的年轻男孩提供了极好的场所。这是卡西尼最早接受教育的地方，之后他在巴黎的马扎林学院学习。皮埃尔·伐里农于1688年被任命为马扎林学院的数学教授，并指导了卡西尼关于光学的学位论文。1691年8月，十四岁的卡西尼答辩了他的学位论文。一位早期传记作者[3]指出，他十五岁时，将一篇关于数学的学位论文献给勃艮第公爵。不清楚这两处对学位论文的提及是指同一作品还是两个不同的作品。

1694年，卡西尼被接纳进入Académie des Sciences，并开始在他父亲正在进行的项目中从事科学工作。次年，他与父亲一起在意大利旅行，他们进行了大量大地测量观测，并访问了博洛尼亚，在那里他们修复了卡西尼的父亲近三十年前设计的圣彼得罗尼奥教堂的日晷。卡西尼像他父亲一样，对进行天文观测和精确测量都感兴趣。意大利之行后，卡西尼在大约1698年访问了佛兰德斯，然后访问了英格兰。在英格兰期间，他会见了艾萨克·牛顿、约翰·佛兰斯蒂德和爱德蒙·哈雷，并当选为伦敦皇家学会会士。回到法国后，他发表了旅行中收集的天文和大地测量数据。

1700年，卡西尼的父亲承担了一个测量从巴黎到佩皮尼昂的子午线的项目，佩皮尼昂位于地中海海岸以西13公里处。卡西尼协助父亲进行这个项目，他们得到的结果错误地表明地球在两极是伸长的。1713年，他提出了一种利用月球遮掩恒星和行星来确定经度的新方法。应用这种方法，并使用1700年巴黎到佩皮尼昂的测量数据，他声称已经证明了[1]：-

……地球子午线的度数从赤道向两极逐渐变小。

不幸的是，卡西尼终生坚决坚持这一立场，尽管有提出的科学证据，他仍拒绝承认地球的扁平。这是否是由于一种错误的爱国主义感，认为他是在支持法国观点以对抗作为艾萨克·牛顿引力理论（卡西尼从未接受）结果的英国观点，还是由于一种错误的家庭忠诚感支持他父亲的观点，我们永远无法知道。也许两者都起了作用，而卡西尼并未完全意识到它们影响了他的科学判断，或者也许他如此确信1700年测量的结果是正确的，以至于他永远无法接受相反的结果。正如1733年Giovanni Poleni所指出的，测量数据的真正问题在于，卡西尼提出的伸长或其他提出的扁平都落在所用仪器的实验误差范围内。

卡西尼接受过一些法律培训，但不足以让人期望他在这一领域发挥重要作用，然而，除了科学工作外，他确实从事了第二项平行职业。他于1706年被任命到审计法院，这是一个财政法庭，具有与国王账目相关的行政和法律职责，特别是与王室拥有的土地有关。他在这个职位上因诚实而享有很高声誉，但有时因优柔寡断而受到批评。后来，在1716年，他是法院的辩护律师，并在1722年成为国务参事。

他的科学地位极为重要，在Académie des Sciences中发挥了重要作用，并从父亲手中接任巴黎天文台台长。约1709年，他父亲的健康开始衰退，卡西尼承担了越来越多的台长职责。值得注意的是，当时并没有正式的巴黎天文台台长职位，理论上天文台由Académie des Sciences管理，但卡西尼像他父亲之前一样实际上管理着它。次年1710年，他与来自沙尔穆瓦的Suzanne-Françoise Charpentier结婚。他们有三个儿子，Dominique-Jean、后来被称为卡西尼三世或塞萨尔-弗朗索瓦·卡西尼·图里的César-François，以及Dominique-Joseph，还有两个女儿Suzanne-Françoise和Elisabeth-Germaine。长子Dominique-Jean跟随父亲进入审计法院，而最小的儿子则从军。

卡西尼继续他的天文和测量工作。1718年，他进行了巴黎子午线向北到敦刻尔克的测量。1722年，他在De la grandeur et de la figure de la terreⓉ（论地球的大小和形状）中发表了结果，这些结果再次支持了他关于两极伸长的错误理论。这部重要论著综述了此前五十年间测量地球所获得的结果。然而，像皮埃尔·莫佩尔蒂这样相信地球在两极扁平的人越来越强烈地反对卡西尼的理论，为了获得进一步支持自己观点的证据，卡西尼在1733年组织了另一个项目，这次是测量从圣马洛到斯特拉斯堡的子午线垂线。正如他年轻时曾陪同父亲一样，在这个测量项目中，卡西尼得到了自己儿子César-François 卡西尼（在本档案中以塞萨尔-弗朗索瓦·卡西尼·图里之名出现）的协助。他们在1733-34年间收集的数据似乎支持伸长理论，但这只是鼓励了Académie中那些支持牛顿观点的成员，如皮埃尔·莫佩尔蒂，组织进一步的科学考察，试图使争论朝着有利于他们的方向解决。到1738年，皮埃尔·布格和夏尔·马里·德·拉孔达米纳于1735年在秘鲁以及皮埃尔·莫佩尔蒂于1736年在拉普兰进行的测量子午线一度长度的大地测量，已经为两极扁平提供了非常有力的证据。尽管卡西尼从未偏离自己的信念，但他从1740年开始逐渐减少自己的科学工作。

至于他对天文学的贡献，卡西尼在Académie des Sciences的期刊上发表了许多论文，并在1740年出版了两部重要论著，即Elements of Astronomy和Astronomical Tables of the Sun, Moon, Planets, Fixed Stars, and Satellites of Jupiter and Saturn。这些论著显示了他作为观测者的卓越能力，他在研究木星和土星的卫星以及研究土星环结构方面做出了令人印象深刻的贡献。他最著名的发现是1738年发现的恒星自行。在这里，古代关于恒星天球不变的信念第一次被直接测量证明是错误的。

卡西尼的工作并不局限于天文学和大地测量学。他撰写了关于数学应用的各种其他主题的论文，特别是关于电、气压计、枪炮后坐力和镜子。尽管1740年后他的科学活动减少了，但他确实协助儿子塞萨尔-弗朗索瓦·卡西尼·图里进行了制作法国精确地图的项目。

最后让我们强调，卡西尼对他那个时代的主要科学辩论做出了极其重要的贡献。尽管他支持一个关于地球形状的错误假说，但他的贡献仍然极其重要。提出假说并加以检验是科学进步的本质。我们绝不应因为卡西尼在辩论中站在错误的一方而认为他的贡献不那么重要。

1756年4月15日，当他访问自己在图里的城堡时，马车翻倒，他于次日因这次事故去世。他被安葬在图里的教堂里。

Jacques Cassini was the son of Jean-Dominique Cassini and Geneviève de Laistre. To avoid confusion with other members of the Cassini family (his father, son and grandson), he is sometimes known as Cassini II. Jean-Dominique Cassini was head of the Paris Observatory at the time of his marriage to Geneviève in 1674 and, two years previously, had become a French citizen, changing his name from Giovanni Domenico Cassini. Geneviève was the daughter of the lieutenant general of the compté of Clermont and they owned the Château de Thury in the Oise which became their summer residence. Jacques, the second of his parents two sons, was born at their home at the Observatory in Paris. It was a wealthy family and Jacques received the finest education possible.

The Paris Observatory where his family lived provided an excellent place to educate a young boy with enthusiastic interests in science. This is where Jacques had his earliest education, after which he studied at the Collège Mazarin in Paris. Varignon had been appointed professor of mathematics at the Collège Mazarin in 1688 and he supervised Cassini's thesis on optics. In August 1691, at the age of fourteen, Cassini defended his thesis. An early biographer [3] states that, when he was fifteen years old, he dedicated a thesis on mathematics to the Duc de Bourgogne. It is unclear whether these two references to theses refer to the same work or to two separate ones.

In 1694 Cassini was admitted to the Académie des Sciences and began to undertake scientific work on projects which his father was carrying out. The following year he travelled with his father in Italy and they made numerous geodesic observations, as well as visiting Bologna where they repaired the gnomon at the Church of San Petronio which Cassini's father had designed nearly thirty years before. Cassini, like his father, was interested in making both astronomical observations and also interested in accurate surveys. After the Italian trip, Cassini visited Flanders and then England in around 1698. While in England he met Newton, Flamsteed, and Halley and was elected to the Royal Society of London. After returning to France he published the astronomical and geodesic data which he had gathered on his travels.

In 1700 Cassini's father undertook a project to measure the meridian from Paris to Perpignan, which is 13 km west of the Mediterranean coast. Cassini assisted his father on this project and they obtained results which wrongly suggested that the Earth was elongated at the poles. In 1713 he proposed a new method for determining longitude by means of the eclipses of the stars and planets by the moon. Applying this method, and using data from the 1700 Paris to Perpignan survey, he claimed to have proved [1]:-

... that the degrees of the terrestrial meridian grow smaller from the equator towards the pole.

It was unfortunate that Cassini resolutely stuck to this position throughout his life, refusing to acknowledge the flattening despite the scientific evidence which was put forward. Whether this was due to a false sense of patriotism, believing that he was supporting the French view against the English view which was a consequence of Newton's gravitational theory (which Cassini never accepted), or whether it was through a false sense of family loyalty supporting his father's views, we shall never know. Perhaps indeed both may have contributed without Cassini being fully aware that they were affecting his scientific judgement, or perhaps he was so convinced that the results of the 1700 survey were correct that he could never accept the contrary. The real problem with the data from the survey was, as pointed out in 1733 by Giovanni Poleni, that both the elongation proposed by Cassini or the flattening proposed by others, fell within the experimental error of the instruments used.

Cassini had some legal training but not so much that one would expect him to play a substantial role in this field but, in addition to his scientific work, he did undertake a second parallel career. He was appointed to the Chambre des Comptes in 1706, which was a financial court with administrative and legal duties relating to the King's accounts and in particular to the land owned by the Crown. He gained a high reputation for honesty in this position but was sometimes criticised for being indecisive. Later, in 1716 he was an advocate in the Court of Justice, and in 1722 he became a Councillor of State.

His scientific role was one of major importance, playing a major role in the Académie des Sciences and taking over as head of the Paris Observatory from his father. Around 1709 his father's health began to fail and Cassini took on more and more of the duties of head. It is worth noting that at this time there was no official post of Director of the Observatory, which was in theory run by the Académie des Sciences, but Cassini like his father before him effectively ran it. In the following year 1710 he married Suzanne-Françoise Charpentier from Charmois. They had three sons, Dominique-Jean, César-François who was later known as Cassini III or Cassini de Thury, and Dominique-Joseph, and two daughters Suzanne-Françoise and Elisabeth-Germaine. The eldest son Dominique-Jean followed his father into the Chambre des Comptes while the youngest son followed a military career.

Cassini continued both his astronomical and surveying work. He undertook the measurement of the Paris meridian north to Dunkerque in 1718. He published the results, which again supported his incorrect theory of elongation at the poles, in De la grandeur et de la figure de la terre Ⓣ in 1722. This important treatise surveyed the results which had been obtained on measuring the Earth over the preceding fifty years. However, those like Maupertuis who believed that the Earth was flattened at the poles argued ever more strongly against Cassini's theory and, in an attempt to gain further evidence to support his case, Cassini organised another project in 1733, this time to measure the perpendicular to the meridian from Saint-Malo to Strasbourg. As he had accompanied his father when he was a young man, on this measuring project Cassini had the assistance of his own son César-François Cassini (who appears in this archive under the name Cassini de Thury). The data they collected during the years 1733-34 seemed to support the elongation theory but this only encouraged those members of the Académie such as Maupertuis who supported the Newtonian view, to organise further scientific expeditions in an attempt to settle the argument in their favour. By 1738 the geodesic measurements carried out in Peru by Bouguer and La Condamine in 1735 and Lapland by Maupertuis in 1736 to measure the length of a meridian degree had produced very strong evidence for the flattening at the poles. Although Cassini never deviated from his belief, he began to scale back his scientific work from 1740.

As to his contributions to astronomy, Cassini published many papers in the journals of the Académie des Sciences and two major treatises in 1740, namely Elements of Astronomy and Astronomical Tables of the Sun, Moon, Planets, Fixed Stars, and Satellites of Jupiter and Saturn. These showed his excellent abilities as an observer and he made impressive contributions with his study of the moons of Jupiter and Saturn and his study of the structure of Saturn's rings. Most notable of his discoveries was that of the proper motions of the stars which he made in 1738. Here for the first time the ancient belief in the unchanging sphere of the stars had been shown to be incorrect by direct measurement.

Cassini's work was not restricted to that of astronomy and geodesy. He wrote papers on a variety of other topics on the applications of mathematics, particularly on electricity, barometers, the recoil of firearms, and mirrors. Although after 1740 his scientific activity decreased, he did assist his son Cassini de Thury in his project to produce an accurate map of France.

Let us end by stressing that Cassini made an extremely important contribution to the major scientific debates of his time. Although he supported an incorrect hypothesis regarding the shape of the Earth, nevertheless his contribution is extremely important. It is in the nature of scientific progress that hypotheses get put forward and tested. We should in no way consider Cassini's contribution any the less important because he was on the wrong side in the debate.

On 15 April 1756 when he was on a visit to his Château at Thury, his carriage overturned and he died as a result of this accident on the following day. He was buried in the Church at Thury.