詹姆斯·布拉德利James Bradley

英国天文学家兼牧师，詹姆斯·布拉德利是William Bradley（生于1640年）和Jane Pound（生于1656年）的第三个儿子，他们于1678年结婚。马克斯·玻恩于1693年3月出生，同年10月3日受洗，均在格洛斯特郡的舍伯恩，布拉德利曾就读于诺斯利奇的韦斯特伍德文法学校，之后进入牛津大学贝利奥尔学院，作为自费生被录取，即没有奖学金的学生。他于1714年10月获得学士学位，1717年6月获得硕士学位。此后，他开始了教会生涯。

他于1719年5月24日被任命为执事，同年7月，在被任命为牧师后，成为威尔士东南部蒙茅斯郡布里茨托的牧师。然而，布拉德利的教会生涯并不长久。他于1721年10月31日辞去神职人员职务，当时他被任命为牛津大学萨维尔天文学讲席教授。后来有人邀请他重返教会并接管格林威治教区，但他拒绝了这一邀请，选择致力于天文学并继续留在当时所在的格林威治天文台。

考虑到布拉德利后来的天文学职业生涯，他早年最重要的经历是与他的舅舅布拉德利 Pound牧师的接触，后者是英格兰领先的天文观测者，曾与爱德蒙·哈雷和艾萨克·牛顿合作。正是通过这位舅舅，布拉德利结识了爱德蒙·哈雷。

到1715年，布拉德利已加入他的舅舅布拉德利 Pound进行天文观测。Pound偶尔向布拉德利提供经济援助，并被认为影响了布拉德利对天文学的兴趣。尽管Pound于1724年11月去世，布拉德利继续使用他舅舅在万斯特德的天文台。虽然没有留下遗嘱，布拉德利保持使用同样的仪器，并与Pound的遗孀同住。

在1716年3月曾就极光写信给皇家学会之后，布拉德利的两项观测由爱德蒙·哈雷于1718年发表在皇家学会的Philosophical Transactions上。爱德蒙·哈雷赞扬了布拉德利的能力，1718年11月，布拉德利在爱德蒙·哈雷的推荐下当选为皇家学会会士。

1721年被任命为牛津大学萨维尔天文学讲席教授，于1722年4月26日发表就职演讲，他对木星卫星的运动表现出极大兴趣。将他从伦敦对四颗主要卫星（伽利略卫星木卫一、木卫二、木卫三和木卫四）食的观测与从纽约和里斯本进行的观测进行比较，并观察每颗卫星在木星背后经过时消失和重现时间的差异，他得以推断出三个观测地点之间的经度差。1726年，他向皇家学会报告了这些结果。

正是在这一观测计划期间，他注意到每颗卫星遵循着不同的路径。于是他着手改进卫星运动表，并理解这些运动差异。进入18世纪30年代后，他基于四颗卫星之间的相互引力吸引，提出了一个描述它们运动关系的公式。

布拉德利最为人所知的是天文学中的两项重要发现：光行差和地轴章动。光行差是一种天文现象，它使天体在其真实位置附近产生视运动，取决于观测者的速度。章动是一种导致旋转天体自转轴取向随时间变化的现象。它由附近天体作用于旋转天体的引力引起。章动（自转轴的短期变化）与岁差（长期变化）之间有所区别。

作为Samuel Molyneux的朋友，Samuel Molyneux对测量恒星的周年视差感兴趣，由此可以计算到最近恒星的距离，布拉德利于1725年12月应Molyneux之邀前往其位于伦敦基尤的庄园，协助他观测天龙座γ星，具体而言，是试图确定其周年视差。

之所以特意选择天龙座γ星，是因为它几乎从头顶经过子午线，从而使观测者可以忽略地球大气折射光线所造成的任何问题。根据理论，这颗恒星将在12月于其最南点中天，其中天点每天向北移动，到6月达到其最北点，并在接下来的六个月中返回。

然而实际观测到的是，12月之后，天龙座γ星继续在更偏南的点中天，在3月而非12月达到其最南点，并在9月而非6月达到其最北点。六个月中位置的总变化约为百分之一度。

因此，从1727年8月起，为了解释这一运动的原因，布拉德利决定在一年中观测若干恒星。他发现它们各自都表现出类似的运动。

在泰晤士河上航行时，他注意到尽管风一直朝同一方向吹，但船转向时桅杆顶上的风向标却改变了方向。布拉德利提出，类似地，观测到的恒星位置的视偏移是光以有限速度从恒星传播与地球绕太阳周年运动共同作用的结果。这就是所谓的光行差。布拉德利写给爱德蒙·哈雷提及这一现象的信于1729年1月在皇家学会上宣读。

根据他的观测，他能够计算光速，声称来自太阳的光需要8分12秒到达地球（今天，我们认为是8分20秒）。他还得出结论，他和Molyneux原本要发现的周年视差的计算超出了当时可用仪器的精度。因此，到最近恒星的距离必定比之前认为的还要大。

1729年，他在牛津大学承担了实验哲学讲师的额外角色，在阿什莫林博物馆授课。三十多年来，他给平均超过五十名学生的听众讲授这些课程。

他还研究土星。在土星环侧对地球因而不可见的一段时间里，他于1730年观测到土星环的重新出现，他的记录是那个日期唯一留存下来的。

1732年，布拉德利搬到牛津，入住新学院桑德斯·麦克兰恩的一所房子，这是他的教授职位赋予他的权利。他的姑母，即布拉德利 Pound 的遗孀，与他同住直到1737年去世。Pound 在 Wansted 家中的许多天文仪器也被搬来。一个显著的例外是格雷厄姆天顶仪，它留在了那里。布拉德利经常回访 Wansted 使用它。正是在那里，他继续工作，最终发现了章动。

搬到牛津后，布拉德利继续在 Wansted 进行观测，旨在收集他观测到的地球极轴章动的证据。通过探测和测量这一点，他将能够计算地球的扁球性，从而解决牛顿派和雅克·卡西尼追随者之间关于地球在两极是扁平还是拉长的争论。

布拉德利 通过坚持观测证明了他对这一问题的执着，尽管面临一个重大障碍：地球自转轴完成一个章动周期需要超过18年。他在必要的时间内持续工作，每次需要观测时都返回万斯特德。这是必要的，因为所有观测都必须使用同一台仪器在同一地点进行。

到1747年，他已经完成了他的计划，并写了一封信，该信于1748年2月在皇家学会上被宣读。在信中，他能够证实地球的牛顿模型，即地球在两极是扁平的。这封信极好地展现了布拉德利的工作方式和思维方式，我们已在THIS LINK上提供了该信的第一部分。

然而，到那时，法国赴瑞典和秘鲁的探险队也提供了独立的观测证实，但布拉德利在这里的工作，以及他早期关于光行差的工作，为日心说太阳系增添了越来越多的证据。他提供了地球绕太阳运行的第一个观测证据。在此之前，缺乏恒星视差的观测证据是阻碍人们完全接受哥白尼太阳系模型的原因，因为伽利略的所谓“证明”被认为是谬误的。正因如此，布拉德利于1748年被授予皇家学会的科普利奖章。弗里德里希·威廉·贝塞尔后来于1838年测量了天鹅座61号星的周年视差，进一步为哥白尼的论点增添了证据。

布拉德利 自1742年起担任皇家天文学家，是第三位担任此职的人，接替了爱德蒙·哈雷。

他于1744年6月25日与格洛斯特郡查尔福德的苏珊娜·皮奇结婚。他们的女儿也叫苏珊娜，生于1746年。正是她继承了她父亲去世前20年间在格林尼治所做的观测记录，这些观测记录由于种种原因，在布拉德利去世后30多年都未发表。

随着他在章动方面的工作和宣布，他开始获得国际认可；1746年，他成为柏林皇家科学院的成员；1748年7月，他被任命为巴黎Académie Royale des Sciences的外籍通讯院士；1750年，他成为圣彼得堡帝国科学院的通讯成员，1754年成为正式成员；1757年，他当选为博洛尼亚科学院成员。他还得到了王室的奖励，获得1000英镑用于重新装备他的天文台（这是他年收入的10倍）。国家还亲自授予他每年250英镑的养老金（按今天的货币计算约为45000英镑）。

直到1798年，布拉德利的任何观测结果才得以发表。他的女儿苏珊娜成年后，将这些观测结果交给了她的舅舅皮埃尔·萨米埃尔·皮奇牧师，后者在经度委员会索要时拒绝交出。他将这些资料留给了儿子弗瑞兹·约翰，后者也不愿交出。王室就这些文件的所有权提起了诉讼，最终这些文件被呈交给牛津大学校监诺斯勋爵，他愿意将它们交给大学，条件是必须由克拉伦登出版社出版。然而，他们花了20年才出版第一卷观测结果。

如此延迟发表天文数据的一个问题是，迟了四十年，它们已不太有用。直到弗里德里希·威廉·贝塞尔被鼓励这样做，这种滞后所造成的复杂性和繁难才得到解决。弗里德里希·威廉·贝塞尔的工作花了10年，从布拉德利的观测开始，最终产生了他的Fundamenta astronomiaⓉ（《基础天文学》）（1818年），被誉为位置天文学最重要的著作之一。

关于斯蒂芬·彼得·里戈为Miscellaneous works and correspondence of the Rev James Bradley, D.D. F.R.S.（牛津大学出版社，牛津，1832年）所写的序言，见THIS LINK。

布拉德利的影响延伸到20世纪，他的观测结果在1882年至1903年间出版了三卷。有人认为，凭借这些，布拉德利奠定了所有现代恒星位置的基础。

然而，他的影响远不止于对恒星的追踪。他对精确测量时间非常感兴趣。布拉德利收集了从伦敦和牙买加寄给他的数据，并在考虑温度差异后得出结论：用于这些实验的时钟在牙买加时比在伦敦时每天慢约2分钟。这符合艾萨克·牛顿关于地球扁球形状的引力理论。此外，尽管没有文献记载，但据信布拉德利作为皇家天文学家，在1752年英格兰采用格里高利历一事中发挥了作用。

他对局部现象（由所用仪器、天文台周围条件以及地轴和轨道运动所产生的现象）影响的关注，使他以及后来的天文学家能够只专注于天体本身的特征。尽管他不是唯一如此认真对待位置天文学的人，但作为皇家天文学家，他无疑是一位主要权威。

正是他虚弱的身体迫使布拉德利于1761年退休。在此之前，他曾尝试过一次雄心勃勃的远征。1761年，一次罕见的金星凌日即将发生。为了利用对这一现象的观测来提高太阳视差值的精度，他选择爪哇和圣赫勒拿作为两个观测点，因为它们在位置上相距足够远。由于身体太虚弱，无法亲自前往，他派格林尼治的助手查尔斯·梅森前往爪哇，派年轻的天文学家内维尔·马斯基林前往圣赫勒拿。两次远征都没有成功。梅森因七年战争的阻碍未能及时到达爪哇，而内维尔·马斯基林的尝试则因天气恶劣而受挫。

1757年妻子去世后，加上自己身体有病，他前往格洛斯特郡查尔福德投靠妻子的家人。他于1762年7月13日因慢性腹部炎症去世。他被安葬在格洛斯特郡明钦汉普顿圣三一教堂的墓地里，靠近他的母亲和妻子，墓地的一座祭坛式墓上固定了一块黄铜牌。在他去世近70年后，一块铭牌被竖立在邱园，位于邱宫日晷的基座上，以标记他开始观测的地点。铭牌上写着：——

在此地点
1725年
布拉德利牧师
进行了首次观测
这些观测导致了伟大的发现
光行差和
地轴章动
他所使用的望远镜
由萨姆·莫利纽克斯先生架设
在一座后来成为
皇家住所
并于1808年被拆除的房屋中。
为永久纪念
如此重要的地点
此日晷于1852年安置于此
奉
最仁慈的陛下
国王威廉四世之命。

尽管他一生中发表的作品相对较少，但他关于光行差和章动的论文对于这些发现以及天文学中精度概念的发展都至关重要。

据天文学史家、数学天文学家和巴黎天文台台长让·巴蒂斯特·约瑟夫·德朗布尔在其1821年关于18世纪天文学的史书中所述：——

正是[光行差和章动的发现……使我们拥有了现代天文学的精确性。……这一双重贡献确保了[布拉德利]在所有国家所有时代最伟大的天文学家之上（仅次于喜帕恰斯和约翰内斯·开普勒）享有最杰出的地位。

在他一生中备受推崇，并得到当时一些伟人的认可，布拉德利对天文学思想的进步具有高度影响力。爱德蒙·哈雷很快认识到了他的才能，而艾萨克·牛顿则称他为欧洲最好的天文学家。

English astronomer and priest, James Bradley was the third son of William Bradley (born 1640) and Jane Pound (born 1656), who had married in 1678. Born in March 1693 and baptised on 3 October the same year, both in Sherborne, Gloucestershire, James attended Westwood's Grammar School at Northleach, before attending Balliol College, Oxford, where he was admitted as a commoner, that is a student who does not have a scholarship. He received his BA in October 1714 and his MA in June 1717. Following this he began a career in the church.

He was ordained deacon on 24 May 1719, and in July of that year, after being ordained a priest, became vicar of Bridstow in Monmouthshire, South-East Wales. However, James's career with the church was not to be long-lived. He resigned as a clergyman on 31 October 1721 when he was appointed Savilian Professor of Astronomy in the University of Oxford. He was later offered the chance to return to the church and take over at the parish of Greenwich, but he rejected the offer, opting instead to devote himself to astronomy and continue at the Greenwich observatory, where he was located at the time.

The most significant of Bradley's early years, when considering his later career in astronomy, were those spent in contact with his maternal uncle, the Reverend James Pound, a leading astronomical observer in England, who had worked with Edmond Halley and Sir Isaac Newton. It was through this uncle that James would meet Halley.

By 1715, James Bradley had joined his uncle James Pound in carrying out astronomical observations. Pound provided Bradley with occasional financial assistance, and is credited with influencing Bradley's interests towards astronomy. Despite Pound's death in November 1724, Bradley continued to use his uncle's observatory at Wansted. Though no will was left, Bradley maintained his usage of the same instruments, and residence with Pound's widow.

Having previously written to the Royal Society in March 1716 about the aurora, two of Bradley's observations were published by Halley in 1718 in the Philosophical Transactions of the Royal Society. Halley praised Bradley's abilities and, in November 1718, Bradley was elected a fellow of the Royal Society on Halley's recommendation.

Appointed Savilian Professor of Astronomy in the University of Oxford in 1721, giving his inaugural lecture on 26 April 1722, he showed a great interest in the motions of Jupiter's satellites. Comparing his own observations from London of the eclipses of the four main satellites (the Galilean moons Io, Europa, Ganymede and Callisto) with those made from New York and Lisbon, and observing the differences in the times of disappearance and reappearance of each as it passed behind Jupiter, he was able to deduce the differences in longitude between the three observation locations. In 1726, he reported these results to the Royal Society.

It was during this programme of observations that he noted the different paths that each satellite followed. He thus set out to improve the tables of the satellites' motions, and to understand these differences in motion. Continuing into the 1730s, he proposed a formula for the relationship between the motions of the four satellites based on their mutual gravitational attraction.

Bradley was best known for two important discoveries in astronomy: the aberration of light and the nutation of the Earth's axis. The aberration of light is an astronomical phenomenon which produces an apparent motion of celestial bodies about their true positions, dependent on the velocity of the observer. Nutation is a phenomenon which causes the orientation of the axis of rotation of a spinning astronomical body to vary over time. It is caused by the gravitational forces of nearby bodies acting upon the spinning body. A difference is made between nutation (short-term variations in the axis of rotation) and precession (long-term variations).

As a friend of Samuel Molyneux, who was interested in measuring the annual parallax of stars, from which it would be possible to calculate the distances to the nearest stars, Bradley was invited by Molyneux in December 1725 to his estate at Kew, a district of London, to assist him in observing the star Gamma Draconis, specifically, attempting to determine its annual parallax.

Gamma Draconis was specifically chosen since it crosses the meridian almost overhead, thus allowing observers to ignore any problems caused by refraction of light through the earth's atmosphere. According to theory, the star would culminate at its most southerly point in December, with its point of culmination shifting northwards every day to its most northerly point in June, returning over the following six months.

What was actually observed however was that after December, Gamma Draconis continued to culminate at more southerly points, reaching its most southerly point in March, rather than December, and its most northerly point in September, rather than in June. The overall change in position over the six months was about a one hundredth of a degree.

Bradley thus, from August 1727, in an attempt to explain the cause of this movement, decided to observe a number of stars over the year. He found that they each displayed a similar motion.

While sailing the Thames, he noticed how a vane atop the mast changed direction as the boat turned, despite the fact that the wind remained blowing in the same direction. Bradley suggested that analogously, the apparent shift in the position observed in the stars was a result of of a combination of the transmission of light at a finite speed from the star and the annual movement of the Earth around the Sun. This is what is known as the aberration of light. The letter from Bradley to Halley referencing this phenomenon was read to the Royal Society in January 1729.

From his observations, he was able to calculate the speed of light, claiming that light from the Sun took 8 minutes 12 seconds to reach the Earth (today, we make it 8 minutes 20 seconds). He also concluded that the calculation of the annual parallax that he and Molyneux had set out to discover was beyond the accuracy of the instruments available. Thus, the distance to the nearest stars must be even greater than had previously been thought.

He took on the added role of lecturer in experimental philosophy at Oxford in 1729, lecturing in the Ashmolean Museum. For over thirty years he gave these lectures to an average audience size of more than fifty students.

He worked also on studying Saturn. During a time when Saturn's rings were edge-on to the Earth and thus invisible, he observed the reappearance of the rings in 1730, his record of which are the only ones to have survived from that date.

The year 1732 saw Bradley move to Oxford to occupy a house in New College Lane, to which his professorship entitled him. His aunt, widow of James Pound, stayed with him until her death in 1737. Many of the astronomical instruments from Pound's house in Wansted were also moved. A notable exception was the Graham zenith sector, which remained. Bradley made frequent return visits to Wansted to use it. This is where he continued his work that led to the discovery of nutation.

Continuing his observations at Wansted after he moved to Oxford, Bradley aimed to collect evidence he was observing of the nutation of the Earth's polar axis. By detecting and measuring this, he would be able to calculate the spheroidicity of the Earth, thus settling the dispute between Newtonians and followers of Jacques Cassini about whether the Earth was flattened or elongated at the poles.

Bradley proved his dedication to the problem by pursuing his observations, despite a major hurdle: the Earth's axis would take more than 18 years to complete a nutational cycle. He worked for the necessary period, returning to Wansted each time an observation was required. This was necessary since all observations had to be made using the same instrument at the same location.

He had concluded his programme by 1747, writing a letter that was read to the Royal Society in February 1748. In his letter, he was able to confirm the Newtonian model of the Earth, that it was flattened at the poles. This letter gives a wonderful understanding of how Bradley worked and his ways of thinking and we have made available the first part of that letter at THIS LINK.

By this time, however, there was also separate observational confirmation by French expeditions to Sweden and Peru, but Bradley's work here, and his earlier work on aberration, added to the increasing evidence for a heliocentric solar system. He offered the first observational proof that the Earth revolved around the Sun. Prior to this, the lack of observational evidence of stellar parallax was what was holding back full acceptance of the Copernican solar system model, for Galileo's attempted 'proof' was seen to be fallacious. It was thus that Bradley was awarded the Royal Society's Copley medal in 1748. Bessel would go on in 1838 to measure annual parallax for the star 61 Cygni, further adding to the Copernican argument.

Bradley served as Astronomer Royal from 1742, the third to occupy the role, succeeding Halley.

He married Susannah Peach of Chalford, Gloucestershire, on 25 June 1744. Their daughter, also called Susannah, was born in 1746. It was she who would inherit the Greenwich observations her father had made over the 20 years prior to his death, the same observations, that for many reasons, were not published for over 30 years after Bradley's death.

With his work on, and announcement of, nutation, he began to receive international recognition; 1746 saw him become a member of the Royal Academy of Berlin; in July 1748 he was made a foreign associate of the Académie Royale des Sciences in Paris; in 1750, he became a corresponding member of the Imperial Academy of Sciences in St Petersburg, becoming a full member in 1754; in 1757 was when he was elected to become a member of the Academy of Sciences of Bologna Institute. He was also rewarded by the crown, receiving a payment of £1000 to re-equip his observatory (this was 10 times the value of his annual income). The state also personally awarded him a pension of £250 per annum (roughly £45,000 in today's money).

It was not until 1798 that any of Bradley's observations were published. Upon his daughter Susannah's coming-of-age, she passed the observations on to a maternal uncle, the Reverend Samuel Peach, who refused to surrender them to the board of longitude upon their request. He left them to his son, John, who was also unwilling to give them up. A legal case was set up by the crown over the ownership of the documents, resulting in their presentation to the chancellor of Oxford University, Lord North, who would give them to the University on the condition that they were to be published by the Clarendon Press. They took, however, 20 years to publish a first volume of the observations.

A problem with such a delayed publication of astronomical data was that forty years late, they were not overly useful. It was not until Wilhelm Bessel was encouraged to do so that the complications and complexities created by such a lag were tackled. It took 10 years of Bessel's work, which began with Bradley's observations, resulting in his Fundamenta astronomia Ⓣ (1818), lauded as one of the most significant works on positional astronomy.

For Stephen Peter Rigaud's Preface to Miscellaneous works and correspondence of the Rev James Bradley, D.D. F.R.S. (Oxford University Press, Oxford, 1832), see THIS LINK.

Bradley's influence extended into the 20th century, with three volumes of his observations being published between 1882 and 1903. It has been suggested that with these, Bradley had laid the foundation of all modern star positions.

His influence however, stretched far wider than just over the tracking of the fixed stars. He was very interested in the accurate measurement of time. Bradley collected data sent to him from London and Jamaica, and, allowing for the temperature differences, concluded that the clock used for these experiments slowed by about 2 minutes per day when in Jamaica in comparison to when in London. This followed Newton's theory of gravity for the Earth's oblate shape. Moreover, though not documented, it is believed that Bradley, as Astronomer Royal, had a role in England's adoption of the Gregorian calendar in 1752.

His focus on the impact of local phenomena (those resulting from the instruments used, the conditions around the observatory and from the motions of the Earth's axis and of its orbit) allowed him, and later astronomers, to concentrate solely on the objects' characteristics. Though not alone in taking positional astronomy so seriously, he was certainly a leading authority as Astronomer Royal.

It was to be his poor health that would force Bradley into retirement in 1761. Prior to this he had attempted an ambitious expedition. In 1761 a rare transit of Venus was due to take place. With the intention of using observations of this phenomenon to improve the accuracy of the value of solar parallax, he chose Java and St Helena to be the two observation points, since they were sufficiently widely separated in location. Too frail to make the journeys himself, he sent his Greenwich assistant Charles Mason to Java, and the young astronomer Nevil Maskelyne to St Helena. Neither expedition was successful. Mason failed to reach Java in time due to obstruction caused by the Seven Years' War, and Maskelyne's attempt suffered from poor weather.

Following the death of his wife in 1757, and with his own physical illness, he went to stay with his wife's family in Chalford, Gloucestershire. He died on 13 July 1762 from a chronic abdominal inflammation. Buried near his mother and wife in the churchyard of Holy Trinity church, Minchinhampton, Gloucestershire, a brass plate was fixed to an altar tomb in the churchyard. Almost 70 years following his death, a plaque was erected at Kew on the base of the Kew Palace Sundial to mark where he began his observations. It reads:-

On This Spot
In 1725
The Rev James Bradley
Made the First Observations
Which led to the Great Discoveries
The Aberration of Light and
The Nutation of the Earth's Axis
The Telescope which he used
Had been erected by Sam. Molyneux Esq.
In a House which afterwards became
A Royal Residence
And was Taken Down in 1808.
To Perpetuate the Memory of
So Important a Station
This Dial was Placed on it in 1852
By Command of
His Most Gracious Majesty
King William the Fourth.

Though he published relatively little during his lifetime, his papers on aberration and nutation were key for their discoveries and also for the development of the notion of precision in astronomy.

According to the historian of astronomy, mathematical astronomer and director of the Paris Observatory Jean Baptist Joseph Delambre, in his 1821 history of astronomy in the 18th century:-

It is to [the discovery of aberration and nutation ... that we owe the exactness of modern astronomy. ... This double service assures to [Bradley] the most distinguished place (after Hipparchus and Kepler) above the greatest astronomers of all ages in all countries.

Highly regarded during his lifetime, and recognised by some of the greats of the day, James Bradley was highly influential in the advancement of astronomical thinking. Halley had quickly recognised his talent, and Newton had described him as the best astronomer in Europe.