A On 8 June 2004, more than half the population of the world were treated to a rare astronomical event. A 2004 年6 月8 日，世界一半以上的人们有幸观看到一个罕见的天文奇观。 For over six hours, the planet Venus steadily inched its way over the surface of the Sun. 在6 个多小时的时间里，金星缓慢地匀速飞过太阳的表面。 This ‘transit’ of Venus was the first since 6 December 1882. 这次金星凌日于1882 年12 月6 日以来首次出现。 On that occasion, the American astronomer Professor Simon Newcomb led a party to South Africa to observe the event. 那一次，美国天文学家西蒙·纽科姆教授带领一个团队到南非观测这一现象。 They were based at a girls’ school, where - it is alleged - the combined forces of three schoolmistresses outperformed the professionals with the accuracy of their observations. 他们住在一所女校里。据说，三名学校女老师合作的观测准确度居然超过了这些专业人士。

B For centuries, transits of Venus have drawn explorers and astronomers alike to the four corners of the globe. B 几百年来，金星凌日现象吸引着探险家和天文学者聚集到地球的四个角落。 And you can put it all down to the extraordinary polymath Edmond Halley. 这都归功于非凡的学者埃德蒙·哈雷。 In November 1677, Halley observed a transit of the innermost planet, Mercury, from the desolate island of St Helena in the South Pacific. 1677 年11 月，哈雷在南太平洋的偏僻小岛圣海伦娜岛上观测到太阳系最内部的行星水星的一次凌日。 He realised that, from different latitudes, the passage of the planet across the Surfs disc would appear to differ. 他意识到如果在不同的纬度观测，行星凌日的轨迹看起来是不同的。 By timing the transit from two widely-separated locations, teams of astronomers could calculate the parallax angle - the apparent difference in position of an astronomical body due to a difference in the observer’s position. 通过分成不同的团队在两个相距很远的地点记录凌日时间，天文学家们可以计算视差角——在不同地点观测到的天体出现位置的明显差异。 Calculating this angle would allow astronomers to measure what was then the ultimate goal: the distance of the Earth from the Sun. 计算这一角度将使当时的天文学家能够测量地球到太阳的距离，这是当时天文学的终极目标。 This distance is known as the ' astronomical unit' or AU. 这一距离被称作“天文单位”或“AU”。

C Halley was aware that the AU was one of the most fundamental of all astronomical measurements. C 哈雷知道AU 是最基本的天文测量工具。 Johannes Kepler, in the early 17th century, had shown that the distances of the planets from the Sun governed their orbital speeds, which were easily measurable. 约翰尼斯·开普勒在17 世纪早期就已经提出行星到太阳的距离决定了它们的轨道速度，这是很容易测量的。 But no-one had found a way to calculate accurate distances to the planets from the Earth. 但是没有人找到准确计算这些行星到地球距离的方法。 The goal was to measure the AU; then, knowing the orbital speeds of all the other planets round the Sun, the scale of the Solar System would fall into place. 当时人们的目的是通过测量AU，加上已知的所有行星公转的轨道速度，计算并掌握太阳系的大小。 However, Hailey realised that Mercury was so far away that its parallax angle would be very difficult to determine. 然而，哈雷意识到水星距离地球过远，它的视差角很难确定。 As Venus was closer to the Earth, its parallax angle would be larger, and Halley worked out that by using Venus it would be possible to measure the Sun's distance to 1 part in 500. 金星距地球较近，它的视差角更大，并且哈雷还计算出利用金星可以将测量地日距离的误差控制在五百分之一。 But there was a problem: transits of Venus, unlike those of Mercury, arc rare, occurring in pairs roughly eight years apart every hundred or so years. 但是，当时有一个问题：与水星凌日不同，金星凌日非常罕见，每百年左右会出现两次，中间相隔大约八年。 Nevertheless, he accurately predicted that Venus would cross the face of the Sun in both 1761 and 1769 - though he didn't survive to see either. 尽管如此，他准确地预测金星将在1761 年和1769 年飞越太阳表面，但是他没能活到看到这一现象的那一天。

D Inspired by Halley’s suggestion of a way to pin down the scale of the Solar System, teams of British and French astronomers set out on expeditions to places as diverse as India and Siberia. D 受到哈雷准确计算太阳系大小建议的启发，一些英国和法国的天文学家组成探险团队，去往印度和西伯利亚这两个相距遥远的地区。 But things weren't helped by Britain and France being at war. 但是英法之间的战争破坏了这一行动。 The person who deserves most sympathy is the French astronomer Guillaume Le Gentil. 最值得同情的人是法国天文学家纪晓姆·勒·让蒂尔。 He was thwarted by the fact that the British were besieging his observation site at Pondicherry in India. 他的观测受到了阻碍，因为英国人正在包围他位于印度本地治里的观测站。 Fleeing on a French warship crossing the Indian Ocean, Le Gentil saw a wonderful transit - but the ship's pitching and rolling ruled out any attempt at making accurate observations. 在乘坐法国军舰逃离的路上，勒·让蒂尔看到了一次美妙的凌日现象，但是军舰的颠簸使他不可能做出准确的观测。 Undaunted, he remained south of the equator, keeping himself busy by studying the islands of Mauritius and Madagascar before setting off to observe the next transit in the Philippines. 他勇敢地留在赤道以南，靠研究毛里求斯和马达加斯加等岛屿打发时间， 然后去菲律宾观测下一次凌日。 Ironically alter travelling nearly 50,000 kilometres, his view was clouded out at the last moment, a very dispiriting experience. 但讽刺的是，在奔波了将近50,000 公里后，他的视线在最后一刻被天空的云彩遮住了。这真是一次令人泄气的经历。

E While the early transit timings were as precise as instruments would allow, the measurements were dogged by the ‘black drop’ effect. E 尽管凌日现象的计时达到了当时设备允许的极致，但是这种测量一直受到“黑滴现象”的影响。 When Venus begins to cross the Sun's disc, it looks smeared not circular - which makes it difficult to establish timings. 当金星开始飞过太阳表面时，它看起来拖着尾迹而不是圆形的——这使得准确计时非常困难。 This is due to diffraction of light. 这是由于光线的衍射作用。 The second problem is that Venus exhibits a halo of light when it is seen just outside the Sun's disc. 另一个问题是，人们看到当金星邻近太阳时，它会带有光晕。 While this showed astronomers that Venus was surrounded by a thick layer of gases refracting sunlight around it, both effects made it impossible to obtain accurate timings. 尽管这让天文学家了解到金星周围包裹着厚厚的一层可以衍射光线的气体，但这两个效应使他们不可能准确地测量时间。

F But astronomers laboured hard to analyse the results of these expeditions to observe Venus transits. F 尽管如此，天文学家们依然投入大量的精力分析这些观测队的观测结果，试图分析金星凌日现象。 Johann Franz Encke, Director of the Berlin Observatory, finally determined a value for the AU based on all these parallax measurements: 153,340,000 km. 柏林天文台台长约翰·弗朗茨·恩克最终基于所有这些观测到的视差角确定了AU 的值，即153,340,000 公里。 Reasonably accurate for the time, that is quite close to today's value of 149,597,870 km, determined by radar, which has now superseded transits and all other methods in accuracy. 这一当时被认为达到合理精确度的数值非常接近现代的数据——149,597,870 公里。 现代的数据是利用雷达取得的，在精确度上远远超过凌日分析及所有其他的方法。 The AU is a cosmic measuring rod, and the basis of how we scale the Universe today. AU 是一个测量宇宙的尺度，也是我们今天测量宇宙大小的基础。 The parallax principle can be extended to measure the distances to the stars. 视差角方法可以推广到测量地球到其他恒星的距离。 If we look at a star in January - when Earth is at one point in its orbit - it will seem to be in a different position from where it appears six months later. 如果我们在一月份观察一颗恒星——此时地球处于轨道中的某个位置——在六个月以后这颗恒星看起来将出现在另一个位置。 Knowing the width of Earth's orbit, the parallax shift lets astronomers calculate the distance. 在知道地球轨道宽度的情况下，天文学家可以利用视差角的变化计算这颗恒星的距离。

G June 2004’s transit of Venus was thus more of an astronomical spectacle than a scientifically important event. G 因此，2004 年6 月的金星凌日更多的是一次天文奇观而不是重大科学事件。 But such transits have paved the way for what might prove to be one of the most vital breakthroughs in the cosmos - detecting Earth-sized planets orbiting other stars. 但是这些凌日现象为取得宇宙研究最伟大的突破之一——找到那些围绕其他恒星轨道公转的和地球大小相似的行星——铺平了道路。