June 2004 saw the first passage, known as a `transit`, of the planet Venus across the face of the Sun in 122 years. Transits have helped shape our view of the whole Universe, as Heather Cooper and Nigel Henbest explain

2004年6月，122年来，金星再次越过太阳表面构成天文奇观，也就是所谓的“凌日”现象。正如Heather Cooper 和 Nigel Henbest所解释的那样，金星凌日现象影响了我们对整个宇宙的认识。

A 2004年6月8日，全世界—半以上的人都有幸见证了这起罕见的天文现象——经过六个多小时，金星缓缓滑过了太阳表面。这是自1882年12月6日以来的第一次金星凌日现象。彼时，美国天文学家Simon Newcomb教授带领着—堆人去南非观测这—天文现象。他们的观测点设在—所女子学校里，据说这所学校里的三位女教师合力观测出的结果比这组专业人士的还要准确。

AOn 8 June 2004, more than half the population of the world were treated to a rare astronomical event. For over six hours, the planet Venus steadily inched its way over the surface of the Sun. This `transit` of Venus was the first since 6 December 1882. On that occasion, the American astronomer Professor Simon Newcomb led a party to South Africa to observe the event. 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.

B 数百年来．金星凌日现象引起了全球各地的探险家与天文学家的关注，而这—起都要归功于非凡的博学家Edmond Halley。1677年11月，Halley在位于南太平洋的荒芜人烟的圣赫勒拿岛上，观测到了内行星水星的凌日现象。他发现，水星滑过太阳盘面的轨迹因观测纬度不同而有差异。通过计算行星在两个相距甚远的地方之间的运行时间，天文学家小组可以计算出视察角度。视察角度是指天体的位置由于观测者的位置不同而产生的明显差异。计算观察角度让天文学家得以实现当时的最终目标——算出地球与太阳之间的距离，这个距离就是所谓的“天文单位(AU)”。

C Halley知道，天文单位是天文学中测量距离的基本单位之—。在17世纪早期，Johannes Kepler就认为行星和太阳之间的距离控制着行星的轨道速度，这个很容易就能测量到，但是还没有人能找到—种方法来计算行星与地球之间的精确距离。目标是先测量出天文单位然后了解其他所有行星围绕太阳运行的轨道速度，最后就能水到渠成，测出太阳系的规模。然后，Halley意识到水星距离地球太远了以致很难确定其视差角度，而金星则距离地球较近，它的视察角度较大。他发现如果利用进行来计算太阳的距离，其误差很可能只有五百分之—。但是有—个问题，与水星凌日不同，金星凄日现象很罕见，而且总是以两次为—组，每组中的两次大约间隔8年，而两组之间的间隔却有100多年。尽管如此，Halley还是准确预测出金星会在1761年与1769年两次穿过太阳表面，只可惜他有生之年一次也没有看到。

BFor centuries, transits of Venus have drawn explorers and astronomers alike to the four corners of the globe. And you can put it all down to the extraordinary polymath Edmond Hailey. In November 1677,Hailey observed a transit of the innermost planet, Mercury, from the desolate island of St Helena in the South Pacific. He realised that, from different latitudes, the passage of the planet across the Sun`s disc would appear to differ. 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 observers position. Calculating this angle would allow astronomers to measure what was then the ultimate goal: the distance of the Earth from the Sun. This distance is known as the astronomical unit` or AU.

D 在Halley提出的测量太阳系方法的鼓舞下，英国和法国的天文学家组成小组，踏上去往各地的征途，这些地方甚至包括印度与西伯利亚。但是由于那时候英法两国在交战，所以这些观测并没有奏效。最值得同情的是法国天文学家Guillaume Le Gentil。英军包围了他在印度本地治里(Pondicherry)的观秘台，这使他备受打击。在乘坐—艘法国军舰穿越印度洋逃亡的时候，他看到了一次凌日的壮观景象，但是穿的颠簸摇晃使他完全没有机会进行精确预测。他并没有回信，而是留在了南半球，先是忙于研究毛里求斯岛和马达加斯加岛的情况，接着前往菲律宾准备观测下一次凌日现象。然而，具有粉刺意味的是，在跋涉了将近五万公里之后，它的视线居然被一片乌云给遮住了，真是一次令人沮丧的经历。

E 虽然早期对零日事件的观测就当时所用的器材而言已足够精确，但是其测量结果却受到“黑滴”效应的困扰。金星入凌时，看起来有点儿模糊而不完全是圆的，因此很难计算时间。这种现象是由光的衍射造成的。另—个问题是，金星出凌时，它的周围会产生晕环。虽然天文学家可以获知金星是被—层厚厚的，可折射阳光的气体包围，但是黑滴效应和晕环效应都使得他们无法获得金星凌日的准确时间。

CHailey was aware that the AU was one of the most fundamental of all astronomical measurements. 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. But no-one had found a way to calculate accurate distances to the planets from the Earth. 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. However, Hailey realised that Mercury was so far away that its parallax angle would be very difficult to determine. As Venus was closer to the Earth, its parallax angle would be larger, and Hailey worked out that by using Venus it would be possible to measure the Suns distance to 1 part in 500. But there was a problem: transits of Venus, unlike those of Mercury, are rare, occurring in pairs roughly eight years apart every hundred or so years. 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.

F 但是天文学家依然努力分折这些观测结果，以便用来观测金星凄日现象。柏林天文台台长 Johann Franz Encke根据所有这些规差测量最终确定了天文单位的值为153340000千米，这个数值在当时已经相当精确了，也与现在用雷达测到的149597870千米非常接近。当然现在雷达因其精准度已经取代了凌日测量与其他方法。天文单位是—个宇宙测量杆，也是现在我们测量宇宙的基础。视差原理可以延伸应用到恒星之间距离的测量中。一月，当地球处于其轨道的某个电时，我们观测—颗恒星，那么六个月后这颗恒星的位置与当时观测的位置看起来是不同的。了解了地球轨道的宽度后，天问这家就可以利用视差移位计算出这个距离。

G 2004年6月的金星凌日现象不只是—项中大的科学事件，更是一次天文奇观。而这种凌日现象为宇宙中最重大的突破之一铺平了道路，即对围绕其他恒星运行的类地行星进行探测。

DInspired by Haileys 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. But things weren't helped by Britain and France being at war. The person who deserves most sympathy is the French astronomer Guillaume Le Gentil. He was thwarted by the fact that the British were besieging his observation site at Pondicherry in India. Fleeing on a French warship crossing the Indian Ocean, Le Gentil saw a wonderful transit - but the ships pitching and rolling ruled out any attempt at making accurate observations. 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. Ironically after travelling nearly 50,000 kilometres, his view was clouded out at the last moment, a very dispiriting experience.

EWhile the early transit timings were as precise as instruments would allow, the measurements were dogged by the 'black drop' effect. When Venus begins to cross the Suns disc, it looks smeared not circular - which makes it difficult to establish timings. This is due to diffraction of light. The second problem is that Venus exhibits a halo of light when it is seen just outside the Suns disc. 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.

FBut astronomers laboured hard to analyse the results of these expeditions to observe Venus transits. 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. 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. The AU is a cosmic measuring rod, and the basis of how we scale the Universe today. The parallax principle can be extended to measure the distances to the stars. 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. Knowing the width of Earth`s orbit, the parallax shift lets astronomers calculate the distance.

GJune 2004,s transit of Venus was thus more of an astronomical spectacle than a scientifically important event. 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.