引言

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 Healther Cooper and Nigel Henbest explain

自然段A

On 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 girl’s school, where—it is alleged—the combined forces of three schoolmistresses outperformed the professionals with the accuracy of their observations.

自然段A：

2004年6月8日，全球超过一半的人口观赏到了一次罕见的天文事件。在六个多小时内，金星稳定地穿越太阳表面。这次金星凌日现象是自1882年12月6日以来的第一次。据称，在那次观测中，美国天文学家西蒙·纽科姆教授率领一个观测团队前往南非进行观察。他们驻扎在一所女子学校，据说三位女校长联合的观测准确性超过了专业人士。

自然段B

For centuries, transits of Venus have drawn explores and astronomers alike to the four corners of the globe. And you can put it all down to the extraordinary polymath Edmond Halley. In November 1677, Halley observed a transit of the innermost planet, Mercury, from the desolate island of St Helena in the south Pacific. He realized 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 observer’s 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’ or AU.

自然段B：

几个世纪以来，金星凌日吸引着探险家和天文学家前往世界各地。而这一切都归功于杰出的博学家埃德蒙·哈雷。1677年11月，哈雷在太平洋南部荒凉的圣赫勒拿岛上观测到了内层行星水星的凌日现象。他认识到从不同的纬度观测时，行星穿过太阳盘面的方式会有所不同。通过在两个相距较远的位置观测凌日现象的时间，天文学家们可以计算视差角，即由于观测者位置的差异而引起的天体的视差。通过计算这个角度，天文学家可以测量当时的终极目标：地球与太阳的距离，也就是所谓的“天文单位”或AU。

自然段C

Halley was aware that the AU was one of the most fundamental of all astronomical measurements. Johannes Kepler, in the early 17 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, Halley realized 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 Halley worked out that by using Venus it would be possible to measure the Sun’s 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.

自然段C：

哈雷意识到天文单位是所有天文测量中最基本的之一。17世纪初，约翰内斯·开普勒已经证明了行星到太阳的距离决定了它们的轨道速度，而这种速度是容易测量的。但是至今还没有人找到一种准确计算行星与地球之间距离的方法。目标是测量天文单位，然后通过知道其他行星绕太阳的轨道速度，太阳系的比例尺将揭示出来。然而，哈雷意识到水星距离太远，很难确定其视差角。由于金星离地球更近，其视差角会更大，哈雷计算出通过使用金星可以将太阳距离测量到500分之1的精度。但是有一个问题：金星凌日现象与水星凌日现象不同，很少发生，约每隔100多年会有一对。尽管如此，他准确预测金星将在1761年和1769年经过太阳盘面，但他自己没有活着看到。

自然段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. 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. Feeling 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. 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.

自然段D：

受到哈雷对确定太阳系比例尺方法的启发，英国和法国的天文学家们前往印度和西伯利亚等地进行考察。但是英法两国正处于战争状态，这给事情增加了困难。最值得同情的人是法国天文学家吉约姆·勒让蒂尔。由于英国正在围困他在印度波多乍铁里的观测地点，他受挫了。在穿越印度洋的法国战舰上，勒让蒂尔看到了一个奇妙的凌日现象，但船只的摇晃使得无法进行准确观测。不气馁的他继续留在赤道以南地区，在观测完毛里求斯和马达加斯加的岛屿之后，前往菲律宾观测下一次凌日现象。讽刺的是，经过近5万公里的旅行，他在最后一刻被云层遮挡，这是一次令人沮丧的经历。

自然段E

While 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 Sun’s 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 Sun’s 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.

自然段E：

虽然早期的凌日观测时间非常准确，但测量结果受到了“黑滴”效应的困扰。当金星刚开始穿过太阳盘面时，其形状看起来不是圆形的，这使得确定时间变得困难。这是由光的衍射造成的。第二个问题是，金星在离开太阳盘面时会出现一圈光晕。虽然这表明金星被一层厚厚的气体包围，可以将阳光折射到它周围，但这两个效应都使得无法获得准确的时间测量。

自然段F

But astronomers laboured hard to analyse the results of these expeditions to observe Venus transits. John Franz Encke, Director of the Berlin Observatory, finally determined a value for the AU based on all these parallax measurements: 153,340,000km. Reasonably accurate for the time, that is quite close to today’s value of 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.

自然段F：

但天文学家们努力分析了这些观测金星凌日的结果。柏林天文台台长约翰·弗朗茨·恩克最终根据所有这些视差测量结果确定了一个天文单位的值：1亿5334万千米。这在当时是相当准确的，接近今天的测量方法的精确度。天文单位是宇宙中的一个测量尺度，也是我们今天衡量宇宙的基础。视差原理可以扩展到测量恒星的距离。如果我们在一月份观察一颗恒星-当地球处于轨道上的一个点时-它看起来会与六个月后的位置不同。通过知道地球轨道的宽度，视差位移使得天文学家可以计算出距离。

自然段G

June 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.

自然段G：

因此，2004年6月的金星凌日现象更多是一次天文壮观的表演，而非科学上的重要事件。但这样的凌日现象为可能证明宇宙中最重要突破之一铺平了道路——探测绕其他恒星运行的类地行星。