第 4 课 · 课后练习

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.

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 Halley. In November 1677, Halley observed a transit of Mercury from 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 shift of an astronomical object due to 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 unit (AU).

CHalley was aware that the AU was one of the most fundamental astronomical measurements. Johannes Kepler, in the early 17th century, had shown that the distances of 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 Earth. The goal was to measure the AU; then, knowing orbital speeds of all other planets, the scale of the Solar System would fall into place. However, Halley realised Mercury was too distant for precise parallax measurements. Venus, closer to Earth, would show a much larger parallax angle, allowing the Sun's distance to be calculated to 1 part in 500. But transits of Venus are rare, occurring in pairs roughly eight years apart every hundred years or so. He accurately predicted transits in 1761 and 1769, though he did not live to see them.

DInspired by Halley's idea, British and French astronomers set off on expeditions to observe the 1761 and 1769 transits, travelling to India, Siberia and other remote regions. But Britain and France were at war, which disrupted observations. The French astronomer Guillaume Le Gentil suffered terrible misfortune. He was trapped by a British siege in Pondicherry and watched the 1761 transit from a rolling ship, making accurate timing impossible. He stayed in the southern hemisphere to wait for the 1769 event, travelling 50,000 km to the Philippines — only for clouds to block his view at the critical moment.

EWhile early transit timings were as precise as instruments allowed, measurements were hampered by the 'black drop' effect. When Venus begins to cross the Sun's disc, its outline smudges rather than staying circular, blurring timings. This optical distortion comes from light diffraction. A second issue: Venus appears surrounded by a faint luminous halo when just off the Sun's edge, proving it has a thick gaseous atmosphere. Both phenomena distorted the transit timings astronomers needed.

FAstronomers analysed decades of transit data to calculate the AU. Johann Franz Encke, Director of the Berlin Observatory, used all the parallax results to produce a figure of 153,340,000 km. This was reasonably accurate for its era; today's radar-measured AU is 149,597,870 km. The AU acts as a universal cosmic measuring stick. The same parallax principle used for Venus transits can be applied to calculate distances to distant stars: compare a star's position in January and six months later, using Earth's orbital diameter to compute its distance.

GThe 2004 Venus transit was more of a spectacular public event than a vital scientific mission. However, the techniques refined by studying Venus transits have paved the way for one of modern astronomy's most vital breakthroughs: discovering Earth-sized exoplanets orbiting distant stars.