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。