【经济学人】古代日食竟然有这样的作用|2016.12.10|总第767期

AS THE well-known Australian philosopher, Kylie Minogue, once pointed out, it can be a source of comfort to remember that, no matter what else is happening, the world still turns. Unfortunately, things are not quite so simple. Thanks to the moon’s gravitational tug, the speed at which Earth spins has been slowing since the satellite’s birth about 4.5bn years ago. Physicists can calculate from first principles how big the effect should be. It turns out that the moon should be adding about 2.3 milliseconds to the length of the day with each century that passes. This means, for instance, that 100m years ago, when dinosaurs ruled Earth, a day was nearer 23 than 24 hours.

正如著名的澳大利亚哲学家Kylie Minogue曾说过，无论发生什么，世界都不会改变---经常被用作心灵鸡汤之言。不幸的是，世界似乎没有这位哲学家形容的这么简单：由于月球的引力作用，相比于45亿年前月球成为地球卫星时，地球的自转一直都在变慢。物理学家能够利用第一性原理计算出地球自转变慢的影响有多大。结果是每100年地球日长就增加2.3毫秒。这意味着一亿年前恐龙时代，地球自转一周大概只需要23小时而不是现在的24小时。

• gravitational tug：引力作用

• satellite：n. 卫星

• dinosaur：n. 恐龙

But that 2.3 milliseconds is only an average. Geological events within Earth can speed the process up, or slow it down. Tracking changes in day length over time is thus of interest. And that requires data. Thanks to the development of super-accurate atomic clocks in the 1950s, and to laser range-finding equipment left on the moon by the Apollo astronauts, researchers have plenty of such data from the past half-century. But more information is always welcome. And extra data are exactly what a team led by Leslie Morrison, a retired professional astronomer, have just provided. In a study just published in the Proceedings of the Royal Society they use observations made by ancient Chinese, Babylonian, Greek and Arab astronomers to reconstruct the history of Earth’s rotation over the past two and a half millennia.

不过2.3微秒只是一个平均值。地球上每一次地质变化都能加快或减慢这个速度。追踪日的长短不仅需要兴趣，还需要数据。多亏了二十世纪五十年代的超高精度原子钟的发展，以及阿波罗宇航员在月球上安装的激光测距设备，研究人员得以掌握了近半个世纪以来的大量的数据。不过信息肯定是越多越好的。一位名为Leslie Morrison的退休的前专业天文学家及其领导的团队恰好证实了这一点并于近日在《英国皇家学会学报》发表了他们的研究成果。他们利用古代中国，巴比伦，希腊以及阿拉伯几个国家近两个半世纪的观测数据揭露了地球自转的历史。

• Observation：n. 观察值

日食

Plenty of ancient cultures were keen on astronomy, for the patterns of the heavens were the basis of their calendars and timekeeping systems (not to mention their systems of astrology). But Dr Morrison and his colleagues were interested in two particular phenomena: eclipses of the sun and of the moon. A solar eclipse happens when the moon moves in front of the sun, as seen from a particular spot on Earth, and blocks out its light. A lunar eclipse happens when the moon moves behind Earth in such a way that Earth blocks the sunlight that, when reflected from the lunar surface, renders the moon visible.

大部分古代的文化都跟天文学息息相关，因为历朝历代的历法和计时系统（更不用说古代还有天体系统）都是基于天体的轨迹得出的。不过Morrison博士和他的同事们兴趣点主要集中于两种天体现象：日食和月食。从地球上某个特定位置点观察，当月球运行到太阳与地球之间时，月球遮住了太阳的光芒，日食便会发生。同样的，当地球运行到日月之间时，地球遮挡住了太阳照射到月球上使月球可见的光芒，月食便会发生。

• Keen: adj. 敏锐的

• Timekeeping: n. 计时；时间记录

• Eclipse: n. 日食，月食

Eclipses were often viewed as portents (usually bad ones), or as battles between moon or sun gods and the forces that sought to overthrow them. For that reason, records of hundreds of historical eclipses survive to the present. Building on work begun in the 1990s, Dr Morrison and his colleagues collated 424 such observations of 250 eclipses that happened between 720BC and 1600AD. The records were scattered across Babylonian clay tablets preserved at the British Museum, translations of histories of Chinese dynasties and Japanese emperors, records from ancient Greece preserved in works like the “Almagest”, and observations made by Arab astronomers during what was, in Europe, the Dark Ages. (The researchers note that no record survives of an eclipse seen in Europe for almost a thousand years following one recorded in 364AD.)

日食和月食在过去往往会被看成是一种不祥之兆，解读为月神和太阳神之间的力量之争。因为上述原因，从古至今关于日食和月食的历史记载多达成百上千。Morrison博士和他的同事们从上世纪90年代开始着手这个研究，并整理了从公元前720年到公元1600年的两百五十份关于日月食的观察记录。这些记录来自不同地方，包括：收藏于大英博物馆的巴比伦泥板，中国及日本皇室两国的翻译文献，古希腊收藏的诸如《天文学大成》之类著作里的记录，以及阿拉伯天文学家在欧洲中世纪（又成为“黑暗时期”）的观测记录。（研究人员指出,从公元364年最后一个记录的发现起，将近一千年的黑暗时期，欧洲几乎没有一份日月食的记录被保留下来。）

• Portents: n. 前兆

• Battles: n. 战争

• Overthrow: vt. 推翻

• Collate: v. 整理

• Babylonian: adj. 巴比伦的

• Almagest: n. 《天文学大成》，又名《至大论》，是希腊天文学家托勒密(Ptolemy)在西元140年前后编纂的天文学和数学百科全书

月食

The idea was to combine those observations with modern computer models of the solar system. These are sophisticated enough to let researchers reconstruct the positions of Earth, moon and sun at any given date in the past, and therefore to work out when and from where any past eclipses should have been visible. That means such models can be used to confirm recorded sightings. But because ancient astronomers reckoned the passage of time by the motion of the heavens, and modern models are based on the unvarying output of atomic clocks, the gradual slowing of Earth’s rotation will produce a disagreement in timing between the two.

运用现代计算机技术将这些观测记录对太阳系进行建模。模型的复杂程度之高甚至可以让研究员们再现过去任何一天地球、月亮和太阳的位置关系，因此也能重现过去日食和月食发生的时刻和地点。这就意味着这个模型可以检测观测记录的准确性。不过因为古代天文学家是依靠天体的运动来推算时间，而现代的模型的时间是基于不变的原子钟，所以地球自转逐渐放缓将通过这二者的差异得出。

• Sophisticated: adj. 复杂的

• Reckone: v. 认为

Many a mickle makes a muckle

积少成多

Given the tiny differences in day length involved (a few dozen milliseconds at most) and the primitive equipment (such as water clocks) available to ancient researchers, it might be thought that any difference in timing would be too small to detect. But Dr Morrison points out that this is not actually a problem. The error introduced by Earth’s deceleration is cumulative. Today may be only fractionally shorter than a day was 2,700 years ago, when the earliest records that the team looked at were collected, but between then and now almost a million days have elapsed. Each passing day adds any daily discrepancy to the total discrepancy, and those repeated daily discrepancies add up to significant fractions of an hour—periods well within the accuracy available to ancient astronomers.

考虑到每一天长度的微小差异（最多就只有几十个微秒）以及古代研究员使用的原始的设备（比如说水钟），可以想当然地理解任何时间上的差异之小足以让人无法发现。不过Morrison博士指出，这实际上并不成问题。由于地球的减速过程而产生地时间误差是不断积累的。根据团队收集的记录来看，今天可能只比两千七百年前的一天多无法感知的一点点，但是从两千七百年前到现如今，中间的无数个日日夜夜里几乎发生了百万次的日月食。日复一日的不明显的误差加到一起，最后累计成了明显的小时——这绝对是连古代的天文学家都能够察觉得到的精度范围了。

• Detect: v. 观测，察觉

• Deceleration: n. 减速

• Cumulative: adj. 累积的

• Discrepancy: n. 不符；矛盾；相差

After crunching the numbers, the team found that the actual rate at which days have been lengthening over the past couple of millennia is 1.8 milliseconds per century, considerably slower than the 2.3 milliseconds predicted. The main reason for the difference, they reckon, is the lingering effect of the most recent ice age, during which the mass of ice at the planet’s poles was sufficient to deform its shape and thus alter its rate of spin. That is not, though, the only thing which is happening. The researchers also found small but cyclical patterns in the rate of change that repeat themselves over decades—as well as intriguing hints of longer cycles with time periods of thousands of years. Exactly what geophysical goings-on such cycles represent is one for the geologists to work out.

处理完数据后，该团队发现每一天的增加的时间在过去两千年里平均每一百年可差出1.8微秒，比之前预估的2.3微秒要少了些。他们认为造成这个改变的主要原因是最近一次的冰川时代的影响，地球两极的冰块重量改变了地球的形状，从而造成了自转的变缓。不过这并不是唯一的改变。研究人员同时也发现了在过去的数十年中不断重复的、微小但是可循的变化规律，这也表明如果成千上万年积累循环周期会更长。（期待您的翻译，明天会有针对这句话的长难句解析哟~）

• Crunch: vt. 处理

• Lingering: adj. 拖延的

翻译 ▍妖精

审核 ▍阳光下的冰叶

编辑 ▍毛毛