Tom Murphy wants to know how far it is to the moon. Not just how many miles， or how many feet， or even how many inches.
He wants to know it within a millimeter①， and that’s less than a 25th of an inch.And he wants to know exactly how far it is every moment of every day. That’s despite the fact that the distance changes minute by minute because the moon is in an elliptical② 28-day orbit ranging from 220，000 to about 252，000 miles from Earth.
To achieve his goal， he will use the latest in laser technology， a large telescope in New Mexico， a team of other experts， a little funding from NASA， and a whole lot of luck.
If he doesn’t success， he could spend the rest of his life trying to live down “Murphy’s folly.” If he does， he might disprove part of Einstein’s theory of General Relativity， or he might discover evidence of an unseen celestial③ body in our solar system， either of which would earn him a page in the history books. What Murphy is really doing is using the Earth and the moon as his laboratory， because the questions he is asking can’t be answered in an ordinary lab. The tools just aren’t big enough.
Murphy’s primary method will be something called “laser ranging.” Light from a laser fired from the Earth could hit a reflector on the surface of the moon and bounce back. The time it took for the light to travel from Earth to the reflector and back would reveal the exact distance or at least within a few inches. That would reveal much about the lunar orbit， and that data could in turn be used to test some of the tantalizing④ ideas in Einstein’s theories.
By using a laser system mounted on a 3.5-meter telescope at Apache Point， and equipped with a sophisticated array of detectors that can capture and isolate every photon⑤ of light that bounces back from the moon， Murphy hopes to get the number down well below a millimeter.
The laser will blast a 1-billion-watt “bullet” of light at the moon 20 times every second. But the Earth’s atmosphere will distort⑥ the beam once it leaves the telescope， so by the time it gets to the moon the beam will cover an area more than a mile wide. Murphy is hoping that at least one out of every 30 million photons hits a reflector and bounces back toward Earth. That would send about a billion photons back from each bullet.
But by the time the reflected laser beam reaches Earth， it will have spread out to nearly 10 miles in diameter， so probably only about one out of 30 million reflected protons will actually be captured by the detectors. That data will be fed into a powerful computer， but all it will tell is the distance between the telescope and the reflector.
① millimetern. 公厘，毫米［C］
② ellipticaladj. （=elliptic）椭圆的
③ celestialadj. 天的，天空的
④ tantalizev. 逗惹，折磨，强烈地诱惑
⑤ photonn. ［物］光子
⑥ distortv. 扭曲，扭歪，使失真，使(电视等的声音、影象)变形
月亮有多高
Tom Murphy想知道地球到月亮有多远，有多少英里，多少英尺，多少英寸。
他想用毫米来测量这一距离，而一毫米不到一英寸的1/25。Murphy还想知道每天的每分每秒地球与月亮的距离是多少。尽管事实上月球是在一个周长为28天的椭圆形轨道上运行，与地球的距离每分钟都在变，在22万~25.2万英里不等。
为了实现目标，他将利用最新的激光技术、一个设在新墨西哥州的大型望远镜、一队专家、一笔来自美国航空航天局的微薄资金和还有全部的运气。
如果失败，他可能会在“Murphy的遇蠢 ”的耻辱下度过余生。如果成功了，他也许证明爱因斯坦广义相对论的部分理论不成立，或者他也许会发现太阳系新天体存在的证据，无论如何他都将名垂青史。Murphy真正要做的事，就是把地球和月亮作为自己的实验室，因为他不可能在普通实验室找到答案，没有这么大的实验工具。
Murphy的基本方法将是“激光测距”，从地球上发出的激光打射到设在月球表面的反射器上，然后反射回地球。光从地球到达反射器后再反射回来的所用的时间，可以揭示月球与地球的实际距离，至少可以将误差降到几英寸以内。这就可以揭示许多关于月球轨道的奥秘。这些数据继而又可以用来检验爱因斯坦理论中一些令人困扰、捉摸不透的概念。
依靠使用安放在新墨西州Apache Point市3.5米望远镜上的激光系统，并装备一组能捕获分离从月球反射回来光子的精密探测仪，Murphy希望能把月地距离的测量精确到1毫米之内.
这种激光将以每秒20次的速度向月球发射10亿瓦特的光“子弹一百万千瓦的光束”，但是地球的大气会扭曲这些光束，所以等光束到达月球时，所经过的地带将达一英里多宽。Murphy希望在每3，000万光子中至少有一个能碰到反射器后回地球. 这就可以使每一个光弹反射回大约10亿个光子。
但是当反射的激光束到达地球时，他的直径将扩散到大约10英里，所以在事实上很可能每3，000万反射光子中才大约有一个光子能被探测仪捕获到. 这数据将输入功能强大的计算机处理，但所有这些将阐明的，只是望远镜和反射镜的距离。
