How far to the Moon? Tom Murphy of the University of Washington is engaged in a five-year project to measure very accurately - to a millimetre or so - the distance from the Earth to the Moon. We have long known that the centre of the Moon lies about 384 790 km from the centre of the Earth. By the 1970s this distance was known to within 25 cm, and by the mid-1980s the uncertainty had been reduced to 2 cm,' but Murphy wants to know it more accurately than that. He is using a laser attached to the 3.5 m telescope at Apache Point. New Mexico, to send laser beams to the lunar surface where they will be reflected back to Earth by reflectors left there by various lunar missions, including Apollo 11. Each laser 'bullet' will be aimed at one of five retro-reflectors, banks of 100-300 prisms the size of a suitcase. The beams leave Earth with a diameter of about 2.5 cm, although because of atmospheric distortion they hit the Moon with a diameter of 2 km. Thus only one of every 30 million photons sent out will find the reflector,' and of those that do, only one in 30 million will return to the detector on Earth. Currently, for every 100 laser pulses launched, only one photon can be expected to get back, although Murphy hopes to increase that to 5-10. All these complications explain why this laser-ranging project is expected to last five years.
展开▼
机译：到月球有多远？华盛顿大学的汤姆·墨菲（Tom Murphy）参与了一个为期五年的项目，旨在非常精确地测量从地球到月球的距离（大约一毫米）。我们早就知道，月球中心距地球中心约384 790公里。到1970年代，这个距离已知在25厘米以内，到1980年代中期，不确定性减小到2厘米，”但墨菲想比这更准确地知道它。他正在使用一台安装在Apache Point 3.5 m望远镜上的激光。新墨西哥州，将激光束发送到月球表面，然后由包括阿波罗11号在内的各种月球任务留下的反射器将其反射回地球。每个激光“子弹”将对准五个后向反射器之一，分别是手提箱大小的100-300棱镜。光束离开地球的直径约为2.5厘米，尽管由于大气变形，光束以2公里的直径撞击月球。因此，每发出的3000万个光子中只有一个能找到反射器，而在发出的光子中，只有3000万个中的一个会返回地球上的探测器。目前，每100个发射的激光脉冲中，只有一个光子可以返回，尽管墨菲希望将其提高到5-10。所有这些复杂因素解释了为什么这个激光测距项目预计将持续五年。
展开▼
