The world is smaller than first thought, German researchers at the University of Bonn said.
They took part in an international project to measure the diameter (直径) of the world that showed it is five millimeters (毫米) smaller than the last measurement made five years ago.
Dr Axel Nothnagel, who led the Bonn researchers, said the difference was crucial in the study of climate change.
“It may seem a very small defference, but it is essential for the positioning of the satellites that can measure rises in sea level.”
“They must be accurate to the millimeter. If the ground stations tracking the satellites are not accurate to the millimeter, then the satellites cannot be accurate either.”
The scientists rounded the number up to 12,756.274 kilometers for the general public.
The system of measurement used by the Bonn scientists in the two-year project consisted of radio waves that were transmitted into space.
“A network of more than 70 radio telescopes worldwide receives these waves. Because the gauging (测量) stations are so far apart from each other, the radio signals are received with a slight time lag (时滞),” dr Nothnagel said.
“From this difference we can measure the distance between the radio telescopes to the accuracy of two millimeters per 1,000 kilometers.”
The procedure is called VLBI, which stands for “Very Long Baseline Interferometry”.
The technique can be used, for example, to prove that Europe and North America are moving apart at a rate of about 18 millimeters a year.

德国伯恩大学的研究人员指出，世界比先前的要小些。
他们在参与的一项国际合作项目中测量了地球的直径，研究表明现在地球比五年前的最后一次测量时直径小了5毫米。
Axel Nothnagel博士曾经是伯恩研究人员的领导者，他说这个变化对于研究气候变化非常重要。
“这也许看起来是个很小的不同，但是这对于测量海平面升高的卫星定位是非常重要的。”
“他们必须精确到毫米。如果地面站跟踪卫星不是精确到毫米级，那么卫星也不能保持精确。”
科学家们对外公布的数据为12，756.274公里。
在为期两年的项目中，伯恩科学家们使用的这个测量系统由发射到太空的无线限电波组成。
“一个由超过七十台射电望远镜组成的世界性网络接收了这些无线电波。由于测量站之间间隔比较远，接收到的无线电波会有轻微的时滞”Nothnagel 博士说。
通过这些不同，我们可以测量设点雷达之间的距离，测量精度（误差）在每一千公里两毫米。
这个方法被称为VLBI，取意于“超长基线的干涉测量”。
这个技术可以被实际运用。例如，可以证明欧洲大陆和北美大陆正在以每年19毫米速度反向运动。