你的地图到午饭时就会出错

你的手机之所以知道你在哪里，是因为它一直在“聆听”太空中的时钟。卫星不断广播时间信号，你的手机根据每个信号到达所用的时间，计算出自己的位置。说到底，这其实是一个披着地图外衣的计时问题。

Your phone knows where you are because it is listening to clocks in space. Satellites broadcast the time, over and over, and your phone works out its position from how long each signal took to arrive. It is really a timing problem wearing a map.

麻烦的地方在于，这些时钟走动的速度和你手腕上的表并不一样。在太空中，离地球质量更远，时间会走得稍快一些；而卫星在轨道上高速运动，又会让时间稍微变慢。这两种效应并不能完全抵消，结果卫星上的时钟每天大约会快出百万分之三十八秒。

Here is the awkward part. Those clocks do not tick at the same rate as the clock on your wrist. Up there, further from the mass of the planet, time runs slightly faster. Moving fast in orbit pulls the other way and slows them slightly. The two effects do not cancel, and the satellites end up gaining about thirty eight millionths of a second every day.

听起来微不足道，其实不然。光在十亿分之一秒内大约能传播三十厘米，如果不加以修正，这样的误差每天会累积成大约十公里的定位偏差。到晚饭前，地图上代表你的那个蓝点就会先偏离街道，再偏离整座城市。因此，这些时钟在地面上被特意设定成“走得不准”，好让它们一旦上了太空，反而能走得恰到好处。

That sounds like nothing. It is not. Light travels about thirty centimetres in a billionth of a second, so an uncorrected error of that size grows into roughly ten kilometres of position error per day. Your blue dot would drift off the street, then off the town, before dinner. The clocks are deliberately built to tick at the wrong rate on the ground, so that they tick at the right one once they are up there.