Like the planets, the stars are in motion, some of them at tremendous speeds, but they are so far away form Eaarth that their apparent positions in the sky do not change enough for their movement to be observed during a single human lifetime.

最近代词指代好晕，直接把正确的句子敲上去了，这个them指代不会造成指代planets或者stars的歧义吗？还是the planets是单数？？问题很白痴，求助。。。。。