英语时差：什么是视差和如何计算它（下）

This is an exciting discovery, because we can now use parallax to judge the distance to objects out in space. Here’s how. Because the earth moves around the sun in one year’s time, every six months it is as far away from where it was six months ago as it can get. That distance is like the distance between your eyes. Images taken of a far off object in January can be compared with images of the object in July. The apparent motion of the object due to parallax tells us how far away it is from earth. But wait! How do you know that your telescope is pointed in exactly the right direction? By aligning it with the so-called “fixed stars” — stars so far away they show no parallax motion whatsoever. Against that unmoving background, the closer stars can be picked out.