Parallax

视差就是从有一定距离的两个点上观察同一个目标所产生的方向差异。从目标看两个点之间的夹角，叫做这两个点的视差，两点之间的距离称作基线。只要知道视差角度和基线长度，就可以计算出目标和观测者之间的距离。同位置看同一天体的方向之差。比如，当你伸出一个手指放在眼前，先闭上右眼，用左眼看它;再闭上左眼，用右眼看它，会发现手指相对远方的物体的位置有了变化，这就是从不同角度去看同一点的视差。视差可用观测者的两个不同位置之间的距离(基线)在天体处的张角来表示。

You may have heard of parallax, but what is it? Find out on this Moment of Science.

Parallax is phenomenon by which we can judge the distance to things just by looking at them. Astronomers use parallax to calculate the distance to stars. Do you need a gigantic computer-based observatory to use it? Not at all. Your finger will do nicely.

Put one finger up in front of your face and close one eye, then the other. Your finger seems to jump back and forth between two positions. That’s because the perspective from each eye is slightly different, and your brain combines these images to give you a sense of three dimensional space.

Notice, however, that the farther you move your finger away the less discrepancy there is between the images. Parallax must decrease with distance! 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 June. 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(恒星)”–stars so far away they show no parallax motion whatsoever. Against that unmoving background, the closer stars can be picked out.

译文：

视差

视差是我们目测事物的距离时所产生的现象。天文学家利用视差来计算我们到恒星的距离。你需要使用一个庞大的以计算机为基础的天文台吗?根本不用。你的手指就可以做得很好。

放一只手指在你的面前，闭上一只眼睛，然后换另一只眼睛。你的手指似乎在两个位置之间来回移动。这是因为每只眼睛的角度稍有不同，大脑合成的这些影会给你一种三维空间感。

但是，请注意，你的手指放得越远，这些图像之间位置的差异就越小。视差会随着距离增加而减少。这是一个令人兴奋的发现，因为我们可以利用视差来判断与外太空物体的距离。下面我们来说说怎样做到。

因为地球围绕太阳公转一圈是一年的时间，每六个月后它的位置离六个月前最远。这之间的距离就像你两眼之间的距离一样。在一月拍摄的远距离物体可与六月拍摄的相比较，根据视差所表现的物体的明显运动轨迹能够告诉我们它离地球有多远。

但是等等，你怎么知道你的望远镜是指向的正确的方向?通过对准所谓的“恒星”-星星如此之远以至于它们没有视差运动的现象。与静止的背景形成对比，比较近的星星就可以被挑选出来。