How can they calculate the distance between the sun and the earth?
There are various methods and combinations of methods. One method mentioned is parallax (see source 1). The best way to explain it is to hold your thumb out and look at it first with one eye and then the other, noting its position against the background. By measuring that difference (in the object’s position relative to the background), and knowing the difference between the two observation points (the distance between your two eyes), you can compute the distance to the object (your thumb). This lets us measure the distance to the moon or to nearby planets like Venus and Mars by using two observation points many miles apart (as long as you know exactly how far apart they are), and measuring the position of the object relative to some background object (like a star). Using parallax measurement with the sun is difficult because it is so bright it is impossible to see background objects (i.e. you can’t see distant stars when the sun is shining). However, during an eclipse you can. And the
