The History Channel has this great Albert Einstein Documentary.

A large part of this documentary revolves around measuring the deviation of where a star is from where it is seems to be observed to be when the light from the star passes close by to the sun. In order to see a star close to the sun, you need to take a photograph at a total solar eclipse. That is the only time when the light from the sun doesn’t drown out the light from a nearby star.

What has bothered me for years about this experiment, and the documentary never touches on in an hour and a half, is how do you know where the star is compared to where it seems to be?

It just dawned on me how you know. You take pictures of the stars when the sun isn’t positioned between you and the stars. You then compare that picture to the one you took at the solar eclipse. The stars that are in your picture but farthest away from the sun in the eclipse picture will be least affected by the sun’s gravity. The stars closest to the sun in the eclipse picture will be the most affected. So if you align the stars far from the sun in the the eclipse picture with the ones in the non-eclipse picture, then the deviation of the stars close to the sun in the overlay of the two pictures is what you are trying to measure.