Certainly we can not hire a survey crew from the Illinois Department of Transportation to survey the route and compute the distance. Astronomers, geologists and others are often confronted with questions about the distance and sizes of objects that they can see but for various reasons may not be able to travel to. An astronomer can see the stars in the sky but we do not yet know how to travel to them. A volcanologist can see an eruption in the distance but can not safely travel to it.
Astronomers use two basic techniques to measure distances.
We will explore the first of these two methods in this exercise. Recall from geometry that if you know the length of one side of a triangle and the two angles which adjoin it you can compute the length of the other sides. Parallax is the process of finding the distance to an object by measuring its angular position from two different places. Place a small object a few meters from your device along a line extending nearly perpendicularly from the line connecting the protractors. Look through one of the straws and carefully center the object.