This article will be the first of a series on the Cosmic Distance Ladder, which calibrates the distance to everything outside our solar system.
Astronomers face a simple problem.
Fortunately in the solar system, we can do nearly as well by bouncing radar signals off planets to determine their distances from Earth. To explain the method, I will use a simple analogy. First hold out your thumb at arm's distance and close one eye. Focus on a distant object behind your thumb, such as an outside building. Then close that eye and open the other.