Cosmic distance ladder
succession of methods by which astronomers determine the distances to celestial objects / From Wikipedia, the free encyclopedia
The cosmic distance ladder (also known as the extragalactic distance scale) is the way astronomers measure the distance of objects in space. No one method works for all objects and distances, so astronomers use a number of methods.
A real direct distance measurement of an astronomical object is possible only for those objects that are close enough to Earth (within about a thousand parsecs). It is the larger distances which are the problem. Several methods rely on a standard candle, which is an astronomical object that has a known standard luminosity.
The ladder analogy arises because no one technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung.