<Li> Solid black lines: Well calibrated ladder step . </Li> <Li> Dashed black lines: Uncertain calibration ladder step . </Li> <P> The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects . A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about a thousand parsecs) to Earth . The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances . Several methods rely on a standard candle, which is an astronomical object that has a known luminosity . </P> <P> The ladder analogy arises because no single 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 . </P>

How do we measure the distance to other galaxies