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The Cosmic Distance Ladder is the succession of methods by which astronomers determine the distances to celestial objects. The ladder analogy is used because no one technique can measure distances over the entire range required in astronomy. Instead, one method can be used to measure nearby distances, a second (the next rung of the ladder) can be used to measure nearby to intermediate distances, and so on.


distance ladderDistance ladder. Source: Bennett,


Each rung of the ladder provides information that can be used to determine the scale for distances at the next higher rung. Because the more distant steps of the cosmic distance ladder depend upon the nearer ones, the more distant steps include the effects of errors in the nearer steps, both systematic and random ones.

Therefore the overall distance scale used in astronomy, is prone to systematic effects in any of these individual measurement techniques and these affect our knowledge about the scale of the Universe.

Measuring reliable distances in astronomy is one of its biggest ongoing challenges, and ultimately defines what we know about the size of the Universe.




A simplified overview of various techniques on the Cosmic Distance Ladder, most of which will be described in more detail in this EBook, is given in this series of 9 videos by PhysicistMichael.






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