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 with methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known 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.
Read more about Cosmic Distance Ladder: Direct Measurement, Standard Candles, Galactic Distance Indicators, Extragalactic Distance Scale, Overlap and Scaling
Famous quotes containing the words cosmic, distance and/or ladder:
“Put on the whole armor of God, so that you may be able to stand against the wiles of the devil. For our struggle is not against enemies of blood and flesh, but against the rulers, against the authorities, against the cosmic powers of this present darkness, against the spiritual forces of evil in the heavenly places.”
—Bible: New Testament, Ephesians 6:11-12.
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—William Shakespeare (1564–1616)