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:
“We made a silence, for teacups, once
After cosmic gall had bittered a lidless night....”
—Allen Tate (1899–1979)
“And then he heard some old forgotten talk
At a short distance like a hundred miles
Filling the air with its secrecy,
And was afraid of all the living air....”
—Allen Tate (1899–1979)
“But ‘tis a common proof
That lowliness is young ambition’s ladder,
Whereto the climber-upward turns his face;
But when he once attains the upmost round
He then unto the ladder turns his back,
Looks in the clouds, scorning the base degrees
By which he did ascend.”
—William Shakespeare (1564–1616)