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The cosmic distance ladder (or cosmological distance ladder, or extragalactic distance scale) is a sequence of methods culminating in those used for measuring distance to distant objects such as the oldest galaxies. The more accurate methods, typically workable only for relatively short distances, are used to calibrate additional methods which handle longer distances, step-by-step, like climbing a ladder. The precision and accuracy of the methods generally decline with increasing distance, some methods being presumed no more accurate than 50% (but if such a method is fairly consistently within 50%, that is much better than no information). The shortest of the following methods are limited to the Milky Way, but are key "lower" steps in the ladder. To gain as much accuracy as possible, efforts are made to confirm distances and calibrations using multiple methods. Methods:
Among the standard candles:
A ladder of three steps has been commonly used: parallax for nearer distances within the Milky Way, Cepheids, to relate those nearer distances to longer Milky Way distances and nearby galaxies, and either redshift or Type Ia supernovae to relate those distances to those of further galaxies. Other methods address particular circumstances, and provide means to confirm the commonly-used methods. The methods continue to be devised and improved.
Since redshift-derived distance depends upon the Hubble constant, which is subject to improvement, cosmological distances are often cited divided by the Hubble parameter, assisting future readers in using future improved Hubble constant determinations.