A central problem in astronomy is the determination of distances. Without a knowledge of distances, sizes would remain nothing more than angular diameters, and stellar brightness could not be converted into true radiated power or luminosity. Astronomical distance measurement began with a knowledge of the Earth's diameter, which provided a base for triangulation. Within the inner solar system, some distances can now be better determined through the timing of radar reflections. For the outer planets, triangulation is still used. Beyond the solar system, distances to the closest stars are determined through triangulation, with the diameter of the Earth's orbit serving as the baseline and shifts in stellar parallax being the measured quantities. Stellar distances are commonly expressed by astronomers in parsecs (pc; 1 pc = 3.086 10{sup 18} centimetres) or in kiloparsecs. They can be measured out to about 50 parsecs by trigonometric parallax.

Only two general methods for galactic distances are described here. In the first, a clearly identifiable type of star is used as a reference standard because its luminosity (total radiated power) has been well determined. Such a star is termed a "standard candle." The distance to a standard candle can be calculated from its known luminosity and its measured intensity. Identification of the standard candle can be made through its spectrum or regular variations in brightness. (Corrections may have to be made for the absorption of starlight over great distances.) This method forms the basis of measurements of distances to the closest galaxies. It has been found that such distances generally correlate with the speeds of recession (as determined from Doppler-shifted wavelengths; see above). This correlation is expressed in the Hubble law, velocity = H distance, with H denoting Hubble's constant, the best value of which is thought to lie between 50 and 100 kilometres per second per megaparsec (km/sec/Mpc). It is currently used to determine distances to remote galaxies in which standard candles have not been found. Application of the Hubble law, however, has been questioned as it relates to some quasars (energetic galactic nuclei), and the value of Hubble's constant cited above is still a subject of disagreement.

Excerpt from the Encyclopedia Britannica without permission.