What is Cosmic distance ladder?
The '''cosmic distance ladder''' (also known as the '''extragalactic distance scale''') is the way [[astronomer]]s measure the [[distance]] of objects in [[Outer space|space]]. No one method works for all objects and distances, so astronomers use a number of methods. A real ''direct'' distance [[measurement]] of an [[astronomical object]] is possible only for those objects that are close enough to Earth (within about a thousand [[parsec]]s). It is the larger distances which are the problem. Several methods rely on a '''standard candle''', which is an astronomical object that has a known standard [[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. == Direct measures == === Astronomical Unit === The [[astronomical unit]] is the mean (average) distance of the Earth from the Sun. This we know quite accurately. [[Kepler's Laws]] tell the ratios of the distances of planets, and [[radar]] tells the absolute distance to inner [[planets]] and [[artificial satellite]]<nowiki/>s in orbit around them. === Parallax === [[Parallax]] is the use of [[trigonometry]] to discover the distances of objects near to the [[solar system]]. As the Earth orbits around the Sun, the position of nearby stars will appear to shift slightly against the more distant background. These shifts are angles in a right [[triangle]], with 2 [[astronomical unit|AU]] making the short leg of the triangle and the distance to the star being the long leg. The amount of shift is quite small, measuring 1 [[arcsecond]] for an object at a distance of 1&nbsp;[[parsec]] (3.26 light-years) This method works for distances up to a few hundred parsecs. == Standard candles == Objects of known brightness are called ''standard candles''. Most