Teach Astronomy - Distance Scale

http://www.teachastronomy.com/ The distance scale in astronomy is a set of measurements that define distances all the way from the solar system to the most remote galaxies. Conceptually it's a pyramid with nearby methods being direct and fairly accurate while the errors accumulate and grow to the point where the measurement of the distance to galaxies is rarely more accurate than ten percent. Why are many techniques needed to establish a distance scale? In part, it's due to the vastness of space. Any distance indicator that can be found near the Sun must also be found ten thousand times further away to be seen on the other side of the galaxy at which point it's ten to the eight or a hundred million times fainter. Similarly, a distance indicator that can be seen on the other side of the galaxy must also be seen a thousand times further away to be seen in distant galaxies at which point it's a million times fainter. Thus nearby distance indicators become too faint to be useful at some point, whereas the most luminous distance indicators are very rare locally, for example supernovae which only occur once every fifty years or so in the entire Milky Way.