cosmic distance ladder

RS Puppis as imaged with the ESO NTT (from Kervella et al. 2008)

Distances in the universe are so vast that we do not have a simple way of measuring them.  For distances within the solar system we can measure them directly, using radar for example, and some very straightforward trigonometry.  But radar is hard to use when it takes light minutes or hours to cross the solar system; and the nearest star is four light years awa