The cosmic distance ladder refers to the methods by which astronomers determine the distances to objects. Each rung of the ladder provides information which is used to determine the distances at the next higher rung.
Many of the rungs involve standard candles which are objects whose intrinsic brightness is believed known. By comparing the intrinsic brightness with the actual brightness, one derives a distance.
At the base of the ladder are radar observations of Venus, which allow one to determine the distance between the Earth and Venus and by extension, the size of the orbit of the earth.
The succeeding rungs are:
The field of astronomy which measures distances is known as astrometry. The field of astrometry has progressed greatly with the launch of space-based observatories such as Hipparchos, which can make extremely precise measurements that serve to strengthen the links between the different rungs in the cosmic distance ladder.