Another important piece of information regarding objects in the universe is their distance to us. This is not an easy thing to measure since these objects are usually very far apart. I will measure distances in light years: one light year is the distance covered by light during one year, which is about 9.5 trillion kilometers, or about 6 trillion miles.
In order to understand why several steps are needed in measuring distances it is useful to consider a simple example. A student is in her room sitting at her desk and would like to find the distance to the window; she gets a ruler and laboriously measures this distance to be 3 feet. This I will call ``the first rung in the student's distance ladder''
Her next task is to find the distance to a building which she can see through the window. This building is too far away for her to use her 12 inch ruler. What she does is to use sound: she notices that when she claps her hands outside her window there is an echo produced by the sound bouncing off the building in front of her. She has a good watch and so she can determine the time it takes for the sound to get from her window to the building and back. Now, if she can determine the speed of sound, she could use the formula distance = speed × time to get the distance. In order to measure the speed of sound she closes her window and times the echo from her desk to the window. Since we already knows the distance to the window (which she measured using her ruler) and she now knows the time it takes sound to go from her desk to the window and back she can determine the speed of sound. So, using the first measurement she determines the speed of sound and this allows her to measure things that are much farther away. In this way she has ``constructed'' the second rung in her distance ladder.
The same idea is used when measuring far away things in space: one finds a reliable method to determine the distances to near-by stars (the equivalent of using the ruler). Then one devises another method which requires a sort of calibration (the equivalent of determining the speed of sound); once this calibration is achieved the second method can be used to find distances to objects that are outside the range of the first method. Similarly a third, fourth, etc. methods are constructed, each based on the previous ones.