 The distance ladder is a set of techniques for determining an object's distance. Each step works for a particular situation in distance range and supports the steps above it. This is how we work our way from Earth to the stars and onto the most distant galaxies. We have just built a solid first rung for our ladder. We can now use the entire diameter of the Earth, 12,756 kilometers, as our baseline for triangulating off the Earth and into the solar system. Here's an overview of the rest of the ladder. Parallax is the second rung in the ladder. Direct measurement doesn't work for figuring out how far away the Sun is. For that, you use an extension of triangulation called parallax. If you hold your finger up in front of your eyes and close one eye at a time, you see a shift in your finger's relative position with respect to distant objects. This is parallax. It can be used to calculate the distance to our solar system neighbors and nearby stars. All you need for this to work is a more distant star that doesn't move when the target object is viewed from different locations. Parallax accuracy goes down as the distance goes up, and it doesn't work at all for stars and galaxies that don't have more distant objects that remain stationary as the Earth-bound observer moves. For those objects, we use a technique called standard candles. Once we know the intrinsic brightness of a star, we can use the simple inverse square rule for the drop-off in the apparent brightness as the object is seen on Earth. Some objects lend themselves to this technique better than others. Some of the most prominent are Cepheid or Cepheid variable stars, emission nebula, and type 1a supernova. These methods can take us across the Milky Way, out to nearby galaxies, and into local supergalaxy clusters. The final rung on the ladder is redshift and the Hubble constant. The most distant galaxies are too far away to see Cepheid stars or supernova explosions. For these, we owe our thanks to Edwin Hubble, who discovered a relationship between the speed that a galaxy is traveling away from us and its distance from us. Based on the evidence from an expanding universe, the faster a galaxy is receding away from us the further away it is. We get the receding speed from the redshift and the light from the galaxy. This method works all the way back to near the Big Bang around 13 billion years ago. We'll be building the cosmic distance ladder rung by rung as we move through how far away is it video book chapters.