Cosmic Perspectives from Amid the Stars:
Astronomy Blog

You might have heard before that the nearest star is 4.24 light-years from Earth. Or that the closest galaxy is 2.5 million light-years from the Milky Way. Or even that the most distant observed objects are 33.8 billion light-years away. But how do we measure these distances in space? After all, there’s no such thing as a cosmic ruler. That’s where the cosmic distance ladder comes in.