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The Cosmological Distance Ladder

The Problem

It's obvious, (I hope), that we can't measure distances to the planets and stars with a tape measure.  Such an idea should be laughable to anyone.  But seriously, how do we measure the distances to the stars and planets?

Image: Stars and Galaxies

What about radar?  We've measured the distance to the moon with radar.  Don't we measure the distance to everything else with radar too?

Image: Radar dish

The unfortunate answer is no.  Radar can be used to measure some distances, but certainly not all distances.  In fact, when it comes to cosmological distances, radar can be used for only a precious few distances, those being distances to nearby planets within our solar system.  We can't even use radar to measure the distance between the Earth and the Sun because the Sun doesn't reflect radar very well.  Other objects are just too far away.

So if we can't use radar, how do we determine the distance to far away objects?  The answer is that we build a ladder.. a cosmological ladder.  We start small, with distances that we can measure directly.  Then we stand on those results, to reach a littler higher, and build another step.  We repeat this over and over until we've built a ladder that stretches out to the furthest reaches of the universe.

There is a twist though.  Not all the steps are the same.  Each new step provides a new way of measuring distances.  I like the analogy of bridges myself.  Each new bridge allows us to cross another gap in the space between us and the furthest reaches of the universe, and each new bridge is uniquely constructed to bridge the next gap.  The analogy of a ladder is most prevalent in current literature though, so I will retain that analogy.