Cosmic distance ladder

And that simple geometric trick, using a known baseline and a couple of angles to find a distance you can't measure directly, that is the seed of one of the greatest achievements in modern astronomy.

Which is what they call the cosmic distance ladder, right?

Yeah, exactly.

It's the exact system astronomers use to figure out the distance to almost everything in the cosmos, from, you know, our own moon out to galaxies so ancient their light has been traveling toward us since before the Earth even formed.

True.

We have to calibrate the flashbulb, which means we have to wait for a type Ia supernova to go off in a galaxy that is close enough to us that we can also measure the Cepheid variables in that same galaxy.

And that is the defining vulnerability of the entire cosmic distance ladder.

The supernovae are calibrated by the Cepheids.