Standard candle

An object whose true brightness is known, so how faint it looks gives you the distance.

If you know how bright something really is, comparing that to how bright it appears gives its distance directly. Cepheid variables serve for nearby galaxies and Type Ia supernovae for distant ones, each calibrated against the rung below in what is called the distance ladder. The ladder is why the Hubble tension is taken seriously: the disagreement is not between a measurement and a guess, it is between two carefully calibrated chains that both look sound.