distance-ladder

# The Cosmic Distance Ladder There is no single method that measures every astronomical distance. Instead, astronomers chain methods into a ladder: each rung calibrates the next, and errors propagate with the chain. Radar pins down the Astronomical Unit. Parallax measures the nearest few thousand parsecs. Main-sequence fitting and spectroscopic parallax reach galactic-scale distances. Cepheids calibrate galaxies in the Local Group and beyond. Type Ia supernovae push to hundreds of megaparsecs. Redshift carries the rest. This skill covers each rung, what calibrates it, where the error bars live, and how to think critically about a claimed distance. **Agent affinity:** hubble (extragalactic distances), rubin (rotation curves), payne-gaposchkin (spectroscopic parallax) **Concept IDs:** astro-distance-ladder, astro-cepheid-variables, astro-hubbles-law ## The Ladder, Top to Bottom | Rung | Method | Typical range | Precision | Calibrated by | |---|---|---|---|---| | 0 | Radar ranging (planets) | AU scale | 10^-11 | Direct measurement | | 1 | Trigonometric parallax | 0 - 10 kpc (Gaia) | 10-100 microarcsec | Geometric | | 2 | Spectroscopic parallax | 0 - 5 kpc | ~0.5 mag | Main sequence from parallax | | 3 | Main-sequence fitting | 0 - 100 kpc | 0.1-0.3 mag | Parallax | | 4 | RR Lyrae | 0 - 100 kpc | 5-10% | Parallax, MS fitting | | 5 | Cepheid period-luminosity | 0 - 30 Mpc | 5-10% | Parallax, LMC distance | | 6 | Tip of the red giant branch (TRGB) | 0 - 20 Mpc | 5-10% | Parall...