Understand how astronomers measure distances across the universe.
No single method measures all cosmic distances. Instead, astronomers use a "ladder" of techniques, each calibrated by the previous rung: Parallax (nearby stars, <1000 pc), Cepheid Variables (period-luminosity, ~30 Mpc), Type Ia Supernovae (standard candles, ~1000 Mpc), Redshift (Hubble's law, entire observable universe). 1 parsec ≈ 3.26 light-years.