Cepheid variables

Hubble's 1929 redshift observation tied galaxy speed to distance. See what the 24-galaxy data showed, why his constant was off 7x, and the tension today.

Henrietta Leavitt's 1912 discovery of the Cepheid period-luminosity relation gave astronomy its first reliable cosmic distance scale. Her story and legacy.

From parallax to supernovae, astronomers chain overlapping methods together to measure distances from the nearest stars to galaxies billions of light-years away. Here is how each rung of the cosmic distance ladder works, and why its top rung is now in dispute.