Astronomy
Measuring the Rotation of Distant Galaxies
Quick fact
By analyzing the Doppler shift of spectral lines, astronomers can measure the rotation speed of galaxies that are millions of light-years away with remarkable precision, even without resolving individual stars.
Why this is interesting
Ever wonder how we know that galaxies spin? It's not by watching a time-lapse movie. We read the light itself to measure the motion of billions of stars.