Astronomers do it spectroscopically. Astronomers do it telescopically. Astronomers do it under the stars. Astronomers do it universally. Astronomers do it variably.
Astronomers do it while gazing at Uranus.
Astronomers do it with binaries. Astronomers do it with dwarfs. Astronomers do it with giants. Astronomers do it with lenses. Astronomers do it with light.