From Earth, the spectra of light given off by celestial objects shifts depending on their movement toward or away from the Earth. Galaxies that move toward Earth shift toward the blue end of the light spectrum and galaxies moving away from Earth shift toward the red end of the light spectrum. Combining his work with astronomer Vesto Slipher's earlier measurements of the "redshifts," Hubble plotted distances of galaxies and uncovered a direct relationship. The galaxies were receding from Earth at a rate proportional to their distance from Earth. Hubble determined that the farther away a galaxy, the greater its redshift. Or, the greater the distance between a galaxy and Earth, the faster that galaxy is racing away from Earth. The greater the distance between two galaxies, the faster they were moving away from one another. This proportionality between distance and speed of galaxies was formulated in 1929 by Hubble and Humason, and is known as Hubble's Law. It was an extraordinary discovery; one that meant that the universe was constantly expanding.
The mathematical equation for Hubble's law is as follows:
The velocity of recession (V), expressed in km/s, is directly proportional to distance (D), measured in megaparsecs/Mpc, where H0 is the value of the Hubble constant at the time of observation in the history of the universe. (A megaparsec is 3.26 million light-years.)