We discussed this in an earlier lecture. This is one of the most important laws in astronomy. It gives the speed at which a galaxy is moving away from us. We have no way to determine any sideways movement. Hubble's findings, simply stated, suggest that the farther away a galaxy is the faster it is moving away from us.
V is the velocity and d is the distance.
Originally astronomers used observations of nearby galaxies, whose distance they knew, to establish the correctness of the law and to measure Hubble's constant. More often, nowadays, we use Hubble's law to determine the distance to a galaxy. All we have to do is to measure the velocity by measuring the redshift, then divide by Hubble's constant. This is the only method that there is for most of the galaxies in the Universe. We will discuss the implications of Hubble's law when we discuss Cosmology.