Transcript: Scientists use a system of units based on mass, length, and time. Almost every physical quantity in the world can be reduced to some combination of units of mass, units of length, and units of time. For example area is length times length. Volume is length times length times length. Velocity is a distance or a length divided by a time. Momentum is a mass times a velocity. So many of the things you see in astronomy will be simply reducible to combinations of mass, length, and time. This is the way in which astronomers make sense of a complicated world, and in astronomy as in all science we measure mass, length, and time in units of the metric system: kilograms for mass, seconds for time, and meters for length.