Space-time is a mathematical model that joins space and time into a single idea called a continuum. This four-dimensional continuum is known as Minkowski space. Combining these two ideas helped cosmology to understand how the universe works on the big level (e.g. galaxies) and small level (e.g. atoms). In non-relativistic classical mechanics, the use of Euclidean space instead of space-time is good, because time is treated as universal with a constant rate of passage which is independent of the state of motion of an observer. But in a relativistic universe, time cannot be separated from the three dimensions of space. This is because the observed rate at which time passes depends on an object's velocity relative to the observer. Also, the strength of any gravitational field slows the passage of time for an object as seen by an observer outside the field.