"An especially large moon will appear to rise on Saturday in the most extreme example of a so-called super-moon phenomenon in nearly 20 years, according to the U.S. Naval Observatory.
For such a visible super-moon to be visible, the moon must be full and passing through its closest approach to earth, known as the perigee, in its month-long, elliptical orbit, the Observatory said on its web site.
This perigee is the closest one for the entire year, the Observatory said.
While less spectacular super-moons occur several times a year, Saturday's is the best example of a perigee-syzygy, to use astronomers preferred term, since March 8, 1993, it said."