The moon will be full on Sunday when it makes one of its closest approaches to Earth of the year, a co-occurrence some call a supermoon. Supermoons are said to occur when a full moon coincides with “perigee” in its elliptical orbit, meaning a relatively close approach to the Earth. Because the definition of supermoon is subjective, sky-gazers differ on how many there will be this year — some say two, some say three or four.