For those who never saw Porky's or Party (or weren't 15),Mooning is the act of displaying ones buttocks publicly. Originally,Flashing The Buttocks was the preferred term until the 1960s. Mooningthen had referred to romantically longing for a person. Never did discover when and why the definition changed. However I did find that there are 4 types of mooning positions that are approiately named for each lunar phase.
Full Moon – Open and free.
Half Moon – Pants are half way up. Usually occurs when mooning in a moving car.
Crescent Moon – While pulling up the pants (in a hurried fashion), the middle area gets stuck while the sides move up. Creates a crescent shape. Usually occurs when the person is suddenly caught by some type of authority figure.
Gibbous – Similar to a crescent moon except the sides and the middle are all at the same height.