Line-crossing Ceremony

The ceremony of Crossing the Line is an initiation rite in the British Merchant Navy, Dutch merchant navy, Royal Navy, United States Navy, U.S. Coast Guard, United States Marine Corps, Royal Australian Navy, Russian Navy, and other navies that commemorate a sailor's first crossing of the Equator. The tradition may have originated with ceremonies when passing headlands, and become a "folly" sanctioned as a boost to morale, or have been created as a test for seasoned sailors to ensure their new shipmates were capable of handling long rough times at sea. Sailors who have already crossed the Equator are nicknamed (Trusty/Honorable) Shellbacks, often referred to as Sons of Neptune; those who have not are nicknamed (Slimy) Pollywogs (in 1832 the nickname griffins was noted).

Equator-crossing ceremonies, typically featuring King Neptune, are also sometimes carried out for passengers' entertainment on civilian ocean liners and cruise ships. They are also performed in the merchant navy and aboard sail training ships.

Line-crossing Ceremony in Southern Victory
Among Ancient Greek mariners, there was a custom of tying a rope around sailors and dragging them along the bottom of their ship the first time they passed over the equator. This was done as an offering to Poseidon. If the sailors survived the ordeal, Poseidon was believed to have shown his favor and deemed them worthy of a career sailing his oceans. The custom was also adopted by Roman sailors, who renamed Poseidon to Neptune.

Long after serious worship of Poseidon had ceased, a similar, though much more light-hearted and safer, custom persisted in many navies of the world. When a ship crossed the equator, all sailors aboard it who had never made an equatorial crossing before (called polliwogs) were expected to engage in a number of demeaning and embarrassing rituals on the deck of the ship in a ceremony presided over by a sailor who had already made an equatorial crossing (called a shellback) playing the role of King Neptune, and by a number of other shellbacks either playing his attendants or just joining the fun. Polliwogs who completed these rituals were made to kiss King Neptune, who was usually played by a particularly unattractive sailor, thus swearing allegiance to him. Upon completion of this ritual, polliwogs became shellbacks themselves, and were free to torment polliwogs on future equatorial crossings.

Line-crossing Ceremony in Joe Steele
When Mike Sullivan sailed from San Diego to Guadalcanal, he and the rest of the troops of the punishment brigade on board were taken topside when the ship crossed the equator and transformed from polliwogs to sturdy shellbacks by the magical powers of King Neptune (and his court consisting of shellback sailors who drenched them with fire hoses).