The Equator is an imaginary line, which divides the Earth into the Northern and Southern Hemispheres. The line-crossing ceremony is an initiation rite that commemorates a person's first crossing of the Equator, where initiation is a rite of passage marking entrance. Equator-crossing ceremonies are common in the navy. The words 'Tadpoles', Pollywogs', and 'Griffins' are nicknamed for those have not crossed the Equator. The sailors who have crossed the Equator are nicknamed ''Shellbacks''.

In the 19th century, the line-crossing ceremony was often involving beating 'Pollywogs' and throwing them over the side of the ship. During the WW II, the frequency of line-crossing ceremony increased in the Pacific. Today, each 'Pollwog' is expected to a standard initiation rite in order to become 'Shellback'. The rare status is the Golden Shellback, where a person has crossed the Equator at the 180 degree meridian. Another rarest one is the Emerald Shellback, for crossing at the prime meridian near the Null Island. Other unique Shellback designations are The Star Spangled Shellback by Franklin D.Roosevelt, The Iron Shellback by D.Eisenhower and Jacinto, The Ebony Shellback by crossing the Equator on Lake Victoria, The Top Secret Shellbacks are for submariners, the Wooden Shellback for those crossed on a vessel with a wooden hull; the Blue Nose for Arctic Circle. The most recent ceremony was during the summer of 2019, on the TS Empire State VI, for a training vessel of US maritime service.

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