Julian Day. Figure that denotes the serial number of a day as reckoned from an epoch in 4713 B.C. The day begins at noon, G.M.T. Used by astronomers to avoid calculations for intercalary days, and for other factors. Introduced in 16th century by Joseph Scaliger, who named it after his father, Julius.
Jumbo Derrick. Heavy-lift derrick. Used when discharging very heavy items of cargo.
Jumper. Alternative name for a 'Jolly Jumper'. 2. Blouse-like upper garment of R.N. seaman. 3. Man who discharges cargo by jumping.
Jumper Guys. Additional guys for supporting jib boom.
Jumper Stay. Triatic stay. So called because used when jumping cargo. 2. Stays on a yacht's mast which are angled forward by spreaders, or jumper struts, to give support fore and aft as well as athwartships.