Glossary of maritime terminology
In maritime navigation, the zenith distance is 90 degrees minus altitude. This means that if a star has an altitude of 89 degrees, in maritime terms, its zenith distance is 90 - 89, or 1 degree. Thus it is very nearly directly overhead.
The zenith itself is an imaginary point that is directly above a particular location on the celestial sphere. For example, imagine a straight line meeting the center of the earth compared to the navigator's position. To that person, the zenith is the highest point of the celestial sphere.
Celestial navigation uses the stars, moon, sun, and horizon to calculate the position of a vessel and comes in useful in the event that technology such as the GPS isn't working and the ship is in the middle of the ocean, with no landmarks or reference points in sight.
Zulu stands for the letter Z in the International Radiotelephony Spelling Alphabet, which is most often referred to as the NATO Phonetic Alphabet or simply the Phonetic Alphabet. This is the most commonly used group of code words used to clearly communicate the letters of the Roman alphabet, particularly over the radio and is essential in helping seafarers give and receive messages, orders and instructions clearly and correctly.