Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time

A backstaff is a rod and crosspiece used at sea to gauge the height of the sun. By studying the shadows it cast, sailors could estimate the sun’s location and derive the local time. Backstaffs replaced forestaffs. (See “forestaff”)

A chronometer is a clock designed for high accuracy. Developed by John Harrison as a marine timekeeper that could withstand weather changes and rolling seas, chronometers made possible quick, accurate determination of a ship’s position (specifically, its longitude) at sea. The term continues to refer to precision timepieces, including highly accurate wristwatches.

Ephemerides (eh-fuh-MER-ih-deez) are “timetables of astronomical movements” (25): They keep track of the motions of stars, planets, the sun, and the moon across the sky. On any given day, the tables will show where important celestial objects will be, along with the precise time of eclipses and lunar occultations (when the moon passes in front of a star or planet). This helped seafarers determine their location, and ephemerides were used extensively before the invention of a reliable chronometer.