Sailors fix position by timing angles to the sky
Celestial navigation, or astronavigation, finds location using stars, sun, moon, or planets. Navigators take sights—timed angular measures between a body and the horizon—then reduce them with almanacs and math. Sun-and-horizon shots remain the classic daytime method at sea.
At any instant, each star, planet or the Sun sits directly above one spot on Earth, its geographic position, listed in the yearly nautical or air almanac. The angle a navigator measures between that body and the horizon reveals how far away that spot is. After a calculation called sight reduction, the result becomes a line of position on the chart, really a short arc of a huge circle centred on the body's spot. Two such lines cross at the ship. In one worked example from 29 October 2005, Sun and Moon sights intersected both about 350 nautical miles west of Madeira and near Asunción in Paraguay; since ships rarely sail across South America, the Atlantic answer was plainly correct.
Latitude is the easier half. Measure the Sun at its highest point at local noon, or the altitude of Polaris, which stays within one degree of the celestial pole: a reading of ten degrees means you are roughly ten degrees north. Longitude demands knowing Greenwich time, and every four seconds of clock error shifts the fix by a nautical mile. Without a trusted clock, the lunar distance method, timing the Moon against other bodies, can recover the time within a second or two after about half an hour of sights and arithmetic. Joshua Slocum, the first person recorded sailing alone around the world, used lunars to keep his cheap tin clock honest.
A stretched arm gives a rough gauge, since a little finger spans just over 1.5 degrees. Sailors progressed through the kamal, astrolabe and octant to the sextant, whose twin mirrors hold object and horizon steady. A nautical mile, 1,852 metres, equals one arcminute along a meridian, so a sextant readable to a tenth of an arcminute could in theory fix position within 185 metres, though about 1.5 nautical miles is typical from a moving deck. When haze hides the horizon, a mirror or a pan of mercury can stand in.
Besides the sextant, the kit is a chronometer, an almanac, sight reduction tables needing only addition and subtraction, and a chart. Modern crews use sights to check satellite fixes and dead reckoning, and the Apollo missions navigated by stars too.
Source: Celestial navigation
