Longitude by chronometer
, is an astronomical navigation
method of calculating the longitude of an observer's position on Earth. The longitude derived by this method must be combined with the latitude of the observer's position (derived through some other method) to resolve a "fix
" or exact position of the observer on the Earth.
Determining longitude by use of a chronometer
is perhaps the easiest method of determining longitude, albeit one of less accuracy than other methods. However, while other methods of celestial navigation require extensive use of sight reduction tables and tedious, error-prone calculations, determining longitude by use of the chronometer is fairly simple and straightforward. As the name of process implies, other than a sextant
(to determine local apparent noon
), the primary tool required is an accurate chronometer. The only celestial body necessary is a clear shot of the Sun at one's local apparent noon.
The principle of determining longitude by chronometer is based on the regular apparent movement of the Sun across the sky each day. Ignoring the fact that the Earth revolves, the Sun’s apparent orbit around the Earth each day takes 24 hours to pass through the 360 degrees of the Earth’s circumference or 15 degrees each hour (360/24 = 15) or one degree every 4 minutes (60/15 = 4). By determining the difference between the time of one’s local apparent noon with that of local apparent noon at a known reference longitude, a... Read More