LATITUDE

Latitude is the angular coordinate that measures the distance of a point from the equatorial plane, from 0° (equator) to 90° (poles). In marine horology, latitude was historically easier to determine than Longitude: one simply had to measure the height of the Sun at the meridian or the altitude of the Star (wheel) Polaris. Longitude, on the other hand, required precise time — hence the decisive role of marine chronometers in the history of navigation. Latitude indirectly influences the running of mechanical watches: the further one moves from the equator, the greater the seasonal variations in Temperature, which places greater demands on the thermal compensation of hairsprings and balance wheels. In Time sidereal time and astronomical time, the latitude of the observation site is a parameter necessary for the exact calculation of the sunrise and sunset of the Sun or the Moon, a parameter that some fine watchmaking timepieces incorporate into their astral display mechanisms.

The astrolabe, ancestor of timekeeping and navigation instruments, well illustrates the importance of latitude in early horology: this instrument required an engraved disc (the tympan) specific to the latitude of the place of use, as the stereographic projection of the sky it represents varies with it. The simplified nautical astrolabe allowed sailors to measure the height of celestial bodies in order to deduce their latitude, long before the appearance of the marine chronometer. This lineage still inspires certain contemporary complications: astronomical watches based on the principle of the astrolabe. To learn more: Royal Museums Greenwich.