Astronomical and Horological Methods Determination of Longitude

Definition and Importance of Longitude

Longitude is the angular distance of a point on the Earth’s surface, measured east or west from the Prime Meridian (0 degrees), up to 180 degrees.
Determining longitude accurately is essential for navigation, especially at sea, as well as for precise timekeeping and astronomical observations.

Astronomical Methods of Determination

Stellar angle measurements: This technique involves measuring the angular distance between the Moon and a specific star at a certain time. Observers at different longitudes see this angle differently, allowing them to calculate their longitude.
Lunar distance method: It utilises the continuously changing angular distance between the Moon and other celestial bodies. These distances are compared with tables in a Nautical Almanac to determine the local time and hence longitude.
Satellite-based methods: Modern techniques employ technology such as GPS satellites to accurately pinpoint longitude. These satellites transmit signals that allow receivers on Earth to calculate their distance from several satellites, and thus their precise geo-location.

Horological Methods of Determination

Marine chronometer: A clock accurate enough to keep Greenwich Mean Time (GMT) at sea was the horological solution for longitude. Comparing the GMT on the chronometer and the local time determined via the Sun allowed calculation of longitude.
Portable timekeepers: H4, developed by John Harrison in the 18th century, was an advanced portable timekeeper that helped sailors calculate their longitude with unprecedented precision.
Radio and atomic clocks: In the present era, extremely accurate atomic clocks, and coordinated signals broadcasted on radio frequencies can be utilised to set and synchronise time across different longitudes.

Key Concept: The Relationship between Longitude and Time

A full rotation of the earth (360 degrees) takes 24 hours, so for each hour the earth rotates by 15 degrees (360 degrees/24 hours).
As a result, each degree of longitude is equivalent to 4 minutes (60 minutes/15 degrees). Therefore, by knowing how many minutes a local solar time is ahead or behind GMT, one can calculate the longitude.

This understanding combines principles of astronomy, time measurement and geography, providing a comprehensive understanding of longitude determination that was crucial for the development of navigation, timekeeping, and astronomy.