Celestial Navigation - Practical Navigation - Longitude

Longitude

Longitude can be measured in the same way. If one can accurately measure the angle to Polaris, a similar measurement to a star near the eastern or western horizons will provide the longitude. The problem is that the Earth turns 15 degrees per hour, making such measurements dependent on time. A measure a few minutes before or after the same measure the day before creates serious navigation errors. Before good chronometers were available, longitude measurements were based on the transit of the moon, or the positions of the moons of Jupiter. For the most part, these were too difficult to be used by anyone except professional astronomers. The invention of the modern chronometer by John Harrison in 1761 vastly simplified longitudinal calculation.

The longitude problem took centuries to solve and was dependent on the construction of a non-pendulum clock (as pendulum clocks cannot function accurately in a tilting ship or a moving vehicle, of any kind). Two useful methods evolved during the 18th century and are still practised today: lunar distance, which does not involve the use of a chronometer, and use of an accurate timepiece or chronometer.

Presently, lay person calculations of longitude can be made by noting the exact local time (leaving out any reference for Daylight Savings Time) when the sun is at its highest point in the sky. The calculation of noon can be made more easily and accurately with a small, exactly vertical rod driven into level ground—take the time reading when the shadow is pointing due north (in the northern hemisphere). Then take your local time reading and subtract it from GMT (Greenwich Mean Time) or the time in east London. For example, a noon reading (1200 hours) near Central Canada or the U.S.A. would occur at approximately 6 pm (1800 hours) in London. The six hour differential is 1/4 of a 24 hour day, or 90 degrees of a 360 degree circle (the Earth). The calculation can also be made by taking the number of hours (use decimals for fractions of an hour multiplied by 15, the number of degrees in one hour). Either way, you can demonstrate that much of central USA or Canada is at or near 90 degrees West Longitude. Eastern longitudes can be determined by adding the local time to GMT, with similar calculations.