logbookThe chronometer is losing twelve seconds a day
Course WNW
A continuous course record supports dead reckoning.
Speed 6 kn
The knot comes from the historical use of a log line and timer.
Wind rising
Changing weather alters sail handling and watch decisions.
Longitude can be turned into a comparison between two clocks
When the sun crosses the local meridian, it marks local noon. At that same instant, a clock keeping the time of a reference meridian may show morning or afternoon. Earth’s rotation turns that time difference into an angular difference east or west.
At an average 15 degrees per hour, four minutes of time corresponds to about one degree of longitude. The arithmetic is simple enough. The difficult part is knowing both times accurately while the ship has been away from land for weeks.
A shore clock became unreliable when the room began to roll
Ordinary precision clocks worked in comparatively stable rooms. A ship supplied vibration, rolling, humidity and changing temperature. Lubricants, metals and mechanisms reacted, and even a small daily rate error accumulated into a growing positional error.
Marine timekeepers were designed and tested against those conditions. John Harrison’s work was central to that development, but the practical solution also depended on other makers, observatories, trials, maintenance and methods for checking a chronometer’s rate. No single mechanism solved longitude in isolation.
The clock did not replace the sky
A chronometer carried reference time; it did not announce when local noon occurred. The navigator still needed a celestial observation, a usable horizon, tables and corrections. Latitude, dead reckoning and coastal evidence continued to support the position.
Chronometers therefore strengthened a wider system rather than replacing celestial navigation. Earlier and parallel methods also compared local observation with reference time. Lunar distance measured the Moon's angle from the Sun or a selected star and used corrections and an almanac to recover Greenwich time. An eclipse used one phase as a shared event. In 1776 Cook set an eclipse result of 15°35′30″ beside a watch result of 15°26′45″ west, allowing the methods to check one another. Lunar distance avoided waiting for a rare eclipse but demanded a clear horizon, precise angle, trustworthy tables and substantial calculation.
Questions
Continue exploring this subject
Why does one hour correspond to 15 degrees?
Earth turns about 360 degrees in 24 hours. Dividing 360 by 24 gives an average rotation of 15 degrees per hour.
Did John Harrison solve longitude by himself?
Harrison made a decisive contribution to practical marine timekeeping, but the working system also involved astronomical tables, observation, clockmaking, testing and institutional adoption.
How did a chronometer, lunar distance and eclipse each yield longitude?
Each supplied reference-meridian time for comparison with local astronomical time. A chronometer carried it, lunar distance recovered it through angle, corrections and almanac, and an eclipse used a predicted phase but was rare and vulnerable to cloud.
Sources
Continue the research
- Time and navigationRoyal Museums Greenwich
- Navigation tools: then and nowNOAA Mariners Weather Log