Glossary · Regulation

Lunar-distance method

Determining reference time from the angular separation of the Moon and another celestial object.

Explanation

The lunar-distance method compares a measured celestial separation with predicted positions to infer reference time. It requires suitable observations, corrections and astronomical tables, not simply reading a sextant angle as longitude. Royal Museums Greenwich explains the roles of Tobias Mayer’s tables, Nevil Maskelyne and the Nautical Almanac, and the long calculations involved. Astronomical methods and chronometers coexisted; the method should not be described as instantly displaced by Harrison’s watch.

Diagram

Schematic: longitude observation method. Navigation schematic: the measured angle between the Moon and a body is reduced through tables; it is not an instruction for using one preserved instrument.

Photographic reference

E. Barthet No. 10 — gimballed box, L2015-3496 — © The Board of Trustees of the Science Museum / Science Museum Group / The Clockmakers’ Museum, CC BY-NC-SA 4.0 as stated by source; commercial-use permission review pending. Official museum or publication source

E. Barthet No. 10 — dial and canister — © The Board of Trustees of the Science Museum / Science Museum Group / The Clockmakers’ Museum, CC BY-NC-SA 4.0 as stated by source; commercial-use permission review pending. Official museum or publication source

E. Barthet No. 10 — exposed movement — © The Board of Trustees of the Science Museum / Science Museum Group / The Clockmakers’ Museum, CC BY-NC-SA 4.0 as stated by source; commercial-use permission review pending. Official museum or publication source

E. Barthet No. 10, L2015-3496: exact chronometer context, not evidence of a recorded rate or a lunar-distance observation.

Cross-references

Nearby regulation terms

Isochronism adjustment: Work making rate independent of the state of wind.
Lift angle: The balance angle used to describe escapement lift and calculate amplitude on an acoustic timing machine.
Lift angle: The balance arc during which impulse is delivered.
Magnetism: Magnetisation of steel parts affecting rate.
Mean rate: Average daily rate across positions or days.
Mean time: Time based on the average solar day.