lunar

A lunar is a single observation of the angular distance between the Moon and another celestial body, usually the Sun or a bright star. Navigators use this measurement to determine a ship's longitude when the chronometer is unreliable, absent, or needs verification. The method depends on the Moon's rapid motion across the sky relative to other celestial objects, which makes its position an effective natural clock.

The navigator measures the angle between the Moon's limb and the chosen reference body using a sextant, recording the altitude of both objects and their angular separation. Unlike a single altitude sight, which gives latitude or helps establish local time, a lunar requires two separate measurements and more complex calculation. The observer typically shoots several lunars in succession to average out measurement errors and atmospheric distortion.

Working with Lunars at Sea

Lunars work best when the Moon is between roughly 30 and 60 degrees above the horizon, and when both the Moon and reference body are sufficiently separated and elevated. The calculation involves comparing the observed distance to predicted distances from a lunar distance table, which was precomputed and published in nautical almanacs. This comparison yields Greenwich Mean Time, from which longitude follows directly. The method demands careful work: errors of even 10 or 15 minutes of arc in the angle measurement translate to significant longitude error.

Before reliable marine chronometers became affordable in the 19th century, lunars were the primary method for determining longitude on long voyages. Even after chronometers became standard, officers still took lunars as a check. The technique fell into disuse during the 20th century as electronic navigation superseded celestial methods, though the skill remained part of traditional navigator training and appears in historical maritime records.

The term comes from the Latin for Moon. Lunars are sometimes called 'lunar distances' in formal navigation texts, though 'lunar' alone refers to the completed observation and calculation, not merely the distance measurement itself.