INTRODUCTION TO THE LUNAR DISTANCE
A lunar distance is the angular distance as it appears from earth between the moon and a comparing body (sun, star or planet). Developing an almanac of the lunar distance and the tools to observe it at sea was one of the most important scientific quests of the 1700's. If one could both predict and measure the distance at sea, one could determine time and from time one could find longitude. In the age of exploration and empire, great wealth and the finest scientific minds were consumed with the search for an accurate method of finding longitude. The lunar distance and the chronometer emerged together as the two great solutions to the problem.
The utility of the lunar distance derives from the fact that the moon moves steadily across the dome of the stars at a much faster rate than the sun or planets. In the course of a month, it moves 360° around the backdrop of the sky, one half a degree per hour on average. In effect, the moon acts like the giant hand of a clock, moving against a clock face defined by the sun, stars and planets. If one can predict where the moon will be from hour to hour, and then measure its actual position with sufficient accuracy, one can read the clock. The quadrant and then the sextant emerged in the mid 1700's to meet the need for instruments accurate enough to measure the distance. The first practical almanac predicting the distance appeared in the 1760's, along with the tables needed to correct or "clear" the measured distance.
With these tools, navigators could measure and clear the lunar distance, compare it to the predicted value in the almanac and determine time and longitude. By the late 1700's, the lunar distance was helping map the world and guiding the navies and merchant fleets of the great powers. Captain James Cook used "lunars" extensively on his explorations of the Pacific in the 1770's. Lewis and Clark made lunar observations as they crossed North America from 1803-1806. In the 1890's, Joshua Slocum sailed alone around the world relying on dead reckoning and lunars for longitude, but he was one of the last avid practitioners. As inexpensive chronometers became more reliable and radio time checks emerged as a simple way to rate them at sea, the lunar distance fell into disuse. The Nautical Almanac stopped publishing its lunar distance tables in 1907.
While lunars are no longer a primary navigation tool, they are a great topic for study and practice. Navigators who believe that celestial techniques have a place in the modern world will find lunars stretch their understanding of the fundamentals, improve their sextant technique, and can be practiced in the backyard without a horizon. Traditionalists will discover the ultimate in independence from digital and electronic tyranny. Anyone interested in the practical application of astronomy or mathematics will find lunars to be a rich field of study as many brilliant minds applied themselves to developing and improving the technique over almost two centuries. In the words of Joshua Slocum:
"The work of the lunarian, though seldom practiced in these days of chronometers, is beautifully edifying, and there is nothing in the realm of navigation that lifts one’s heart up more in adoration."
For basic theory and instruction in the
practice of
lunars at home, click here.