| "Lunar Distances" or lunars for short were used to determine longitude at sea in the period from about 1767 until 1850 (and rarely after that date). This was a time when chronometers were not yet widely available at a reasonable price and were considered unreliable on long voyages. The principle behind lunars is simple. Think of the Moon in the sky as the hour hand of a great clock. The stars along its path are the numbers on the face of the clock. If we can measure the Moon's position relative to the stars, we can read the time from the clock. Comparing that absolute time with local time (usually determined by a time sight), we have our longitude. A one hour difference in time corresponds to a 15 degree difference in longitude. |
Predicted
Lunars| Lunar distances can be used for navigation only if a navigator has access to predicted distances. These were published in almanacs around the world from the late 18th through the early 20th centuries. Today the predicted distances are no longer published, but they can be calculated without much difficulty. The predictions generated by this web site are extremely accurate and more accurate than several other sources for lunars available on the Internet. The time period covered is 1750 to 2050 so the predictions here can be used for modern lunars or for analysis of historical lunar observations. |
Clear a Lunar Online| The web tool here will analyze a lunar observation and generate your longitude in a fraction of a second. Historically, the analysis of lunars required about twenty minutes of work and a table of logarithms. Even if you decide to work a lunar by hand with pencil and paper, this online calculator will allow you to check your results. |
NEW Online
Nautical Almanac| A complete online Nautical Almanac covering the period 1750 to 2050. The positions of the Sun, Moon, navigational planets and stars are generated for every hour of the day at a high level of accuracy just as in the published almanacs. This online tool permits customization of display options so that data are generated that are relevant for your location and interests. |
Easy Lunars| An account of a simple technique for analyzing lunars using nothing more than the basic corrections and data in the Nautical Almanac and a handheld calculator. Lunars are not difficult. They take a little practice, and you may have to re-familiarize yourself with the use of a calculator, but any navigator can enjoy the challenge and experience of this historical method of navigation. |
About Lunars| More background on the lunar distance method of navigation. |
Links| Various other online resources on lunar distances including links to Slocum's "Sailing Alone Around the World" and Dana's "Two Years Before the Mast", both of which contain references to lunars. |
Lunars in the Nautical Almanac | Samples of lunars tables from old almanacs. The publication of the Nautical Almanac containing lunar distance predictions made long-distance ocean navigation practical. |
Bowditch (1801)
| Nathaniel Bowditch is frequently credited with inventing a simplified method of clearing lunar distances that made lunars practical for any navigator. That's more legend than fact, but Bowditch's navigation manual was the primary source for information on lunars for most American ocean navigators from about 1801 until they were dropped in 1916 (from an appendix at that late date). |