Marine Glossary

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Lunar Distance

The procedure of finding longitude and GMT by measuring angular distance between the moon and another celestial body. The procedure is often called "lunars."

With care and practice, one can find GMT in this manner to within about 1 minute of time, sometimes better, more often not quite that good. The sights are difficult, even on land, and must be accurate, along with accurate index correction.

There are several procedures for solving the sight reduction. We solve this problem in the StarPilot and in that section of our webpage there is an explanation of the procedure we use.

The sights can also be solved with tables. The governement stopped printing these in 1900 or so, but navigation historian Bruce Stark has published a set with instructions. They are available in the books section of our online catalog.

If you care to read about how the navigators of the 19th century used these sights, it is covered in the 1851 edition of Bowditch that we publish as an e-book. See books section of our online catalog.