Bond Marine Chronometer

Bond, William C. · ca 1812-1815

In the 19th century, portable marine timekeepers called chronometers became indispensable instruments for determining longitude at sea. To use a marine chronometer, outbound sailors would set their timepieces to the time of a known port's longitude—say Greenwich, England, or Boston. Once at sea, mariners calculated their position east or west of that place by converting the difference in time on the chronometer and local ship time into distance, 15 degrees of longitude for every hour.