Ancient Navigation

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The Mariners’ Museum has a large collection of very old—to ancient navigation instruments. These include astrolabes in the Age of Exploration gallery, quadrants, cross staffs, back staffs, octants & sextants in several galleries. A reproduction of the even more primitive Kama’l can be found in the Age of Exploration Gallery, and it was one of the earliest devices to estimate the elevation of a heavenly body above the horizon. That observation put the ancients on the same track that we “modern” navigators used—until satellites, computers, and GPS made the satisfying and elegant art of celestial navigation obsolete.

The earliest astronomers through painstaking and detailed observations and record-keeping recognized that the stars seemed to rotate about a single star in the heavens, Polaris, in the constellation Ursa Minor. The ancients eventually realized that the angular elevation (altitude) of Polaris above the horizon corresponded almost exactly with the latitude on the earth from which the altitude measurement was made. From the altitude of Polaris, and a few corrections the Latitude can be easily calculated. (Corrections are necessary since Polaris is not exactly due north, in fact, because of precession of the earth’s axis, Polaris was not always and will not always be the North Star. Around 14,000 CE, the star Vega will be in that position for 500 or 1000 years.)

Latitude can also be ascertained via a different observation of altitude of a heavenly body; the sun. When it crosses the observer’s meridian, an event named Local Apparent Noon (LAN) occurs. This observation does not require precise timing. Even long after the development of pendulum clocks, there was no way to precisely tell time at sea, since ship’s motion interfered with the operation of pendulum clocks. So for many years, the determination of longitude at sea was problematic and work-arounds were developed to facilitate navigators finding their way across the seas. The most common, was sailing north or south to the destination’s latitude, then east or west until landfall was made. This “Parallel Sailing” was used by Arabs and Chinese long before Columbus’ time. This approach was necessary until the invention and development of the marine chronometer by Harrison in the last half of the 18th century. The early exploration and exploitation of North America (e.g., Christopher Newport) was without the benefit of a chronometer.

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