Saturday, February 4, 2012

Celestial Navigation

Quartermaster 3rd Class Jay Kintner uses a sextant to verify the position of USS COWPENS (CG 63) by the stars. Photo by Dennis A Narlock, USN. 
The captain was telling me about a particular clear night on an offshore supply boat  (OSV) in the Gulf of Mexico. He stood on the wing station outside the bridge where the men on watch could see him. He held out his hand with the thumb and index finger extended, and held it up to the sky. He pretended to figure for a minute, wrote something in his notebook, then repeated the exercise a couple of times. He then went into the wheelhouse and, to the astonishment of the men on watch, plotted a very accurate position fix on the chart.  Now here was an old salt, they must have thought, able to plot our position with celestial navigation using only his hand and his memory.

To many, even those who work on ships today, celestial navigation is a bit mysterious. The mystique is intentional, and comes down from the Age of Sail, when the ships’ officers were taught the art, but not the crew. The idea was to make mutineers think twice: if only the officers knew how to find the way home, a rebellious crew might be less likely to slit their throats.

The idea behind celestial navigation is simple: that the objects in the sky – sun, moon, planets, stars – follow regular and predictable paths, and thus a given object’s position over a given point on the earth at a given time can be known in advance, or at least calculated. As Susan P. Howell put it in her book, Practical Celestial Navigation
Each celestial object can be thought of as a lighthouse since, for a specific instant of time, it stands directly over one spot on the earth. For instance, the sun will be 90 degrees high over Honolulu for a split second this summer and therefore marks that one place much as a lighthouse would.
In this example, using a sextant or other instrument, a sailor can compare the angle of the sun above the horizon and its observed direction to those of its known position over Honolulu. From this, the sailor’s current position can be calculated. In many cases, even a lot of math is not required; the calculations have been broken down into easily usable tables that are widely available.

While not mysterious, use of the sextant is an acquired skill, especially on the deck of a moving ship. And the calculations must be done accurately. These days, celestial navigators use worksheets that resemble nothing so much as IRS tax forms. If you can fill out a 1040A correctly, you can probably master the calculations for celestial navigation.

The captain I mentioned above had not, of course, determined his OSV’s position using only his hand and his memory. Before he stepped up on the bridge wing, he had consulted a handheld GPS that he then slipped into his pocket. So while celestial navigation is indeed one of the mariner’s ancient arts, he was practicing one even older: that of the practical joke.

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Strait of Magellan: Celestial Navigation 101: Lesson 1, Circles of Equal Altitude and subsequent posts.

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