Navigation

C

Piloting by Water Depth

Navigators also pilot by determining the depth of the water below the boat using manual or electronic depth sounders (see Sounding). Electronic depth sounders send a signal down and time how long the signal takes to rebound (see Sonar). The longer the delay, the deeper the water. Regular depth soundings help the navigator avoid straying into dangerous, shallow water. They also can help the navigator locate the craft’s position on a chart with relatively good accuracy because many nautical charts show water depth at regular intervals.

Ancient navigators developed a simple but ingenious manual sounding device, called the lead line, that is still in use today. A lead (pronounced “led”) line consists of a long rope with a weight at the end. Knots or colorful markers tied into the rope mark off every fathom—that is, every 1.8 m (6 ft)—or every meter. Navigators can estimate the water depth by counting the markers on the rope as the weight sinks to the bottom. The weight, made of lead, has a dollop of grease or soft wax on its bottom. When the weight drops, it picks up a sample of the muck, gravel, or other material coating the sea or lake bottom. Water depth and seafloor consistency are shown on many charts. This information can be helpful in position finding. For example, if the water measures 25 m (82 ft) deep and has a gravel bottom, navigators look on the chart for an area with those characteristics.

IV

Celestial Navigation

When no landmarks or aids to navigation are visible, navigators may use the Sun, the Moon, or other celestial bodies to fix the craft’s position. In celestial navigation, navigators measure the altitude of a celestial body to derive a circle of position. Altitude of a celestial body refers to its angle, in degrees, above the horizon. From every point on the circle of position, the altitude of the celestial body is the same.

To illustrate the basic concepts of celestial navigation, push a pencil point into the top of an apple so the eraser is several inches above the skin. Secure a string to the eraser with a thumbtack or pin. The apple represents the Earth; the eraser, a celestial body; the string, the line of sight between the vessel and the celestial body. To find the craft’s circle of position, stretch the string down to the apple’s surface and swing a circle around the point where the pencil pierces the apple. The altitude of the eraser is the same from every point on the circle of position.

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In celestial navigation, navigators derive an estimated position by plotting the celestial circle of position on a special chart called a plotting sheet. The estimated position lies on the point where the circle of position intersects the dead reckoning track. To determine a more reliable position fix, the navigator measures the altitude of two different celestial bodies. The point where the two circles of position cross provides the navigator with a position fix.

A

Altitude and Time

Celestial navigators use a unique precision instrument tool for measuring angles, the sextant. To measure the altitude of a celestial body the navigator points the sextant at the horizon, then slides a movable sighting arm up until mirrors in the sextant reflect an image of the body onto the horizon. The horizon provides a base line against which to measure the angle to the Sun, star, or Moon. A traditional sextant cannot be used in dense fog or other times of poor visibility because it requires a clear view of the horizon. Some sextants include a system for creating an artificial horizon to overcome this obstacle.

The navigator also needs an extremely accurate clock or watch called a chronometer. The exact time of the sighting with the sextant must be recorded to ensure accuracy because most celestial bodies change position in the sky as Earth rotates. A five-second mistake can cause an error in position of 1.6 km (1 mi) or greater.

B

Position

After identifying the celestial body, measuring its altitude, and recording the exact time, the navigator refers to a table in the Nautical Almanac. This annual publication provides the body’s exact position at every time of day. The navigator makes small corrections to account for the sextant’s height above the water, the size of the celestial body, and other factors, then consults another set of tables, called sight reduction tables, to pinpoint the craft’s position. These tables comprise many printed pages in a thick book. Today, they are also available as computer software. The software calculates the craft’s position from data entered by the navigator, such as the name of the celestial body, its vertical angle in degrees, minutes, and seconds, and the time the measurement was taken.

C

The Noon Sun Sight

One type of celestial sight, the noon sun sight, does not require navigators to consult the Nautical Almanac or sight reduction tables. With a chronometer set to Greenwich mean time (GMT)—that is, the time at the prime meridian, the navigator can determine a craft’s longitude. When the Sun is at its highest point in the sky (noon local time) the navigator notes GMT. Earth revolves 360° in 24 hours, or 15° per hour, so the time difference multiplied by 15 provides the ship's longitude. For example, a navigator on a ship in the Atlantic Ocean determines that noon on the ship (the minute the Sun reaches its highest point in the sky) occurs at 14:00 GMT (2:00 in the afternoon). By multiplying the time difference, 2 hours, by 15, the navigator calculates that the ship’s longitude is 30°. This simple celestial navigation sight is called the noon sun sight. By plotting the noon sun sight value for longitude on a chart and noting where it intersects with the dead reckoning track or a circle of position from a recent sextant measurement, the navigator can obtain a position fix.

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