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E4 - - - Sight Reduction - - - 10/03/2004

Sight reduction is defined as the process of deriving from a sight the information needed for establishing a line of position or the location of the observer. This entails computing the body's altitude and azimuth, using either the estimated or an assumed position.

For centuries, the only sights the navigator could use were those of bodies transiting his meridian; from these he could obtain his latitude. Otherwise, with the exception of Polaris, which served to indicate latitude and direction in the Northern Hemisphere, without an accurate time source, the celestial bodies were of little use except as steering reference.

With the invention of the chronometer, when the latitude was known, it became possible to compute the longitude, using the time sight method; this method of navigation remained popular into the early twentieth century, as a position could be determined without plotting. The discovery of the line of position (LOP) by Captain Thomas H. Sumner in 1837 heralded a new era in navigation. The Sumner line of position was originally obtained by reducing the same sight twice; the estimated latitude was used for the first reduction. A slightly different latitude, say, 10' or 20' from the first, was then selected to reduce the sight a second time; a line of position was then drawn through the two positions on the chart. With the invention of azimuth tables in the latter part of the nineteenth century, it became possible to work only one time sight, and then draw a line through the resulting position, perpendicular to the body's azimuth.

The era of the "new navigation" came with the introduction of the altitude- difference method of determining a line of position by Commander Adolphe-Laurent- Anatole Marcq de Saint-Hilaire, of the French Navy, in 1875. This method remains the basis of almost all celestial navigation used at sea today.

The Marcq Saint-Hilaire method, as it is generally called, remained in common use on board ships through most of the twentieth century.

A ship is never entirely lost. Even though its position has not been verified for several days, by recording headings and speed and plotting the tracks, the reckoning (DR) position is usually within twenty miles of the correct location.

In this new method the problem is worked backward to simplify the calculation. For example, instead of using the observed data to locate the observer directly, an assumed position is used as a reference as follows.

By using the DR position, or an assumed position (AP) in the vicinity of the DR, the elevation and azimuth of a heavenly body is calculated. From the AP, an azimuth line is drawn on the plotting sheet, and the calculated elevation is compared with the measured elevation and if, for example, the measured value is 5 minutes higher it means that the LOP (line of position) must be 5 nautical miles from the AP, in the direction of the celestial object, and if the measured value is 10 minutes lower, then the LOP must be 10 nautical miles away from the AP in the direction away from the celestial object.

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