CD 157: Investigating the submarine canyons off Portugal


Using a sextant for celestial navigation

Modern technology has given us GPS and satellite positioning, but the good old fashioned sextant is still a reliable instrument. Second officer Peter Reynolds explains all...

The sun, stars and planets change their position relative to us from second to second  throughout the day.Their relative positions can be determined from a book of tables called a Nautical Almanac.

The trick to celestial navigation is to solve the ‘spherical triangle’ formed by the observer’s zenith, the Pole and the body being observed. We use the sextant to measure the body’s altitude. This is a measurement of its angle above the horizon from you, the observer. The time of this observation is recorded and the almanac is used to find the body’s hour angle and declination.

We then use a dead reckoning position [This is an estimated position calculated from knowing the ship’s speed and course] to work out what the altitude of the body should be when viewed from this assumed position. By comparing the altitude from the sextant measurement with the calculated altitude and by using the computed bearing of the body, a line of position can be plotted.

So, if we observed two or three bodies at the same time and plotted their position lines, where these lines cross would be the ship’s position.


Peter Reynolds
Second Officer RRS Charles Darwin

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April 2004
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