Flicrk CC: aresauburn

What you'll need

• A sextant (which can be improvised from a protractor, a fishing line and a weight)
• An accurate chronometer (a watch will do)
 • An almanac showing the positions of celestial bodies
 • Sight navigation tables for calculations (try HM Nautical Almanac Office, hmnao.com)

Simple astronavigation
Locate north and your latitude by finding Polaris (the North Star) or the Sun.

"The simplest navigation instruments - the kamal, cross-staff, quadrant and mariner's astrolabe - were used with the Sun or the North Star," says Richard Dunn of the National Maritime Museum.

• Polaris can be found by following the line created by the two stars, Merak and Dubhe, that form the edge of Ursa Major (the Plough) away from its "handle".

"The North Star is due north in the sky and deviates by only small amounts. It can also be used to determine latitude by measuring its angular elevation from the horizon," Dunn says. Use a sextant to measure the angle between Polaris and the horizon. Your latitude is 90 minus the elevation (if you're at the North Pole, for instance, it will be 90-0).

• The Sun can used for both direction and to find latitude. "For latitude, measure its greatest elevation (at local noon) and apply a correction for the date from a table of declination." (See tinyurl.com/ c8az4a for more on this.) To find direction, the Sun at its greatest height (ie noon) is either due north or south of you - if you're in the northern hemisphere, it is south, and vice versa. Alternatively, you can use the Sun's rising and setting positions to locate east and west.

More complex astronavigation

• "Line of position (LOP) navigation is based on the idea that, if you know from using the chronometer and almanac where a celestial body is at a specific time, you can then work out how far you are from it," says Richard Dunn

If, for instance, you are right below a chosen celestial body, its angle above the horizon will be 90°. The further from its position you go, the smaller that observed angle will be - yet the observed angle defines a circle on the surface of the Earth from any point on which the distance from the object may be calculated. Doing this on two observable celestial bodies is known as the intercept method, and will create two points of intersection between each circle: this is known as a "fix". Because the circles are so wide, it's usually easy to tell which intersection you are located on.

"The intercept method gives you lines of position, which are drawn as straight lines because, over relatively short navigational distances, a straight line is a reasonable approximation of an arc of a circle with a very large radius," adds Dunn.

The Sun can also be used for LOP astronavigation by observing it at two different times of the day, and advancing one of the "plots" by the distance and direction moved in the intervening time. This should provide two different lines on your chart that intersect at your position.


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