Category: Navigation

For a complete description of finding the GP of a Celestial Body in PDF format, click here.

Finding the Geographical Position (GP) of a Celestial Body

 The GP of a celestial body is a position on the earth’s surface that coincides with an imaginary line drawn from the centre of the body to the centre of the earth. The celestial body can be the Sun, Moon, Planet or a Star.

The significance of this point is that we can use a sextant to work out how far away we are from the GP and hence, our position.

 The GP is referenced by its Latitude and Longitude, however, to avoid confusion with earthly positions, we use different names. These are;

 Declination                     (Dec)              (Lat of celestial body)
Greenwich Hour Angle     (GHA)             (Long of celestial body)

Dec is measured just like Latitude i.e. An angle measured in degrees, minutes and decimals North or South of the Equator. The Equator is 0°, the poles 90°

GHA is measured just like Longitude i.e. an angle measured in degrees, minutes and decimals. However, unlike Longitude which is measure East and West of Greenwich, GHA is only measured westwards from 0° to 360°.

 Because of the earth’s rotation, it should be obvious that the GP of a body is continually changing. This is why we need to know the precise time (UT) when we take our sextant sight.

 The earth rotates 15° every hour, which is 1nm every 4 seconds, so you can see the precision required in your deck watch (chronometer). The time used is Universal Time (UT) – formerly Greenwich Mean Time (GMT).

 To use the time of the sight to find the GHA and Dec we need a special book called the ‘Nautical Almanac’. This book is printed every year and you will always need a current edition.

For a complete description of finding the GP of a Celestial Body in pdf format, click here.


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