**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

*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)

**(GHA) (Long of celestial body)**

*Greenwich Hour Angle**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.