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.

Methods and comparison of calculating Tidal Heights

Tidal curve

The Tidal curve is the preferred and most accurate method of calculating tidal heights. It’s reason for accuracy is due to the fact that the curve is created specifically for the harbour in question taking in local and topographical features.

Almost all curves are based upon HW, but be aware that in a few special circumstances where HW is unreliable, then we use LW as our central datum. In this case the curve is inverted.

The setting up of the curve is fairly straight forward;

• Find the time of HW at the harbour required and place in the HW box on the curve. (fill in subsequent hourly boxes, +1,+2,+3,-1,-2,-3 etc.)
   
• Find the height of HW at the harbour required and mark on the scale on the top left of the curve.
   
• Find the height of LW at the harbour required and mark on the scale on the bottom left of the curve.
   
• Join a straight line between HW mark and LW mark.
   

Read more: Methods and comparison of calculating Tidal Heights