Author Archives: Jack Case

Distance To The Horizon

Posted on April 20, 2014 by Jack Case

Pythagoras provides us with a method calculating the distance to the horizon. In the diagram below, A represents a ship’s position. B is the top of the lighthouse. B1 is the base of the lighthouse. C is the Earth’s centre. r … Continue reading →

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Survival – Calculating Altitude Without An Angle Measuring Instrument

Posted on April 4, 2014 by Jack Case

Links:  Astro Navigation in a survival situation.    Latitude from the midday Sun.    Find your longitude.     Calculating declination.    Declination table.     Trig table.    Revise trigonometry  Revise Spherical Trigonometry As the above links demonstrate, there are several methods of employing astro navigation … Continue reading →

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The Relationship Between Altitude and Zenith Distance

Posted on February 18, 2014 by Jack Case

Several readers have raised questions relating to my post dealing with the Intercept Method.  These questions mostly concern the relationship between altitude and zenith distance and also the relationship between angular distance and the nautical mile. Since several chapters of … Continue reading →

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Zone Time

Posted on November 22, 2013 by Jack Case

It would be impossible for a ship at sea to keep to the time of its longitude because (unless it is travelling due north or south) the longitude will be constantly changing.  For this reason, the sea areas of the … Continue reading →

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Lunar Distance

Posted on November 9, 2013 by Jack Case

  With the invention of the chronometer in 1759, mariners were, for the first time, able to calculate longitude by finding the difference between Greenwich Mean Time and Local Time.  However, the marine chronometer did not become affordable until around … Continue reading →

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The Intercept Method

Posted on November 8, 2013 by Jack Case

Although it is usual these days to calculate an observed position in astro navigation by using the Rapid Reduction Method, there are other methods which do not necessitate the purchase of expensive reduction tables.  One of these methods is the … Continue reading →

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Rhumb Line Sailing

Posted on November 4, 2013 by Jack Case

Terminology.  The following terms are used in this exercise:  Rhumb Line, Departure, D.Long, D.Lat, Mean Lat,      Mid. Lat.To find the meaning of these terms, click here:    Short distance sailing is a term which is applied to sailing along a … Continue reading →

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How To Find Longitude

Posted on September 3, 2013 by Jack Case

The longitude problem.  The measurement of longitude was a problem  until well into the 18th. century.  Determining latitude was relatively easy because it could be calculated from the altitude of the sun at noon but for longitude, early navigators had to rely on dead … Continue reading →

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What are Local Hour Angle and Greenwich Hour Angle

Posted on August 17, 2013 by Jack Case

Local Hour Angle (LHA). In astro navigation, we need to know the position of a celestial body relative to our own position. In the diagram below: LHA is the angle BNU on the Earth’s surface which corresponds to the angle ZPX … Continue reading →

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What are Azimuth and Altitude

Posted on July 22, 2013 by Jack Case

Azimuth and Altitude part 1 We can define the position of a celestial body in relation to its local hour angle and declination.  We can also state its position in relation to our own celestial meridian and celestial horizon; in … Continue reading →

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