

Sun's Information and Sight ReductionBelow is a form using JavaScript that provides the Sun's information and sight reduction  for the years between 1979 and 2020: [The error for GHA and Declination of Sun calculated by this script in most of the cases should not exceed 0.3' which is considered as very small when compared with the accuracy of human observation by marine sextant.] I wish to thank Oscar W. Underwood III, a retired American engineer, for information he provided that enable me to extend my Sight Reduction program from the year 2000 through 2020. The Intercept (Marcq SaintHilaire) MethodThe article in the general sight reduction script briefly describes the history of navigation by celestial observations. The practice of position fixing by celestial bodies has been used by navigators in ocean passage until accurate and low cost electronic navigation aids, like transit satellite and GPS, are available. Sun sights during the daylight hours and stars sights during twilight are obtained to determine the ship's position. The Marcq SaintHilaire or intercept method is the most popular way to reduce a sight for a line of position. The intercept method uses the difference between the observed true altitude (obtained by correcting the sextant altitude for index error, reflection, dip, etc) and the calculated altitude (obtained by sight reduction tables or calculations) of the ship's DR position to give an intercept for plotting the line of position on chart or plotting sheets. The figure on the right shows the principle of the intercept method. If we take the celestial body as the centre of a circle, the position circle formed by using the zenith distance (90 degrees  altitude) of the calculated altitude as radius will pass through the DR position and that for the observed true altitude should pass through the observer's position. Given that the celestial bodies are so far away from the earth, we can safely assume that the arc of these position circles passing the observer's vicinity are in straight lines. These position lines are running prependicular to the Azimuth of the celestial body concerned. The result of a sight provides us with the following information :
To make the above simpleThe practice of reducing a sight to a line of position is summerized as below :


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