I have been asked about using ABC tables to calculate azimuth and altitude when planning star and planet observations. Unlike the Sun and the Moon which are easily identified, the approximate positions of stars and planets need to be established before observations are made. The times of rising and setting of the Sun and Moon can be found in the daily pages of the nautical almanac so we know when they will be visible above the horizon. Risings and settings for stars and planets are not listed and so their approximate positions need to be calculated. There are various techniques and devices that can be used for this purpose such as ‘star globes’, ‘star charts’, computer software and of course the ABC tables. The ABC tables can be found in books of nautical tables such as Norie’s and Burton’s and can be used to calculate the approximate azimuth and altitude of celestial bodies.
My book ‘Astro Navigation Demystified’ teaches the use of the ‘Rapid Sight Reduction Tables For Navigation’. The procedures for using these tables includes a ‘Planning Phase’ and a ‘Fix Phase’. In the planning phase, the approximate azimuth and altitude of chosen celestial bodies are calculated before the process of taking the sights is undertaken in the fix phase. So, by using these tables, the need to clutter the chart table with star globes, hefty volumes of nautical tables and other paraphernalia is avoided which can be a blessing in a yacht or other small craft.
Furthermore, the mathematical solution of the triangle PZX by the use of spherical trigonometry is time consuming and gives considerable scope for arithmetical error. Time and accuracy are of the essence in practical navigation and the Rapid Sight Reduction Tables enable us to obtain the solutions of the triangle PZX for all combinations of Declination, Hour Angle and Latitude so that we can calculate altitude and azimuth by relatively simple table operations without the mental torture of making calculations by spherical trigonometry.
Of course, when selecting the celestial bodies required for our observations, it is necessary to make sure that they will be visible above the horizon before we go to the trouble of calculating their expected approximate azimuth and altitude. In ‘Astro Navigation Demystified’ you will find a ‘rule of thumb’ method for doing this without any complicated calculations or the use of complicated nautical tables.
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