Principle
Meridian altitude is the simplest calculation ofMethodology
The estimated time of meridian altitude of the heavenly object is extracted from the nautical almanac. A few minutes before this time the observer starts observing the altitude of the object with a sextant. The altitude of the object will be increasing and the observer will continually adjust the sextant to keep the reflected image of the object on the horizon. As the object passes the meridian a maximum altitude will be observed. The time in UTC of this is observed. The altitude obtained is corrected for dip (the error caused by the observers height above the sea) and refraction to obtain the true altitude of the object above the horizon. This is then subtracted from 90° to obtain the angular distance from the position directly above to obtain the zenith distance. A further correction must then be taken into account to counter the "wobble" of the earth's spin and rotation relative to the sun and planets. This is given in the declination for the body on a particular day in the year (also taken from the nautical almanac). If the declination of the body is in the opposite hemisphere (ie if you are in the northern hemisphere and the declination is in the southern hemisphere) then the declination must be subtracted from your true zenith distance, otherwise the declination is added.See also
*References
*''Nicholls's Concise Guide, Volume 1'', by Charles H. Brown F.R.S.G.S. Extra Master *''Norie's Nautical Tables'', edited by Capt. A.G. Blance *''The Nautical Almanac 2005'', published by Her Majesty's Nautical Almanac Office *''Navigation for School and College'', by A.C Gardner and W.G. Creelman Navigation