Position lines

Position Lines Position Lines • The position of a body on the Celestial Sphere is determined by it’s GHA and Declination, both of which are given for any instant of GMT in the Nautical Almanac • By joining a line from the body to the Earth’s centre it will cut the Earth’s surface at the Geographical Poition of the body • This position is that point on the Earth’s surface which has the body directly overhead Position Lines Z X Pn N S Ps Position Lines • Most observations will produce position circles of thousands of miles in radius so this data is of little use by itself itself • But! If an observer assumes a DR position, then a PZX triangle can be formed and the Zenith Distance calculated on the basis of this DR position position • Due to the immense size of the position circle, that part of its circumference which lies within the vicinity of the DR position can be considered a straight line, and as such also be considered a POSITION LINE at right angles to the Azimuth Azimuth Position Lines • A comparison of the true and calculated Zenith Distances will indicate the distance of the POSITION LINE from the DR position i.e The INTERCEPT • The Intercept, in conjunction with the Azimuth will allow the INTERCEPT TERMINAL POINT (or ITP) to be calculated i.e the position through which the position line Z1 Z X True ZX Assumed or DR position Actual position of the observer Geographical position of the body C Rational Horizon True ZX Z1 Z X Actual position of the observer Assumed or DR position C Rational Horizon