On 12 October your vessel is on course 081° T, speed 20 knots. Your 1800 zone time DR position is LAT 26° 11.0' S, LONG 77° 18.0' E. You observed 3 celestial bodies. Determine the latitude and longitude of your 1835 running fix. ZONE BODY's BODY's OBSERVED STAR TIME GHA DECLINATION ALTITUDE (Ho) -------------------- ------- --------------- --------------- ----------- Vega 1810 299° 26.6' N 38° 46.3' 23° 08.7' Fomalhaut 1823 237° 37.0' S 29° 43.2' 50° 23.9' Antares 1835 337° 43.4' S 26° 23.4' 40° 53.1'
• Running fix from multiple celestial LOPs (lines of position) using different observation times • Applying motion (run) from DR track between each sight time to a common time (1835) • Effect of course and speed on DR position change between 1800 and each sight time
• From 1800 to each sight time (1810, 1823, 1835), how far does the vessel travel along course 081° T at 20 knots, in nautical miles and minutes of longitude/latitude? • Once each earlier sight’s LOP is advanced to 1835 by your DR motion, where should the final running fix lie in relation to your 1835 DR position (ahead/astern, slightly north/south)? • Compare the time interval from 1800 to 1835 with the displacement implied by each answer choice from the 1800 DR position—does the change in latitude and longitude match the distance you would expect at 20 knots on 081° T?
• Compute the run correctly: distance = speed × time (in hours) for 35 minutes, 23 minutes, and 10 minutes; then break that distance into ΔLat and ΔLong on course 081° T. • Estimate the 1835 DR position from the 1800 DR and your run over 35 minutes, and see which answer choice is reasonably close (a running fix shouldn’t be wildly different from DR if sights are good). • Check which choice best matches the direction of DR motion: on course 081° T in the Southern Hemisphere, should longitude increase or decrease, and should latitude move slightly more north or more south?
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