On 22 July at 0448 ZT, morning stars were observed, and the vessel's position was determined to be LAT 21° 43.0' N, LONG 158° 39.0' E. Your vessel is steaming at 21.0 knots on a course of 028° T. A sextant observation of the Sun's lower limb is made at 0956 ZT. The chronometer reads 10h 54m 27s, and the sextant altitude is 54° 28.2'. The index error is 1.5' off the arc, and the chronometer error is 01m 38s slow. Your height of eye on the bridge is 56 feet. What is the azimuth (Zn) of this sight using the assumed position?
• Intercept (altitude) sight reduction process using the assumed position • How to correctly apply index error, height of eye, and chronometer error to get Ho and GHA/Dec • Using the Nautical Almanac and sight reduction tables (e.g., Pub. 229) to compute Z and convert it to Zn
• First, focus on cleaning up the raw sextant data: in what order do you correct the sextant altitude, and what is the final Ho you get for the Sun? • Work out the exact GMT of the sight from the chronometer reading and chronometer error, then determine the Sun’s GHA and Declination at that time and form the correct assumed position. • Once you compute the tabular azimuth (Z) from the sight reduction tables, think carefully about which quadrant the Sun is in and how to convert Z into Zn for your latitude and local hour angle sign.
• Be sure you’ve applied index error with the correct sign (“off the arc” vs “on the arc”). • Confirm you’ve converted height of eye in feet to dip in minutes of arc correctly before applying it. • Double-check that the final Zn is consistent with the vessel’s hemisphere (N/S) and whether the Sun is east or west of the meridian; eliminate choices that would place the Sun in the wrong quadrant.
No comments yet
Be the first to share your thoughts!