On 13 November at 0438 ZT, morning stars were observed and the vessel's position was determined to be LAT 22° 14.0' S, LONG 79° 23.0' E. Your vessel is steaming at 13.0 knots on a course of 242° T. A sextant observation of the Sun's lower limb is made at 0822 ZT. The chronometer reads 03h 20m 16s, and the sextant altitude (hs) is 45° 49.7'. The index error is 1.0' on the arc, and the chronometer error is 01m 47s slow. Your height of eye on the bridge is 61.0 feet (18.6 meters). What is the azimuth (Zn) of this sight using the assumed position?
• Sight reduction process for the Sun using an assumed position • How to apply index error, height of eye, and chronometer error to get observed altitude (Ho) and correct time • Relationship between LHA, latitude, declination, and azimuth (Zn) in the sight reduction tables or formulas
• After correcting hs to get Ho, what information do you need from the Nautical Almanac at the exact UTC of the sight, and how do you get Local Hour Angle (LHA)? • Given your latitude and the Sun’s declination, is the Sun north or south of you, and is it rising or getting higher? How does that affect whether Zn is to the east or west and roughly what quadrant it should lie in? • When you compare Hc from the assumed position with Ho, what does the intercept direction tell you about whether your assumed longitude was to the east or west of your actual line of position, and how does that help confirm whether your Zn choice is reasonable?
• Be sure you’ve applied chronometer error correctly (remember it is slow) when converting to UTC and then to GHA • Confirm you used the correct hemisphere sign for latitude (South) when entering tables or formulas • Before choosing, check if your computed Zn quadrant (NE, SE, SW, NW) matches the time of day and the vessel’s approximate position in the Indian Ocean in mid-November
No comments yet
Be the first to share your thoughts!