On 16 November your 1200 ZT DR position is LAT 26° 48.0' S, LONG 124° 32.0' W. Your vessel is on course 078° T, speed 17.0 knots. You observe an ex-meridian of the Sun's lower limb. The sextant (hs) reads 81° 41.3'. The index error is 1.5' off the arc, and your height of eye is 56 feet. The chronometer time of the observation is 08h 15m 32s, and the chronometer is 03m 06s fast. What is your latitude at meridian transit?
• Ex-meridian Sun sight and how it relates to meridian transit latitude (running your DR to LAN time) • Correcting sextant altitude: index error, height of eye (dip), and clearing to Ho (observed altitude) • Using Sun’s declination and the relationship between zenith distance and latitude when the Sun is south of you in the same hemisphere
• First, decide: is this sight taken before or after meridian passage? How does that affect whether your latitude will be greater or smaller than the DR latitude at LAN? • Work out the corrected observed altitude (Ho) step by step. From that, find the zenith distance and compare it with the Sun’s declination. Is the Sun to the north or south of your zenith at transit? • Think about how far the ship moves between the time of the sight and local apparent noon (LAN). In which direction does motion along your course affect your latitude, and by how many miles in that time interval?
• Carefully apply index error: for off the arc, do you add or subtract from hs? • Make sure you convert height of eye in feet to dip in minutes of arc correctly before finding Ha and then Ho. • Double-check the Sun’s declination and its sign (N or S) at the date and GMT of the sight; confirm whether latitude is found by Lat = Dec ± ZD and which sign to use in this hemisphere and date.
No comments yet
Be the first to share your thoughts!