On 17 December your 0600 ZT fix gives you a position of LAT 27°16.7'N, LONG 138°39.2'W. Your vessel is on course 137°T, and your speed is 14.8 knots. Local apparent noon (LAN) occurs at 1207 ZT, at which time a meridian altitude of the Sun's lower limb is observed. The observed altitude (Ho) for this sight is 40°22.1'. What is the calculated latitude at LAN?
• Meridian Altitude (LAN) sight – relationship between Ho, Sun’s declination, and latitude when the Sun is on your meridian • How to determine the Sun’s declination on 17 December and whether it has N or S declination • Effect of run between 0600 and LAN on your DR latitude before applying the sight
• First, update your DR position from 0600 to 1207 using the given course and speed. How many miles did you travel north/south in that time? • For a meridian altitude, think: is the Sun due south or due north of you at LAN in December at about 27°N? How does that affect whether you add or subtract (90° − Ho) and declination? • Once you write the meridian altitude latitude formula, plug in Ho and the correct declination sign and see which answer best matches your computed latitude.
• Be sure your time run from 0600 to 1207 is converted correctly to hours and fractions of an hour before computing distance steamed. • Confirm the sign of declination for the Sun on 17 December (north or south of the Equator) – this controls whether you add or subtract it from (90° − Ho). • Check that your final latitude is reasonable compared with your DR latitude at LAN (it shouldn’t jump by many degrees).
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