On 28 July your 0800 zone time (ZT) fix gives you a position of LAT 25°16.0'N, LONG 71°19.0'W. Your vessel is on course 026°T, and your speed is 17.5 knots. Local apparent noon (LAN) occurs at 1150 ZT, at which time a meridian altitude of the Sun's lower limb is observed. The observed altitude (Ho) for this sight is 82°28.7'. What is the latitude at 1200 ZT?
• Meridian passage (LAN) sights give latitude directly from the Sun’s observed meridian altitude and declination • How to compute latitude from meridian altitude: Lat = (90° ± Ho) ± Dec, with signs depending on Sun’s declination and whether it’s north or south of you • The vessel has moved between 0800 and 1150 ZT, so you must account for distance run in latitude before applying the LAN sight to get the 1200 ZT latitude
• First, determine the Sun’s declination on 28 July and whether it is north or south; how does that compare with your approximate latitude? • From 0800 to 1150 ZT, how long (in hours and minutes) has the vessel been steaming, and what is the distance made good along the track? Then, what is the north-south component of that track? • After you find the latitude at LAN from the sight reduction, how do you adjust it to find the latitude at 1200 ZT, given your course and speed?
• Be sure to use the correct sign for declination (N or S) and apply the proper formula for latitude from meridian altitude • Confirm the run between times: convert the time interval to decimal hours and multiply by speed in knots to get nautical miles, then resolve into northing/easting • After computing the final latitude, check that it is reasonable compared to the 0800 latitude and the direction of travel on course 026°T (mostly north, slightly east)
No comments yet
Be the first to share your thoughts!