PROCEDURE FOR L. A. N.
You can see that latitude by LAN is rather a simple , requiring only the most elementary knowledge of arithmetic. However, there are certain procedural aspects of the sittuation which you must know. You must be able, first of all, to calculate the time of local apparent noon, when the sun will be at its maximum altitude, exactly on the meridian. Watch time of local apparent noon may be determined in advance by what is called the FIRST ESTlMATE, SECOND ESTIMATE method. Since the navigator usually begins the observation of the sun several minutes before the actual time of meridian passage, many navigators do not consider the increased accuracy of a second estimate necessary.
Sample Problem
On 12 February 1981 your 0542 zone time (ZT) fix gives you a position of LAT 26° 42.0' N, LONG 60° 18.0' W. Your vessel is on course 300° T, and your speed is 9.8 knots. Local apparent noon (LAN) occurs at 1220 ZT at which time a meridian altitude of the Sun's lower limb is observed. The observed altitude (Ho) for this sight is 49° 10.0'. What is the calculated altitude at LAN?
A. 27° 13.5' N
B. 27 16.3' N
C. 27° 17.6' N
D. 27° 19 .2 N
BODY SUN
DATE 12 FEB. 1981
DR LAT 27-14.5 N
DR LONG 61-14.3 W
Hs
IE
Hs
Dip
Ha
AlT CORR
Ho 49- 10.0
ZT 1220
ZD +4
GMT 1620
CT
CE
CCT
GHA
M&S
SHA
v
GHA
DR/A LONG
LHA
TAB DEC S 13- 34.0
d corr (0.8) - 0.3
DEC S 13- 33.7
90° = 89- 60.0
Ho 49- 10.0
z 40- 50.0
Dec - 13- 33.7 S
Lat 27- 16.3 N
The rule for this case is LAT = Z - dec
STEP 1
Setup plotting sheet, and DR ahead from 0542 to 1220 and get your DR position and enter it in your format:
1220
0542
6 38 x 9.8 kts = 65.0 miles
STEP 2
Enter the Nautical Almanac and compute the declination for 1620 GMT.
STEP 3
Enter your Ho in your format and subtracted it from 90° to get "Z"
STEP 4
Enter your declination and subtract it from "Z". To get near your DR Latitude you would have to subtract. Result is your latitude
27° 16.3'N. Then plot this latitude on your plotting sheet and label it as such
Answer is "C"
