Wednesday, February 6, 2008


Local apparent noon (LAN) is a sight taken on the Sun at the instant it transits an observers meridian. At that time, the Sun is at its highest altitude. The purpose of observing the Sun at LAN is that it allows you to establish your latitude.

The purpose of determining watch time of LAN is merely to enable you to arrive on the bridge within a few minutes of the time you should take your sight. Since zone time of LAN is based on your dead reckoning (DR) longitude, the exact moment it transits over your meridian will seldom be precisely calculated, but you should be able to figure it very closely.

The Greenwich Mean Time of meridian passage, or GMT Mer. Pass, is found on the lower right daily pages of "The Nautical Almanac" just below the sunset table. This is also the time that the sun will be over any standard meridian, no matter what zone you maybe in. Below is an excerpt from the Nautical Almanac that shows you how this information appears. Be sure that you use the page for the appropriate date. 12 Febuary, 1981. All the Coast Guard test question’s are based on using the 1981 Nautical Almanac.

Excerpt from Nautical Almanac time of meridian passage for Febuary 12, 1981
Eqn. of Time Pass.
00h 12h
12 1417 1417 1214
13 1417 1416 1214
14 1415 1414 1214

To compute the time of the 1st Estimate of LAN, I use a step by step process using the form shown below.

Problem 1 : On 12 Febuary 1981, your 0930 zone time DR position is LAT. 25° 20.0 N, LONG. 30° 40.0 W. Your vessel is on course 135° T at a speed of 11.2 knots. What is the zone time of local apparent noon (LAN) ?

A. 1210

B. 1215

C. 1220

D. 1224

DATE 12 Feb. 1981
MER. PASS. 1214
DR. LONG. 30° 08.0 W
d Long.(arc) 0° 08.8
d Long. (time) + 0m 32s
ZT LAN ( 1st Est. 12h 14m 32s = 1215

Step 1: Enter the appropriate date.

Step 2: Go to The "Nautical Almanac" and extract the time of meridian passage for 12 Febuary 1981.

Step 3: (a) Set up universal plotting sheet.

(b) Plot 0930 DR position and DR ahead to 1214.


- 0930

3h 44m at 11.2 knot = 41.8 miles

Step 4: Enter your DR longitude for the time of meridian passage you extracted from the almanac (1214).

Step 5: Enter the nearest standard meridian.

Step 6: Calculate the difference in arc between the standard meridian and your DR longitude.

Step 7: Convert the arc in into time using the "Conversion of Arc to Time" from the Nautical Almanac.

Step 8: Now apply the time correction to the time of meridian passage from the almanac. Because you are west of our standard meridian, you must add the difference. By applying this correction you will have the time of LAN at the meridian for your DR position. This is called the first estimate of LAN.

Note: if you clocks are set to daylight savings time (DST), add one hour to your zone time or use the standard meridian that your clocks are set to.