Category: Navigation

Meridian Passage  (mer pas) or Noon Sight

A Meridian Passage is a quick method of getting a position line that happens to be your Latitude. This occurs once a day at your local noon, when the sun is directly overhead. In fact, its GHA will be the same as your Longitude. It is a lot simpler to work out, and does not require precise time.

The first step is to find the time of the ‘mer pas’. This is always around noon, but not
precisely, as the sun is an irregular timekeeper. In fact ‘noon’ varies from approximately
15 minutes before 12.00 to 15minutes after 12.00. Look in the NAUTICAL ALMANAC
for the day in question. At the bottom of the right hand page, you will see a table giving
the time of ‘mer pas’ for the 3 days on the page.

 time of mer pas

This is the time the sun passes overhead at Greenwich, and also the same time it passes overhead your position by your LOCAL TIME (see time zones).

Take your Sextant on deck approximately 20 minutes before your local mer pas. Bring the sun down to the horizon, and keep monitoring it. As the sun rises slowly, keep bringing it down to the horizon. Eventually, the sun will appear to stop moving, and will stay this way for 5 minutes or so. Eventually you will note the sun start to drop back into the horizon. DO NOT ADJUST THE SEXTANT! Write down the maximum sextant reading. Now correct the sextant reading for the normal errors;

 i.e (Index error)
DIP (Height of Eye)
Altitude Correction (Refraction, UL/LL etc.)

The maths is relatively simple.

Latitude = (90° - Sextant Angle) + Declination of the Sun if SAME
Latitude = (90° - Sextant Angle) - Declination of the Sun if CONTRARY

Note:    SAME               – We are in the same hemisphere as the Sun
              CONTRARY      – We are in the opposite Hemisphere as the Sun

Note: There is a special rule if you are in the tropics;
Latitude LESS than Declination (SAME Name)                     
LAT = Declination – ZD

The Declination of the sun is found in the daily pages of the NAUTICAL ALMANAC  for the UT time and date of the mer pas. Time to the closest minute is good enough.

Example 1
In this example we assume that we are in the Northern Hemisphere and the Sun is in the Northern Hemisphere (SAME)


                                    90° 00’.0
Sextant Angle     -45°  12’.5
                                   44° 47’.5
Declination           +06° 11.1  (SAME)
Latitude                 50°  58’.6 N

Example 2
In this example we assume that we are in the Northern Hemisphere and the Sun is in the Southern Hemisphere (CONTRARY)


                                    90° 00’.0
Sextant Angle      -45°  12’.5
                                    44° 47’.5
Declination             -06° 11.1  (CONTRARY)
Latitude                   38° 36’.4 N


Find my Pro-Forma on the web site;

 mer pas pro-forma



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