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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.
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Celestial Navigation - Compass Checking
Even in this age of electronic wizardry, your steering compass is still a vital piece of equipment. We use it steer a compass course when we are far from land, or just to glance at to make sure the autopilot is keeping us on track.
Normally we employ the skills of a compass adjuster to set up our compass and to create a ‘Deviation Card’. We can also do it ourselves when we are close to land and have many visual references. It is a relatively easy task to take transits and compare these to readings on our compass.
However, once at sea, we are devoid of lighthouses, headlands and useful transits. We are forced to look to the skies and use the celestial bodies to check our compass.
The 2 main methods employed are to compare our steering compass against either;
- Azimuth (bearing) of a celestial body (usually the Sun)
- Amplitude (bearing of the Sun at Sunrise / Sunset)
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