## In Defence of Mer Pas

The noon sight for latitude is a method of calculating latitude from the altitude of the sun at the instant it crosses your meridian and for this reason, the method is also known as ‘Meridian Passage’ or ‘Mer. Pas’.  I am often asked “what’s the point of this when we already know our latitude”; my reply is “unless we are using GPS, we won’t know our exact latitude and if we are using GPS, why are we bothering with astro navigation anyway?  The whole point is that, as I have pointed out in previous posts, we can no longer depend of GPS for a variety of reasons so prudent navigators will keep up their skills in astro navigation.

When we calculate a position fix using astro navigation, our starting points are our DR position or our EP which are only approximate positions so how can we say that there is no point in taking a noon sight when we already know our latitude.  The fact is that we don’t know our exact latitude and the noon sight is a way of calculating just that.

Why stop at latitude?  In the past, before the advent of the chronometer, the noon sight would enable the navigator to only calculate latitude but of course we can now use it to calculate longitude as well.  At the instant the sun crosses our meridian, we will know the exact local mean time (LMT) from the time of Mer Pas as listed in the Nautical Almanac for the day and by combining this with the Greenwich Mean Time which we take from the deck watch at the same instant, we are able to calculate our longitude as the following example taken from my book ‘Celestial Navigation’ shows:

 Step 8. Calculate time difference between LMT and GMT of Local Mer Pas. (Latitude West: GMT Best.  Latitude East: GMT Least). GMT at Local Mer Pas: 16h 08m 20.1s        (Subtract from LMT if DR East) LMT Mer Pas: 12h 02m 00.0s      (Subtract from GMT if DR West) Time Diff: 04h 06m 20.1s Step 9. Calculate Longitude at Local Mer Pas.  (Convert time difference to Arc) Time Difference = 04h 06m 20.1s Multiply the hours by 15 and divide the minutes and seconds by 4. Convert decimals to units of arc. degs      mins      secs Convert hours: 4h  = 4 x 15  = 60o 60o        00’     00” Convert minutes: 6m = 6 ÷ 4  = 1o.5 1o     30’     00” Convert seconds: 20.1s = 20.1 ÷ 4 = 5m.025 0o        05′     01″.5 Total 61o        35′     01”.5 Calculated Longitude: 61o  35’ 01″.5 W

So the noon sight can give us a position fix in terms of both latitude and longitude.

In the summer months, in the middle latitudes, there can be up to 16 hours from morning nautical twilight to evening nautical twilight so that means that we can go up to 16 hours between star and planet sights.  However, a noon sight will give us an accurate fix between twilight times and the good news is that there is only one celestial body to shoot and no ‘cocked-hat’.

Of course, there is also the ‘Intercept Method’ or ‘Sun Run Sun’ which will give us another fix during daylight hours.  The whole point is that we never know when poor visibility will prevent us from taking morning and evening sights so the more tricks that we have up our sleeves the better.

If you wish to learn about the Meridian Passage method try this link: