Back in the age of sail a solemn ceremony would take place around noon in all vessels of the British navy in clear days.
The captain, the officer of the watch, the ship’s master and the midshipmen would meet on the quarterdeck to measure with their sextants the height of the sun at that magic moment when the sun crosses the ship’s meridian.

The captain would record the latitude thus estimated in the ship’s log, check the progress of the ship based on her course and speed during the past 24 hours, and then write down the official date and day of the week on the log-board, thus marking the beginning of a new day in the ship’s life.

Today it is still common practice even in small sailboats during long passages to calculate a position fix at noon and estimate the distance covered by the boat from the fix obtained 24 hours earlier.

Whereas few sailors nowadays use the sextant as their primary navigation tool, many still practice their celestial navigation skills to check GPS data and as a precautionary measure. After all, the GPS receiver can break down. Or the US military may suddenly turn its satellites off, in response to some national security threat.

While celestial navigation has the reputation of a complex and esoteric field, some techniques are really quite simple. And of these the simplest and quickest is the noon sight. The reason for its simplicity is that at local noon the sun and the boat are on the same meridian: if you can estimate the angular distance between the two ( which is 90º minus the height of the sun over the horizon) and you know the sun’s declination at the time of the sight, you just add or subtract one from the other and, voila, you have your latitude.

Anyone who is interested in the theory involved may wish to consult either the American Practical Navigator (1995), chapter 20 or, in Italy, the Manuale dell’Ufficiale di Rotta (1998), published by the Istituto Idrografico della Marina, Section 6.
However, if all you want to know is how to do it, these are the key steps:

1) Estimate the time of the sun’s meridian passage in your location.
The Nautical Almanac (or, in Italy, the Effemeridi Nautiche) will give you the time of sun’s meridian passage at Greenwich on each day of the year. Based on your dead reckoning position and your time zone, you can quickly figure out the time of local noon. For instance, if your D.R. longitude is 7º 30’.0 E, you know that your time zone is one hour before GMT (each time zone being 15º wide) and that the sun will cross your meridian exactly one half hour before it crosses Greenwich’s ( in its apparent movement around the earth, the sun moves 360º in 24 hours, or 15º in one hour, or 1º in 4 minutes; anyway, do not worry about detailed calculations: the Nautical Almanac has a neat table on "Conversion of Arc to Time" that will help you translate any distance into the time the sun will take to cover it). Therefore, if the time of the sun’s meridian passage at Greenwich in the given day is 12: 10 GMT, your local noon will be thirty minutes before, i.e. 11: 40 GMT or 12:40 local time.

2) Take sextant sights. About ten minutes before your estimated local noon (in our example, around 12:30 local time) start taking sights of the sun. You will notice that the altitude first increases, then stays approximately constant for a little while, then starts declining, as the sun goes through an arc in the sky centered around your meridian. Choose an average of the 3 highest estimates as your sextant altitude (Hs). In our example, let Hs be 70º 30’.0.

3) Correct the sextant altitude for index, dip and altitude error. The index correction is to eliminate the instrument error that affects virtually every sextant (this is often provided by the manufacturer and can be either positive or negative). The dip correction is necessary because you measured the height of the sun from deck and not from the surface of the sea and the horizon appears lower the higher you are; this correction is always negative and can be derived from a table at the beginning of the Nautical Almanac. The altitude correction is to eliminate the effect of light refraction at different altitudes of the sun and to correct for the fact that you measured the sun’s altitude from its lower limb rather than from its center; this correction is also provided by the Nautical Almanac. To summarize, if your sextant altitude (Hs) is 70º 30’.0, the index correction is +2’.0, the dip correction is -3’.0 and the altitude correction is +15’.0, the corrected sextant altitude (Ho) will be 70º 44’.0.

4) Determine the sun’s declination. This is given for each day of the year and each hour of the day in the Nautical Almanac. As declination varies little from one hour to the next, interpolation is easy. For instance, if on the day you took your sights the sun’s declination was N 21º 34’.8 at 11:00 GMT and N 21º 34’.4 at 12:00 GMT, you can estimate that at 11:40 GMT (or 12:40 local time) the sun’s declination was about N 21º 34’.5. Always remember that the declinations shown in the Nautical Almanac are for Greenwich time (GMT) and therefore must be converted to local time.

5) Calculate the zenith distance. This is simply 90º minus the corrected sextant altitude, in our case 90º- 70º 44’.0= 19º 16’.0. Name the zenith distance North or South depending on the relative position of the sun and the observer at local noon. If the observer is to the North of the sun (as is always the case in the Mediterranean), name the zenith distance North, in this instance N 19º 16’.0.

6) Calculate the latitude. Compare the names of the zenith distance and the declination. If they are both North (as they will be in the Mediterranean in spring and summer) or both South, simply add the two values together to obtain the latitude. If the name of the zenith distance is contrary to the name of the sun’s declination (e.g., in the Mediterranean in fall and winter, the sun’s declination will be South while the zenith distance will be North), subtract the smaller of the two from the larger and name the latitude thus obtained with the name of the larger of the two quantities. In our case, as we took our sun altitude in summer, both zenith distance and sun’s declination are North and therefore the latitude will be: N19º 16’.0 +N 21º 34’.5 = 40º 50’.5 N. If we had taken our sight in winter and the sun’s declination was, say, S 15º 00’.0 with a zenith distance of N 55º 00’.0, the latitude would be 55º - 15º = 40º and of course would be labeled North.

This looks complicated but really is not. All you need is a sextant, the Nautical Almanac and, perhaps, a simple form summarizing the various steps. An accurate watch is useful but not indispensable, as you can always determine the time of meridian’s passage just by observing when the sun reaches its highest altitude. Actually, the whole exercise boils down to a few additions and subtractions and will not take more than a couple of minutes.

The only danger is to make some stupid mistakes like reading the wrong page of the Almanac or making arithmetic errors. On the other hand, with just the latitude estimate thus obtained and some good dead reckoning records, you can go a long way.

After all, this is what navigation was mainly about for centuries, before John Harrison invented the marine chronometer.

Today,in order to get the accuracy of a marine chronometer, you do not need to go beyond your wrist watch. With that, your sextant and a few sight reduction tables you can complement your latitude estimates with good estimates of longitude and thus have a position fix, even if your GPS does not work.
We’ll talk about that in a next article.