To take a sextant noon sight you first have to figure out when the sun will pass over your meridian.

It is good enough to work it out to the closest minute. Start by taking a sight of the sun a couple of minutes before the calculated meridian passage.

Remember, it’s important that you don’t make any adjustments to the sextant as you do this. Wait a minute before taking another shot. You’ll notice that the sun will now be a little bit higher in the sky. Keep repeating this procedure until you can see that the sun no longer rises.

At that point you will have taken a noon sight.

Working example

If you are unsure of the exact time of the meridian passage, you can start observing as early as 1330 UTC (Coordinated Universal Time, equivalent to Greenwich Mean Time) in the following hypothetical example.

The meridian passage at Greenwich (on our hypothetical day) will be found in a nautical almanac in the daily pages for the sun in the column named TRANSIT or MER.PAS. For this working example, it turns out to be 1157 UTC. Because your position is approximately 25°W longitude, the meridian passage at your longitude will occur about 1hr and 40min later, at around 1337 UTC.

For the sake of this example, let’s imagine it’s five hours after your previous morning shot.

If we assume you have had an average cruising speed of five knots, you will have covered around 25 miles in that time. If you mark this on the chart along your course line you will arrive at a probable longitude at 1337 UTC, which would turn out to be approximately 24°47’. This results in a passage of 1hr 39min 8sec, rounded to 1hr 39min. This will give you a slightly more accurate meridian passage of 1336 UTC.

Let’s say the noon sight is noted as 70° which must be corrected as usual.

True altitude turns out to be 70°10.8’ which is rounded off to 70°11. If you subtract this from 90° and add the declination you will have your latitude. Note that 90° is often written as 89°60’ in order to make it easier to subtract. This is a horizontal position line which is also a segment of a parallel of latitude. Beware that you don’t adjust the other way and follow the sun down again!

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