This technique uses an extended noon site to give both latitude and longitude. It is far more simple but may be less accurate than the more orthodox methods using intercepts and azimuths. Accuracy is reduced if sailing rapidly north or south, the adjustments for this are not given here.
In brief, a number of sights are taken from 30mins before noon until 30 mins after noon . These are plotted on graph paper to show the altitude and time when the sun was highest in the sky (i.e. local noon). The sun’s altitude at noon gives the ship’s latitude. The exact time of local noon gives the ship’s longitude.
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- Estimate the time of local noon – estimated longitude (your EP) in degrees x4 gives time difference from Greenwich in mins – add this to time of noon at Greenwich (from tables) to give approx local noon. (subtract if long is easterly) e.g. Here at the Canaries I know my long. is around 013deg west, 13x4=52 so I add 52 mins to Greenwich noon time today of 1144 to give an approximate local noon of 1236.
- Take extended noon sight – every 4 mins from about 30mins before until 30mins after noon take and record the sun’s altitude and time using sextant and accurate watch. You can correct the sextant readings for height of eye etc. later.
- Plot sights – on graph paper plot the results with time along the bottom (x-axis) and altitude up the side (y-axis). You will have to convert mins of time into decimals e.g. 0.5 mins rather than 30secs. Draw a curve to fit the plotted points as accurately as you can.
- Extract results – judge the highest point of the graph and note the time and the altitude at this point; this is your local noon time and altitude.
- Calculate longitude – convert the resulting time from the graph back from decimal to secs then use the tables to extract the sun’s GHA ( Greenwich hour angle) at this time. The GHA is your longitude.
- Calculate latitude – correct the altitude from the graph for height of eye etc using the tables, convert to zenith distance (ZD) using formula ZD= 90 minus altitude, add ZD to sun’s declination at this time (from tables) to give latitude (assumes ship in northern hemisphere and it’s winter i.e. ship is north of equator and sun is south of it).
- Estimate local noon as before.
- Take a sight around 20 mins before noon , record altitude and time.
- Close to noon study sun through sextant and record altitude when it is at its highest point – this is used to measure latitude as above.
- About 20 mins after noon set sextant to the altitude recorded during the pre-noon sight, record the time when the sun falls to this altitude, calculate the time half way between the pre noon and the post noon sights. Use this time to calculate longitude as described earlier.
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