NOON SIGHTS
The noon sight is a simple calculation and the result provides your exact latitude.
CALCULATING LOCAL NOON TIME
This is based on the longitude of your rough dead reckoning position. The sun revolves around the earth, which means 360° of longitude in every 24 hours. That means the sun moves 15° of longitude per hour or four minutes of time for each degree of longitude. That’s one minute of time per 15 minutes of longitude. Figure these out with paper and pencil and remember them by heart – it will make the calculations easier.
The sun crosses the Greenwich Meridian (000º) roughly at 1200 each day. The exact time for each day is called Meridian Passage (MP), and the value is found at the lower righthand corner of each page of the Nautical Almanac (NP314, £44, UKHO), which covers three days per page.
For the local noon time at your longitude on a certain day you first check the Meridian Passage time for the day, e.g. 1146.
You then take your Dead Reckoning (DR) longitude, e.g. 62º 20’ E. As we are on eastern longitudes, our noon will be before London. The time difference will be 62º x 4 minutes per degree = 248 mins, which is four hours and eight minutes.
The additional 20 minutes of longitude make for roughly one more minute of time, so the local noon will be four hours and nine minutes before MP, which gives us a local noon time of 0737 UTC (=GMT).
TAKING THE NOON SIGHT
Next you get on deck with the sextant at least five minutes before the calculated local noon time and start taking sun sights once every 30 seconds. The sun will be slowly ascending, so you’ll keep getting slightly bigger values each time. The rate of increase slows down and at noon it will not change. A while later the lower limb of the sun will be below the horizon when you look at it through the sextant. Don’t adjust the sextant in the other direction, but instead read the last and highest value when the sun stopped going up. That is your Noon sextant height (H/s).
CALCULATING NOON LATITUDE
You’ll need to correct your Noon H/s for index error and height of eye, but these are the same every time, get Noon H/o (observed height). From the H/o we need the so-called Zenith Distance (ZD) for the calculation. Instead of the angle between the horizon and the sun, the ZD is the angle between the sun and the Zenith which is vertically above your head. This is easily calculated by subtracting the H/o from 90°. When you do the subtraction, instead of 90°, write 89° and 60 minutes (which is the same thing, but easier to work) and subtract your H/o from that to work out the ZD.
Next, you get the declination of the sun at the time of your local noon. Check it for the closest whole hour from the Nautical Almanac and add or subtract the minutes according to the d= value at the bottom of the declination column and the hourly increase or decrease according to the minutes of time of your local Noon time.
You get your Noon Latitude by either adding the ZD and declination or subtracting declination from your ZD. If the latitude and declination are the same name (both S or both N), you add the declination to the ZD. If they’re different, you subtract the declination from the ZD. One more thing: if the latitude is less than declination but the same name, then subtract the Zenith Distance from the declination. And voila! There’s your Noon Latitude!