01-15-2017, 09:12 PM
(02-29-2016, 02:18 PM)CelNav57 Wrote: The following is copied from part of JeremyParker's post over at; [/url]
[url=https://thenauticalalmanac.com/Forum/showthread.php?tid=14]
Does anyone really shoot the Moon?
His question follows;
My follow up question perhaps belongs in a different thread - Who bothers with Noon sights, and why?
I ask because there seems to be an almost religious attachment to the noon sight amongst many navigators and a belief that it is necessary to determine latitude to obtain a fix! Consequently we hang around on deck for 20 minutes awaiting the magical zenith moment while our lunch is going cold in the galley below, only to have the only clouds of the day obscure the sun right about LAN. Why not snap off and reduce a quick series of sights whenever we please and cross the resultant LOP with that of our advanced morning sight. This still renders our position; we can still determine Lat and Long. We only NEED a Latitude if we are performing a Time Sight - an observation by which we determine LHA and thence Longitude - but when using the intercept method precise Lat is not necessary.
Many sailors still use noon sights though it is not necessary with modern methods using position line plotting of a single body, or for fixing position if more than one celestial body is observable.
It was/is traditional for various reasons.
Masters would use the noon to noon positions to find daily distance run for their write-up of the ship's log which was/is a legal document and therefore important. The great clipper ship races were assessed on their daily distance run as well as who finally got home first.
In the days of practically everyone except the very wealthy traveling any real distance by ship (which was believe it or not up to about the 1960s when air travel became affordable for the average working person), and the daily noon to noon run became of interest to not only the ship's master but also to the passengers, and there would probably be a lottery available for the passengers too.
It also has an astronomical and practical significance.
At (local) noon the Sun is by definition on the observer's meridian, which means the Sun is at it's maximum altitude, there is no longitudinal component in the PZX triangle which would require the usual more elaborate calculations, and so the calculation of an accurate latitude becomes very easy with simple addition/subtraction only needed using the declination of the Sun for the time at the local noon.
Although for more abstruse technical reasons (because the declination is changing all the time, and the ship is moving too) it is not strictly true that the maximum altitude is the time of exact noon, it is sufficiently close to be of no great significance for practical assessment of the noon sight for obtaining latitude.
The procedure for maximum accuracy should be to first calculate when noon actually is (can be done easily within a minute of time) for your expected location i.e. DR position for noon, - then take the sight (Sun's altitude) at that time.
You do not therefore have to hang around waiting for maximum altitude if the procedure is done properly. You just take a sight at the correct calculated time.
Most sailors I believe do not bother with this and just wait and take the maximum altitude and use that.
If the noon sight cannot be take until after (or before) the local noon because of cloud for instance, then there are various ways of still using the sight taken, but including corrections to allow for it not being exactly noon.
These corrections are to be found in what are called Ex-Meridian tables. Meaning ex- (i.e. outside) the Meridian .. correction tables.
Once the Marcq St Hilaire method was discovered in 1875, any observable sight of a celestial body at any time (within wide limits) can be plotted as a position line on a chart or plotting sheet, and noon sights in effect became relegated to more of a traditional technique. . . though still useful for a simple method for finding latitude.
Douglas Denny. Bosham. England.