Noon sights by sextant

[Skylark]

Greenwich Mean Time - 12:00 is the mean average time that noon occurs at Greenwich.

I've just looked at the almanac (see my post number 11) and the Meridian Passage of the sun was 11h56 today. A quick flick through the pages shows the MP will be 12h07 on July 25th and 11h44 on November 10th.

 

[cliffdale]

Hi

I don't know it this helps but I found this site useful.

http://celestial-navigation-course.com/

Cliff

 

[alant]

I don't make a habit of disagreeing but the noon sight is the simplest way of getting Lat and long. With any Long you need accurate time so assuming you have an accurate watch set to a known time zone you can determine your local noon by tracking the sun sights up to its high point (zenith for those learning the sextant) No tables or formulae needed.

As the time needs to be accurate and the curve is quite flat at the top of the arc the sun makes, drawing a graph of heights and times before and after noon can help. You get a nice parabola of fixes from which you can draw a fairly accurate curve, from this pick a n angle either side of noon and note the time from the graph divide by two and you get your local time of noon (when the sun was exactly over head as far as you were concerned) Noon moves around the planet at 360deg every 24hrs so every minute is 0.25 of a degree of longitude.

In Plymouth at roughly 4deg west of London our noon is 16 minutes after Greenwich/London. So if you watched the sun you would see it reach it's highest point at 12.16

But, if you were in Plymouth, your watch & radio time signals, would be on Greenwich Time anyway.

 

[Bilgediver]

But, if you were in Plymouth, your watch & radio time signals, would be on Greenwich Time anyway.

However as the original poster pointed out this was not noon for sight purposes at his location. So about 16 minutes after noon on the radio he could start looking for max altitude if in Cornwall. Blimey he is only playing and not trying to get a fix to 1 mile

 

[John Barry]

But, if you were in Plymouth, your watch & radio time signals, would be on Greenwich Time anyway.

And, as has been said previously, the Sun would reach its high point at 12:16 minus the equation of time - currently +3m41s - so 12:12:19.

Actually, the Sun doesn't reach it's highest point at meridian passage, because the Sun's declination is changing. This means that the highest point is before (declination moving away from you) or after (declination moving towards you) meridian passage. Currently for Plymouth (50°N) maximum altitude is 7.8 seconds (1'.95) after meridian passage. It's more at the equinoxes, and more with a lower maximum altitude (higher latitudes).

Effectively it makes no difference to the meridian sight as you're not trying to get longitude from it - you're trying to get latitude and the maximum error for that is less that 0'.1.

Nerdy bit over

John