Finding Solar Noon While Sailing

[Teebeedee]

How do you find out when the sun is the highest point in the sky during a passage for finding latitude and longitude? Is there a way to find Langitude without knowing solar noon if you are in the Southern Hemisphere?

Thank you for any help

[StuM]

Take a series of sights as the sun passes from slightly east to slightly west.

[Teebeedee]

What time on average should your first be at to be safe?

Thank you for the quick reply.

[Bruce K]

Take a series of sights starting about 1/2 hour before and continue till angle starts to decrease, then a few more, plot on graph paper, you will then be able to extrapolate to get local noon.

[Bruce K]

Originally Posted by Bruce K

Take a series of sights starting about 1/2 hour before and continue till angle starts to decrease, then a few more, plot on graph paper, you will then be able to extrapolate to get local noon.

Should have said time on horizontal axis and angle on vertical axis

[SeanPatrick]

Originally Posted by Teebeedee

Is there a way to find Langitude without knowing solar noon if you are in the Southern Hemisphere?

Time sight. Take a sight of the Sun when it is close to due East or West.



cos(LHA) = (sin(Ho) - sin(Lat.) ∙ sin(Dec.)) / (cos(Lat.) ∙ cos(Dec.)))


If the Sun is east of your meridian (i.e. before noon), subtract LHA obtained above from 360°.


If you are in west longitudes, subtract LHA from GHA of the Sun to get your longitude. If you are in east longitudes, subtract GHA from LHA.


The closer the Sun is to due East or West, the less error you will have due to using an assumed latitude.


If you'd rather use tables than a calculator, I recommend Martelli's time sight tables.

[ZULU40]

Originally Posted by StuM

Take a series of sights as the sun passes from slightly east to slightly west.

and record the precise time

[copaco]

What I read, was to note height of sun many minutes before high noon (witch stays put for 4 minutes).Keep the sextant set on this height and wait until sun commes down to this height and divide by two.

[waterman46]

Originally Posted by StuM

Take a series of sights as the sun passes from slightly east to slightly west.

Yes. To save taking too many shots and using an assumed Longitude, you can look up in Almanac what the GMT, hence ship's time will be for you to expect for the meridian passage of sun.

Take sun's altitude about every 2 minutes starting about 15 minutes before it peaks (yes it is tedious) and the a couple after it peaks, then plot them up on paper, altitude vs. GMT, and choose the highest point of a smooth curve connecting the points. You now have time and sextant altitude of local noon.

Do all corrections (sextant, U/L limb, HE) as normal.

Then look in Nautical Almanac to get LHA of Sun for this date and time. To get Latitude you look up Sun's declination and compare to your peak altitude.

[barnakiel]

I never use noon to find long. This simply gives crazy error rates that are so easy to avoid with morning LOP moved towards the afternoon one.


My fave method, in brief is this:


1) get your lat at or around noon,
2) keep on going E or W for about 3-4 hours,
3) make afternoon LOP, plot.


You are where the afternoon LOP cuts your lat.


Now if I see the wx is less stable, I will make:


1) morning LOP,
2) noon lat,
3) afternoon LOP.


I cannot remember last time I did not get position this way. The amt of work is minimal as are any calculations.


Cheers,
b.

[Snowpetrel]

Originally Posted by barnakiel

I never use noon to find long. This simply gives crazy error rates that are so easy to avoid with morning LOP moved towards the afternoon one.


My fave method, in brief is this:


1) get your lat at or around noon,
2) keep on going E or W for about 3-4 hours,
3) make afternoon LOP, plot.


You are where the afternoon LOP cuts your lat.


Now if I see the wx is less stable, I will make:


1) morning LOP,
2) noon lat,
3) afternoon LOP.


I cannot remember last time I did not get position this way. The amt of work is minimal as are any calculations.


Cheers,
b.

Funny thing, I'd always thought the same, but I tried a longitute by Local apparent Noon and my first go was only 1' out from the GPS. Subsequent sights weren't as good more like 5-15 miles but the technique worked well enough for me if I averaged and plotted multiple sights carefully. But Sun-Run-Sun is quicker and more accurate.

[GrowleyMonster]

Yes, longitude by time of LAN can give you a significant error. Or maybe NOT so significant. In midocean, a few miles is no big deal. And on a slow moving sailboat, careful observations carefully graphed with accurate shot time should be within a few miles of correct Longitude. However, a running fix from two or more observations advanced or retarded to one fix time is going to be spot on the money. Relatively speaking, anyway. This of course depends very much on keeping a good DR track through the day. Time sights of a body very close to East or West of you can give a pretty accurate Longitude. The Time Sight was a very common navigation technique before Altitude/Intercept methods became popular around 1910 or so and it still works today, though the altitude/intercept method will usually give you a more accurate LOP than a Longitude from the time sight.


One problem with a time sight of the Sun is that when it is most nearly E or W of you, the Sun is very low and refraction can vary significantly with temperature and barometric pressure, especially at low altitudes. Errors can amount to a few miles in worst case scenarios when the Sun is very low, especially if you just use the standard correction table in non standard conditions. The further away from E or W the sun is, the more a time sight depends on having an accurate Latitude, also.



Myself, I much prefer to not take a Sun observation (or any observation, besides an azimuth or amplitude for compass check) when it is below 15deg and to use the more modern altitude/intercept method of reduction to an LOP which will generally not be a N-S line. If nothing else, if the LOP passes very close to my DR position for shot time, it at least gives my DR position a greater degree of certainty.


Once again, a good DR is the very cornerstone of navigation. It will make a big difference in the accuracy of your celestial navigation.