Sextant to Learn Navigation Old School

[barnakiel]

;-)


This is what I imagined.


I simply take one morning shot then the noon one. Then move the morning line along the track.


When no sun in morning, I take a noon shot then an evening one. Then move the noon line.


In fact, because I exercise mostly between Cabo Verde and Barbados, I hardly ever need to do any line moving as I tend to do parallel sailing all day long and thus our noon lat = our morning lat = our evening lat.


A simple trick for E-W-E crossings.


b.

[sailorman14]

Originally Posted by SeanPatrick

May I ask what technique you use to determine the time of local noon?

It’s very simple. I start shooting the sun about 10 minutes before noon and record my reading. Then continue until I get the highest sextant reading. That gives me my latitude. Then I set my sextant back to the reading I took before noon and watch the sun until it drops to that reading. I log the time for both the morning and afternoon sites. Calculate the time difference and divide by 2. Then add the time to the morning site. That gives me an exact high noon time reading.

[john61ct]

"very simple" 8-)

[SeanPatrick]

Originally Posted by sailorman14

It’s very simple. I start shooting the sun about 10 minutes before noon and record my reading. Then continue until I get the highest sextant reading. That gives me my latitude. Then I set my sextant back to the reading I took before noon and watch the sun until it drops to that reading. I log the time for both the morning and afternoon sites. Calculate the time difference and divide by 2. Then add the time to the morning site. That gives me an exact high noon time reading.

Good. I was hoping you weren't simply using the time of your highest observation as the time of local noon. As I'm sure you know, the sun can appear to "hang" at its highest apparent altitude for several minutes making it very difficult to judge when the exact time of culmination occurs, thus making it difficult to determine an accurate longitude. Of course, this phenomenon is helpful when determining latitude because in that case it is the altitude you are after, not the time at which it occurs.

However, the method you are using is also prone to some error. Consider that at certain times of the year, the declination of the sun can be changing almost as fast as one arc minute per hour of time. Add to that the fact that your vessel is most likely moving. These two factors will both change the apparent altitude of the sun. Therefore, dividing the difference in time between two equal observations on either side of LAN [local apparent noon] will not necessarily result in the "exact" time of LAN.

Perhaps this is one reason that the noon site was rarely (if ever) used to determine longitude in the heyday of celestial navigation. A much more accurate and commonly used method was to shoot a "time sight" in the morning for longitude, a noon sight for latitude, and another time sight in the afternoon. Each of these sights would be "run up" to the following one to give the position of the ship. The timing of the time sights is not even of much importance, as long as they occur within a few hours of when the sun is due east or west of the observer.

The calculations for the time sight are relatively simple and can be completed in a couple of minutes with a scientific calculator or a set of log tables such as Martelli's. The latter only requiring the ability to add and subtract.

[barnakiel]

Originally Posted by sailorman14

It’s very simple. I start shooting the sun about 10 minutes before noon and record my reading. Then continue until I get the highest sextant reading. That gives me my latitude. Then I set my sextant back to the reading I took before noon and watch the sun until it drops to that reading. I log the time for both the morning and afternoon sites. Calculate the time difference and divide by 2. Then add the time to the morning site. That gives me an exact high noon time reading.

...


OK.


I never do this as there are a number of limitations that do not work for me on a passage.


One that you need to measure three times. Two that we assume the third measurement will happen (e.g. no cloud). And three that it is darn easy to make small mistakes at each side that may add or deduct to the overall error of our calculated meridian passage.


With this method you can go one step further and get your longituude from sunset or sunrise time, eh? No need for sextant then.


b.

[GrowleyMonster]

Originally Posted by barnakiel

...


OK.


I never do this as there are a number of limitations that do not work for me on a passage.


One that you need to measure three times. Two that we assume the third measurement will happen (e.g. no cloud). And three that it is darn easy to make small mistakes at each side that may add or deduct to the overall error of our calculated meridian passage.


With this method you can go one step further and get your longituude from sunset or sunrise time, eh? No need for sextant then.


b.

Yeah I know, should be simple with a good calculator, right? LOL not very accurate due to refraction at such low altitudes. I tried it. Sure, you can get close enough to not run over very many continents, and it is a fun thing to try, but it is not any more accurate than comparing time of LAN. You would think it would be, but nah. Try it next time you get a chance.

[Steve Bean]

Argentine sailor Alberto Torroba navigated across the Pacific to the Philippines using a plumb bob. He knew which star to stay under. (In a 15-foot dugout canoe!)
See also Marvin Creamer, who circumnavigated - with crew - without clock, compass, sextant, or other instruments. He's in Wikipedia.

[barnakiel]

Yep.


There were also those French twins who sailed from Africa to Caribbean without a watch, a sextant, charts.


They did make it allright and they did find the landfall they aimed at!


Quite some feat!


b.

[GrowleyMonster]

(Guy with no navigation equipment with boat on the beach) "No, really! I meant to make landfall here, honest!"

[CaptBobR]

The trade winds and current does help you to reach the Lesser Antilles though without any navigation

[barnakiel]

Originally Posted by GrowleyMonster

(Guy with no navigation equipment with boat on the beach) "No, really! I meant to make landfall here, honest!"

;-)


hard luck ;-)


But I think they landed on Antigua. If I remember well, no sandy beaches on the windward coast there!


b.