Another confessional - Astro?

Uricanejack said:
Both your Boat and the Sun moved during the interval.
Click to expand...

Thus ushering in the 'MOO' and 'MOB' corrections much disputed among V-Force astro-navigators and those rarified individuals who had done the 'Spec N' course..... and Mear's Slides for Acceleration Corrections..... and Fix-Monitored Azimuth techniques..... and.... and....

Yes. "Get a GPS."

But you still need to make good decisions based on the fix info you derive from that.... and your evaluation of what to 'allow' and in what direction, should it be degraded in any unknown way.... and that's a whole different 'ball of chalk'!
 
Uricanejack said:
I think you are mixing up two different things.
The method of determining longitude by taking a sight a equal altitude before and after noon as described by several poster's is imprecise for the reasons you have posted along with two other important errors being introduced.
Both your Boat and the Sun moved during the interval.

This is not the Longitude by Chronometer method of calculating a sight.
While I Personally chose to use the Marc St Hillarie. or intercept terminal point method. which is more modern. than the older longitude by chronometer method.
both have similar accuracy the difference is in the calculation not the observation.

In the Longitude by chronometer method.
You take the sight in the same way.
Instead of calculating the DR distance from the sun and comparing to your observed distance from the sun to find the nearest point through which your position line passes.
You use a different calculation to determine the longitude on your DR latitude through which your observed position line passes. you can then run this up to Noon for a noon DR to use the same way as with the intercept method.

I have used both with equal success or lack of success. In the end I preferred the intercept method finding it easier.
Click to expand...

Most, understand the Marc St Hilaire intercept method, which alone, doesn't actually determine longitude, but uses an 'assumed longitude' with which you calculate backwards to find a calculated altitude, thus enabling you to find the difference between that & observed sextant altitude, from this you plot a position line.
Where you are on that position line, can only then be determined using a second position line, thus giving your longitude.

What is the "different calculation" you refer to, that determines longitude?
 
I don't know if it is still in print but Eric Hiscock's "Voyaging under sail" gives a wonderfully clear introduction to the subject that even I understood.
 
alant said:
Most, understand the Marc St Hilaire intercept method, which alone, doesn't actually determine longitude, but uses an 'assumed longitude' with which you calculate backwards to find a calculated altitude, thus enabling you to find the difference between that & observed sextant altitude, from this you plot a position line.
Where you are on that position line, can only then be determined using a second position line, thus giving your longitude.

What is the "different calculation" you refer to, that determines longitude?
Click to expand...

Long By Chron is just another calculation to find a position line.
With Marc St Hilaire method, you calculate the "Calculated Zenith Distance" of the body, using Lat, Dec, and LHA.
With Long by Chron, you calculate the LHA using Lat, Dec, and True Altitude

The calculation is

Cos(P) = Sin(TrueAltitude) (-/+) Sin(Latitude) X Sin(Declination)
Cos(Latitude) X Cos(Declination)

P is the LHA

Having calculated P, (LHA), and knowing the GHA from the almanac, you can determine the longitude of the position line where it crosses the DR Latitude

The calculation works best when the observed body is on or close to the prime vertical (bearing is close to 090 or 270)
 
alant said:
Most, understand the Marc St Hilaire intercept method, which alone, doesn't actually determine longitude, but uses an 'assumed longitude' with which you calculate backwards to find a calculated altitude, thus enabling you to find the difference between that & observed sextant altitude, from this you plot a position line.
Where you are on that position line, can only then be determined using a second position line, thus giving your longitude.

What is the "different calculation" you refer to, that determines longitude?
Click to expand...

The calculation has been posted Nigel 1.

Long by chronometer. Just finds a point through which a single position line passes.
Theoretically if you were to take a sight and do both calculations you would determine two different points through which the same single position line would pass.
you would still have to cross this with another position line in order to fix your position.
Your noon position is still a running fix.
As said the closer the azimuth is to 090 or 270 the closer the longitude.
This is also true by Marc St Hillarie

If anyone is interested Il see if I can dig out my old sight book and post a layout.
 
For those interested in such things, can I recommend the wonderful book "The Barefoot Navigator" which explores the history of navigation and describes methods used throughout history from the ancient Egyptians, Polynesians, Vikings, etc. A fascinating and well written book.

One thought... someone mentioned that the "long by chronometer" method is inherently imprecise and that the best that could be hoped for would be about 10 miles. 10 M seems inaccurate now but, for the likes of James Cook, travelling where there were unknown currents and sometimes blown dozens of miles off course by Southern Ocean storms that lasted for days, a 10 mile fix would have been very welcome. Just a thought.
 
I think with a bit of practice you'll do better than 10nm which for the purposes of land fall with charts with features and accurate depth data it would be good enough to be safe. Based on a landfall on the North coast of Spain when we had an uncertainty of 20 miles or more (we were bordering on lost)
 
Neil_Y said:
I think with a bit of practice you'll do better than 10nm which for the purposes of land fall with charts with features and accurate depth data it would be good enough to be safe. Based on a landfall on the North coast of Spain when we had an uncertainty of 20 miles or more (we were bordering on lost)
Click to expand...

Pre GPS, didn't navigators, use the offset method of reaching landfall, so not requiring 'exact' position?
ie making sure they hit landfall, to one definite side of their destination, then able with certainty, to turn port/starboard to reach it.
 
alant said:
Pre GPS, didn't navigators, use the offset method of reaching landfall, so not requiring 'exact' position?
ie making sure they hit landfall, to one definite side of their destination, then able with certainty, to turn port/starboard to reach it.
Click to expand...

There is an interesting analysis of this in the aforementioned 'Barefoot Navigator'. As you describe Vikings would know their lat - typically by observing that the pole star was adjacent to a notch in their mast when they stood at the helm. Then they would sail East and West in ever increasing legs until they reached their home port. Of course, one of them sailed a bit far East and discovered America!
 
Yes the offset landfall was one of the methods in the arsenal.
I used it occasionally, more frequently when sailing.
More often I would plan to make land fall near a significant navigational feature. A major light.

You could choose to offset intentional as you neared landfall based on the existing conditions. And how long it was since your last good fix. In practice I rarely found it necessary. It is a technique I have used more when sailing in coastal waters in poor visibility.

I do recall one particular occasion where we had not been able to get a reliable observation for almost a week. We adjusted for an offset

Sailing down from the Clyde to Brittany arriving in poor visibility we used one of those old hand held RDF sets. It worked quite well
 
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