Astro: My Disconcerting discrepancy

[ARO]

Lunar distances have never been used to fix a position. This was a technique to find GMT. You observe the distance between the center of the Moon an a star. At a given time, this angle is the same, regardless of the observers position. The observed distance was then used to calculate the true time. Nautical tables like Norris contained tables to support the calculation. Typical use was a chronometer check, just like we still check our steering compass using celestial bearings.

 

[nigel1]

And a quote from this website
http://oceannavigation.blogspot.com/2009/01/how-to-use-marine-sextant.html


Moon Sights
When observing the Moon, use the same procedure as for the Sun. Sights of the Moon are best made during daylight hours or twilight when it is easier to see it and the horizon. At night you can get false horizons below the Moon because the Moon illuminates the water below it.

So I guess my old Captain from many moons ago did know what he was talking about.
Just to digress, he was the Master of the "Gothic" which was used by the our present Queen and hubby for a royal tour.
Later in the vessels life, she suffered a major fire, which gutted the bridge. I believe that the skipper got the ship back to NZ using some very rudimentary nav techniques. He got the OBE out of it anyway.

 

[estarzinger]

Lunar distances have never been used to fix a position. This was a technique to find GMT. You observe the distance between the center of the Moon an a star. At a given time, this angle is the same, regardless of the observers position. The observed distance was then used to calculate the true time. Nautical tables like Norris contained tables to support the calculation. Typical use was a chronometer check, just like we still check our steering compass using celestial bearings.

Sorry but lunars were in fact used before chronometers were common (from about 1750 to 1850), to determine time, which then determined longitude, which is then half a position fix.

From Slocum "I found from the result of three observations, after long wrestling with lunar tables, that her longitude agreed within five miles of that by dead-reckoning."

More here if you are interested. . . "The lunar distance method is a way of finding one's longitude without an accurate clock."

 

[taifun]

A bit of googling led me to the "Sextant Handbook" by Bruce Bauer, of which a small part of the book is shown on google books. A quoye from which is:
A moon illuminated horizon can be used but the navigator must be careful not to be tricked. More on this in Chapter 6"

Predictably, I cannot find Ch 6, so, if anyone out there has a copy of the book, please put us out of our misery.

I don't have access to the book but I'm pretty sure the author refers to the fact that a false horizon can sometimes occur under a moon illuminated horizon. What appears to be the horizon is closer to the observer than the true horizon. Think about taking a sight at midnight when there is a full moon. The general recommendation is to use binoculars to make sure that what you see is the true horizon, in other words be "careful not to be tricked".

From one of your earlier posts

The captain on the ship, who held an Extra Master's certificate and had been Examiner of Masters and Mates fior the BOT (as it was known then), explained to me that he moon should not be used as by some trickery, the moon light would cause an apparent lowering of the sea horizon.

This fits in with the "Irradiation correction" that at one time was used in the Nautical Almanac (from 1958 - 1970) but later investigations did not support an average value. There are to many factors involved, including the individual observer.
Irradiation is a psychological effect in the eye that causes a brighter area, observed next to a darker one, to appear to be larger than it is.

This is treated in Bowditch from 1977 Vol 1 p 423 §1609, but unfortunately not in the latest pdf edition.

 

[nigel1]

Nice to know there is some truth behind all this, thanks.

 

[Skylark]

Thanks for the explanation, taifun.

I have Bowditch Pub 9, 2002 bicentennial edition. Irradiation correction is defined within the glossary but not in the index, hence is not covered within the main body of text.

Irradiation is the apparent enlargement of a bright surface against a darker background.

Irradiation correction is applied to sextant altitude and is caused by apparent enlargement of the bright surface of a celestial body against the darker background of the sky.

His description of moon sight reduction follows the same sequence as other heavenly bodies and assumes a clearly defined horizon is available. Moon-specific correction have already been posted.

So the conclusion seems to be "no later than twighlight"

Section 1614 refers to the use of an artificial horizon. After observation, apply index correction. Take half of the remaining angle and apply all other relevant corrections except dip (height of eye).

 

[Neil]

So would the use of an artificial horizon mitigate against irradiation errors? Both images of the moon would be subject to the same effect, canceling each other out?

On a related note, is a moon-run-moon sight ever used?

 

[estarzinger]

So the conclusion seems to be "no later than twighlight"

Practically speaking, when taking late twilight and into night sights, its best to pre-compute and pre-set the sextant. This make it MUCH easier to find a specific star/planet and bring it down to a dim horizon. If you pre-compute you are reasonably certain to bring the moon down to the correct horizon and not to a shorter false horizon. So, I personally find the Moon to be a perfectly usable object, assuming proper procedural hygiene is used to avoid the normal possible errors, and at night, pre-computing is one of these hygiene factors.

Again practically speaking, the irradiation effect can be forgotten. First, its tiny and most of us shooting from the deck of a small sail boat are not achieving pinpoint accuracy. Second, irradiation effect re the moon only applies to the upper limb sight. Third, the correction has been discontinued in the almanac so you are unlikely to have it on hand in any case.